Murata Electronics North America DNT900 User Manual

IC Radio Standards Specification: RSS-210

ACS Report Number: 08-0361 - 15C

Certification Exhibit

FCC ID: HSW-DNT900

IC: 4492A-DNT900

FCC Rule Part: 15.247

Manufacturer: Cirronet Inc.

Model: DNT900C, DNT900P

Manual

5015 B.U. Bowman Drive Buford, GA 30518 USA Voice: 770-831-8048 Fax: 770-831-8598

www.RFM.com

DNT900 Series

900 MHz Spread Spectrum Wireless

Industrial Transceivers

Integration Guide

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Important Regulatory Information

FCC ID: HSW-DNT900

IC: 4492A-DNT900

THIS DEVICE COMPLIES WITH PART 15 OF THE FCC RULES. OPERATION IS SUBJECT TO THE FOLLOWING TWO CONDITIONS. (1) THIS DEVICE MAY NOT CAUSE HARMFUL INTERFERENCE, AND (2) THIS DEVICE MUST ACCEPT ANY INTERFERENCE RECEIVED, INCLUDING INTERFERENCE THAT MAY CAUSE UNDESIRED OPERATION.

This Class B digital apparatus complies with Canadian ICES-003.

Cet appareil numérique de la classe B est conforme à la norme NMB-003 du Canada.

FCC User Information

“NOTE: This equipment has been tested and found to comply with the limits for a Class B digital device, pursuant to Part 15 of the FCC Rules. These limits are designed to provide reasonable protection against harmful interference in a residential installation. This equipment generates, uses, and can radiate radio frequency energy and, if not installed and used in accordance with the instructions, may cause harmful interference to radio communications. However, there is no guarantee that interference will not occur in a particular installation. If this equipment does cause harmful interference to radio or television reception, which can be determined by turning the equipment off and on, the user is encouraged to try to correct the interference by one or more of the following measures:

• Reorient or relocate the receiving antenna.

• Increase the separation between the equipment and receiver.

• Connect equipment to an outlet on a circuit different in which the receiver is connected.

• Consult the dealer or an experienced radio/TV technician for help.”

Warning: Changes or modifications to this device not expressly approved by RFM Inc.

could void the user’s authority to operate the equipment.

RF Exposure

In accordance with FCC requirements of human exposure to radiofrequency fields, the radiating element shall be installed such that a minimum separation distance of 23cm shall be maintained from the user and/or general population.

Industry Canada

This Class B digital apparatus meets all requirements of the Canadian Interference Causing Equipment Regulations. Operation is subject to the following two conditions: (1) this device may not cause harmful interference, and (2) this device must accept any interference received, including interference that may cause undesired operation.

Cet appareillage numérique de la classe B répond à toutes les exigences de l'interférence canadienne causant des règlements d'équipement. L'opération est sujette aux deux conditions suivantes: (1) ce dispositif peut ne pas causer l'interférence nocive, et (2) ce dispositif doit accepter n'importe quelle interférence reçue, y compris l'interférence qui peut causer l'opération peu désirée.

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“To reduce potential radio interference to other users, the antenna type and its gain should be so chosen that the equivalent isotropically radiated power (e.i.r.p.) is not more than that permitted for successful communication.”

“This device has been designed to operate with the antennas listed below, and having a maximum gain of 6 dBi. Antennas not included in this list or having a gain greater than 6 dBi are strictly prohibited for use with this device. The required antenna impedance is 50 ohms.”

Cushcraft S8963B 5 dBi gain dipole

Astron 918-2 6 dBi gain yagi

OEM Installation and Compliance Labeling

The DNT900 module is labeled with its own FCC ID number, and, if the FCC ID is not visible when the module is installed inside another device, then the outside of the device into which the module is installed must also display a label referring to the enclosed transmitter module.

This exterior label can use wording such as the following:

“Contains Transmitter Module FCC ID: HSW-DNT900” or

“Contains FCC ID: HSW-DNT900”

Any similar wording that expresses the same meaning may be used. The Grantee may either provide such a label, an example of which must be included in the application for equipment authorization, or, must provide adequate instructions along with the module which explain this requirement. In the latter case, a copy of these instructions must be included in the application for equipment authorization.

The antenna connections from the module to the certain antennas approved with this device are not unique and require Professional installation.

See section 3.8 of this manual for regulatory notices and labeling requirements. Changes or modifications to a DNT900 not expressly approved by RFM may void the user’s authority to operate the module.

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Table of Contents

1.0 Introduction ........................................................................................................................................... 5

1.1 Why Spread Spectrum?.................................................................................................................................... 5

1.2 Frequency Hopping versus Direct Sequence..........................................................................................6

2.0 DNT900 Radio Operation ......................................................................................................... 7

2.1 Network Synchronization and Registration........................................................................................ 7

2.2 Transparent and Protocol Serial Port Modes..........................................................................................8

2.3 RF Data Communications........................................................................................................................ 8

2.4 RF Transmission Error Control ......................................................................................................... 9

2.5 Network Configurations .................................................................................................................... 9

2.5.1 Point-to-Point Network Operation.................................................................................................. 9

2.5.2 Point-to-Multipoint Network Operation ....................................................................................... 10

2.6 Full-Duplex Serial Data Communications ...................................................................................... 10

2.7 Channel Access ................................................................................................................................ 10

2.7.1 CSMA Modes..................................................................................................................... 11

2.7.2 TDMA Modes ...........................................................................................................................12

2.8 Network Configuration Planning ..................................................................................................... 13

2.9 Serial Port Operation ...................................................................................................................... 15

2.10 Sleep Modes...................................................................................................................................... 16

2.11 Encryption................................................................................................................................................ 18

3.0 DNT900 Hardware ............................................................................................................. 19

3.1 Specifications............................................................................................................................................20

3.2 Module Interface.......................................................................................................................................21

3.3 Input Voltages...........................................................................................................................................22

3.4 ESD and Transient Protection .................................................................................................................22

3.5 Interfacing to 5 V Logic Systems .............................................................................................................23

3.6 Power-On Reset Requirements ..............................................................................................................23

3.7 Mounting and Enclosures...............................................................................................................................23

3.8 Connecting Antennas......................................................................................................................................23

3.9 Labeling and Notices.......................................................................................................................................24

4.0 Protocol Messages........................................................................................................................25

4.1 Protocol Message Formats.............................................................................................................................25

4.1.1 Message Types ....................................................................................................................................25

4.1.2 Message Format Details.............................................................................................................. 26

4.1.2 Escape Sequence ..................................................................................................................................27

4.1.3 CFG Select Pin .......................................................................................................................................27

4.1.4 Flow Control ....................................................................................................................................27

4.1.5 Protocol Mode Data Message Example .............................................................................................28

4.2 Configuration Registers ............................................................................................................................28

4.2.1 Bank 0 - Transceiver Setup..................................................................................................................28

4.2.2 Bank 1 - System Settings............................................................................................................. 30

4.2.3 Bank 2 - Status Registers ............................................................................................................ 31

4.2.4 Bank 3 - Serial.......................................................................................................................................33

4.2.5 Bank 4 - Host Protocol Settings ................................................................................................. 34

4.2.6 Bank 5 - I/O Peripheral Registers ........................................................................................................35

4.2.7 Bank 6 - I/O Setup........................................................................................................................................36

4.2.8 Bank FF - Special Functions ................................................................................................................38

4.2.9 Protocol Mode Configuration/Sensor Message Example.................................................................38

4.2.10 Protocol Mode Event Message Example............................................................................................39

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5.0 DNT900DK Developer’s Kit ................................................................................................................ 40

5.1

5.2

5.3

5.4

5.5

5.6

DNT900DK Kit Contents................................................................................................................ 40

Additional Items Needed ............................................................................................................... 40

Developer’s Kit Default Operating Configuration .......................................................................... 41

Development Kit Hardware Assembly........................................................................................... 41

DNT900 Wizard Utility Program .................................................................................................... 43

DNT900 Interface Board Features ................................................................................................ 50

6.0 Demonstration Procedure ................................................................................................................... 53

7.0 Troubleshooting .................................................................................................................................. 54

8.0 Appendices ......................................................................................................................................... 55

8.1

8.2

8.3

Ordering Information...................................................................................................................... 55

Technical Support.......................................................................................................................... 55

DNT900 Mechanical Specifications............................................................................................... 56

9.0 Warranty.............................................................................................................................................. 58

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1.0 Introduction

The DNT900 series transceivers provide highly reliable wireless connectivity for either point-to-point or point-to-multipoint applications. Frequency hopping spread spectrum (FHSS) technology ensures maximum resistance to multipath fading and robustness in the presence of interfering signals, while operation in the 900 MHz ISM band allows license-free use in the US, Canada, Australia and New Zealand. The DNT900 supports all standard serial data rates for host communications from 1.2 to 460.8 kb/s. On-board data buffering and an error-correcting air protocol provide smooth data flow and simplify the task of integration with existing applications. Key DNT900 features include:

• Multipath fading resistant frequency hop- ping technology with up to 50 frequency channels (902 to 928 MHz).

• Support for point-to-point or point-to- multipoint applications.

• FCC 15.247 certified for license-free operation.

• 40 mile plus range with omni-directional antennas (antenna height dependent).

• Transparent ARQ protocol with data buffering ensures data integrity

1.1 Why Spread Spectrum?

• Selectable 1, 10, 100, 250, 500 or 1000 mW transmit power with a firmware interlock of 85 mW maximum for 500 kb/s operation.

• Optional AES encryption provides protection to eavesdropping

• Nonvolatile memory stores DNT900 configu- ration when powered off

• Dynamic TDMA slot assignment that maxi- mizes throughput.

• Simple serial interface handles both data and control at up to 460.8 kb/s

A radio channel can be very hostile, corrupted by noise, path loss and interfering transmissions from other radios. Even in an interference-free environment, radio performance faces serious degradation through a phenomenon known as multipath fading. Multipath fading results when two or more reflected rays of the transmitted signal arrive at the receiving antenna with opposing phases, thereby partially or completely canceling the signal. This problem is particularly prevalent in indoor installations. In the frequency domain, a multipath fade can be described as a frequency-selective notch that shifts in location and intensity over time as reflections change due to motion of the radio or objects within its range. At any given time, multipath fades will typically occupy 1% - 2% of the band. From a probabilistic viewpoint, a conventional radio system faces a 1% - 2% chance of signal impairment at any given time due to multipath fading.

Spread spectrum reduces the vulnerability of a radio system to interference from both multipath fading and jammers by distributing the transmitted signal over a larger region of the frequency band than would otherwise be necessary to send the information. This allows the signal to be reconstructed even though part of it may be lost or corrupted in transmission.

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Narrow-band versus spread-spectrum transmission

Figure 1.1.1

1.2 Frequency Hopping versus Direct Sequence

The two primary approaches to spread spectrum are direct sequence spread spectrum (DSSS) and frequency hopping spread spectrum (FHSS), either of which can generally be adapted to a given application. Direct sequence spread spectrum is produced by multiplying the transmitted data stream by a much faster, noise-like repeating pattern. The ratio by which this modulating pattern exceeds the bit rate of the base-band data is called the processing gain, and is equal to the amount of rejection the system affords against narrow-band interference from multipath and jammers. Transmitting the data signal as usual, but varying the carrier frequency rapidly according to a pseudo-random pattern over a broad range of channels produces a frequency hopping spectrum system.

Forms of spread spectrum - direct sequence and frequency hopping

Figure 1.1.2

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One disadvantage of direct sequence systems is that due to spectrum constraints and the design difficulties of broadband receivers, they generally employ only a minimal amount of spreading, typically no more than the minimum required by the regulating agencies. For this reason, the ability of DSSS systems to overcome fading and in-band jammers is relatively weak. By contrast, FHSS systems are capable of probing the entire band as necessary to find a channel free of interference. This means that a FHSS system will degrade gracefully as the channel gets noisier, while a DSSS system may exhibit uneven coverage or work well until a certain point and then give out completely.

Because it offers greater immunity to interfering signals, FHSS is often the preferred choice for co-located systems. Since direct sequence signals are very wide, they tend to offer few non-overlapping channels, whereas multiple hoppers may interleave with less interference. Frequency hopping does carry some disadvantage in that as the transmitter cycles through the hopping pattern it is nearly certain to visit a few blocked channels where no data can be sent. If these channels are the same from trip to trip, they can be memorized and avoided. Unfortunately, this is generally not the case, as it may take several seconds to completely cover the hop sequence during which time the multipath delay profile may have changed substantially. To ensure seamless operation throughout these outages, a hopping radio must be capable of buffering its data until a clear channel can be found. A second consideration of frequency hopping systems is that they require an initial acquisition period during which the receiver must lock on to the moving carrier of the transmitter before any data can be sent, which typically takes several seconds. In summary, frequency hopping systems generally feature greater coverage and channel utilization than comparable direct sequence systems. Of course, other implementation factors such as size, cost, power consumption and ease of implementation must also be considered before a final radio design choice can be made.

DNT900 series modules achieve regulatory certification under FHSS rules at air data rates of 38.4, 115.2 and 200 kb/s. At 500 kb/s, the DNT900 series modules achieve regulatory certification under “digital modulation” or DTS rules. At 500 kb/s DNT900 series modules still employ frequency hopping to mitigate the effects of interference and multipath fading, but hop on fewer, more widely spaced frequencies than at lower data rates.

2.0 DNT900 Radio Operation

2.1 Network Synchronization and Registration

As discussed above, frequency hopping radios such as the DNT900 periodically change the frequency at which they transmit. In order for the other radios in the network to receive the transmission, they must be listening to the frequency on which the current transmission is being sent. To do this, all the radios in the network must be synchronized to the same hopping pattern.

In point-to-point or point-to-multipoint networks, one radio module is designated as the base station. All other radios are designated as remotes. One of the responsibilities of the base station is to transmit a synchronization signal to the remotes to allow them to synchronize with the base station. Since the remotes know the hopping pattern, once they are synchronized with the base station, they know which frequency to hop to and when. Every time the base station hops to a different frequency, it immediately transmits a synchronizing signal.

When a remote is powered on, it rapidly scans the frequency band for the synchronizing signal. Since the base station is transmitting on up to 50 frequencies and the remote is scanning up to 50 frequencies, it can take several seconds for a remote to synchronize with the base station.

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Once a remote has synchronized with the base station, it will request registration information to allow it to join the network. Registration can be handled automatically by the base station, or it can be controlled by allowing the base station host application to authenticate the remote for registration. When a remote is registered, it receives several network parameters from the base station, including HopDuration, InitialN- wkID, FrequencyBand and Nwk_Key (see Section 4.2 for parameter details). Note that if a registration parameter is changed at the base station, it will update the parameter in the remotes over the air.

Among other things, registration allows the tracking of remotes entering and leaving a network, up to a limit of 254 remotes. The base station builds a table of serial numbers of registered remotes using their three-byte serial numbers (MAC addresses). To detect if a remote has gone offline or out of range, the registration is “leased” must be “renewed” once every 250 hops. Any transmission from a remote running on a leased registration will renew its lease with the base station.

2.2 Transparent and Protocol Serial Port Modes

DNT900 radios can work in two serial port data modes: transparent and packet protocol. Transparent mode formatting is simply the raw user data. Protocol mode formatting includes a start of packet framing character, length byte, addressing, command bytes, etc. Transparent mode operation is especially useful in point-to-point systems that act as simple cable replacements. In point-to-multipoint systems where the base station needs to send data specifically to each remote, protocol formatting must be used. Protocol formatting is also required for configuration commands and responses, and sensor I/O commands and responses. Protocol formatting details are covered in Section 4.

The DNT900 provides two ways to switch between transparent and protocol modes. If CFG input Pin 18 on the DNT900 is switched from logic high to low, protocol mode is invoked. Or if the ASCII escape sequence “DNT900” is sent (without quotation marks) to the primary serial input following at least a 20 ms pause in data flow, the DNT900 will switch to the protocol mode. When input Pin 18 is switched from logic low to high, or an ExitProtocolMode command is sent to the primary serial input, the DNT900 will switch to transparent operation. Note that if the escape sequence is used to switch to protocol mode, the se-

quence will be transmitted before protocol mode is invoked.

When operating in transparent mode, two configuration parameters control when a DNT900 radio will send the data in its transmit buffer. The MinPacketLength parameter sets the minimum number of bytes that must be present in the transmit buffer to trigger a transmission. The TxTimeout parameter sets the maximum time data in the transmit buffer will be held before transmitting it, even if the number of data bytes is less than MinPacketLength. The default value for both the MinPacketLength and the TxTimeout parameters is zero, so that any bytes that arrive in the DNT900 transmit buffer will be sent on the next hop. As discussed in Section 2.5.2, it is useful to set these parameters to non-zero values in point-tomultipoint systems where some or all the remotes are in transparent mode.

2.3 RF Data Communications

At the beginning of each hop, the base station transmits a synchronizing signal. After the synchronizing signal is sent, the base will transmit any user data in its transmit buffer, unless in transparent mode the MinPacketLength and/or TxTimeout parameters have been set to non-zero. The maximum amount of data that the base station can transmit per hop is limited by the BaseSlotSize parameter, which has a maximum value of 233 bytes. If there is no user data or reception acknowledgements (ACKs) to be sent on a hop, the base station will only transmit the synchronization signal.

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The operation for remotes is similar to the base station, but without the synchronizing signal. The Re- moteSlotSize parameter sets the maximum number of bytes a remote can transmit on one hop, up to a limit of 243 bytes per hop. The RemoteSlotSize must be coordinated with the HopDuration and BaseSlot- Size parameters and the number of registered remotes. The MinPacketLength and TxTimeout parame- ters operate in a remote in the same manner as in the base station.

2.4 RF Transmission Error Control

The DNT900 supports two error control modes: automatic transmission repeats (ARQ), and redundant transmissions for broadcast packets from the base station. In both modes, the radio will detect and discard any duplicates of messages it receives so that the host application will only receive one copy of a given packet. In the redundant transmission mode, broadcast packets are repeated a fixed number of times based on the value of the ARQ_AttemptLimit parameter. In ARQ mode, a packet is sent and an acknowledgement is expected on the next hop. If an acknowledgement is not received, the packet is transmitted again on the next available hop until either an ACK is received or the maximum number of attempts is exhausted. If the ARQ_AttemptLimit parameter is set to its maximum value, a packet trans- mission will be retried without limit until the packet is acknowledged. This is useful in some point-to-point cable replacement applications where it is important that data truly be 100% error-free, even if the destination remote goes out of range temporarily.

2.5 Network Configurations

The DNT900 supports two network configurations: point-to-point and point-to-multipoint. In a point-topoint network, one radio is set up as the base station and the other radio is set up as a remote. In a pointto-multipoint network, a star topology is used with the radio set up as a base station acting as the central communications point and all other radios in the network set up as remotes. In this configuration, each communication takes place between the base station and one of the remotes. Remotes cannot communicate directly with each other. It should be noted that point-to-point operation is a subset of the point-tomultipoint operation, so there is no need to specify one or the other.

2.5.1 Point-to-Point Network Operation

Most point-to-point networks act as serial cable replacements and both the base station and the remote use transparent mode. Unless the MinPacketLength and TxTimeout parameters have been set to nonzero, the base station will send the data in its transmit buffer on each hop, up to a limit controlled by the BaseSlotSize parameter. In transparent mode, if the base station is buffering more data than can be sent on one hop, the remaining data will be sent on subsequent hops. The base station adds the address of the remote, a packet sequence number and error checking bytes to the data when it is transmitted. These additional bytes are not output at the remote in transparent mode. The sequence number is used in acknowledging successful transmissions and in retransmitting corrupted transmissions. A two-byte CRC and a one-byte checksum allows a received transmission to be checked for errors. When a transmission is received by the remote, it will be acknowledged if it checks error free. If no acknowledgment is received, the base station will retransmit the same data on the next hop. Note that acknowledgements from remotes are suppressed on broadcast packets from the base station.

In point-to-point operation, by default a remote will send the data in its transmit buffer on each hop, up to the limit controlled by its RemoteSlotSize parameter. If desired, the MinPacketLength and TxTimeout parameters can be set to non-zero values, which configures the remote to wait until the specified amount of data is available or the specified delay had expired before transmitting. In transparent mode, if the

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remote is buffering more data than can be sent on one hop, it will send the remaining data in subsequent hops. The remote adds its own address, a packet sequence number and error checking bytes to the data when it is transmitted. These additional bytes are not output at the base station if the base is in transparent mode. When a transmission is received by the base station, it will be acknowledged if it checks error free. If no acknowledgment is received, the remote will retransmit the same data on the next hop.

2.5.2 Point-to-Mu ltipoint Network Operation

In a point-to-multipoint network, the base station is usually configured for protocol formatting, unless the applications running on each remote can determine the data’s destination from the data itself. Protocol formatting adds the address of the destination (remote) and other overhead bytes to the user data. If the addressed remote is using transparent formatting, the destination address and the other overhead bytes are removed. If the remote is using protocol formatting, the destination address and the other overhead bytes are output with the user data.

A remote can operate in a point-to-multipoint network using either transparent or protocol formatting, as the base is always the destination. In transparent operation, a remote adds addressing, a packet sequence number and error checking bytes as in a point-to-point network. When the base receives the transmission, it will format the data to its host according to its formatting configuration. A remote running in transparent mode in a point-to-multipoint network will often have the MinPacketLength and TxTimeout parameters set to non-zero values to reduce the chance of transmission collisions.

2.6 Full-Duplex Serial Data Communications

From an host application’s perspective, DNT900 serial communications appear full duplex. Both the base station host application and each remote host application can send and receive serial data at the same time. At the radio level, the base station and remotes do not actually transmit at the same time. If they did, the transmissions would collide. As discussed earlier, the base station transmits a synchronization signal at the beginning of each hop followed by its user data. After the base station transmission, the remotes can transmit. Each base station and remote transmission may contain all or part of a complete message from its host application. From an application’s perspective, the radios are communicating in full duplex since the base station can receive data from a remote before it completes the transmission of a message to the remote and visa versa.

2.7 Channel Access

The DNT900 provides two methods of channel access: CSMA or TDMA. Each method supports several options as shown in the table below. The channel access setting is distributed to all remotes in the base station status packet, so changing it at the base station sets the entire network. Carrier Sense Multiple Access (CSMA) is very effective at handling packets with varying amounts of data and/or packets sent at random times from a large number of remotes. The DNT900 includes a CSMA polling mode for coordinated remotes and a CSMA contention mode for uncoordinated and/or reporting remotes. Time Division Multiple Access (TDMA) provides a scheduled time slot for each remote to transmit on each hop. The default DNT900 access mode is TDMA dynamic mode.

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Access Mode

2.7.1 CSMA Modes

When using CSMA, each remote with data to send listens to see if the channel is clear and then transmits. If the channel is not clear, a remote will wait a random period of time and listen again. CSMA works best when a large or variable number of remotes transmit infrequent bursts of data. There is no absolute to the number of remote radios that can be supported in this mode. For a DNT900 network, a maximum of 254 remotes can be supported if base station join-leave tracking is required, or a maximum of 1024 remotes is suggested if base station join-leave tracking is not required. Figure 2.7.1 .1 below compares TDMA to CSMA operation.

0 CSMA polling 1024 manual

1 CSMA contention 1024 manual

2 TDMA dynamic slots up to 16 automatic

3 TDMA fixed slots up to 16 automatic

4 TDMA with PTT up to 16 automatic

Description

Max Number of Remotes

Slot Size

Table 2.7.1

TDMA and CSMA operation

Figure 2.7.1.1

There are two important parameters related to CSMA operation. The CSMA _MaxBackoff parameter defines the maximum time that a remote will wait after a collision before attempting to send the packet again (back-off interval). The CSMA_ Predelay parameter controls the maximum time that a remote will randomly backoff when it finds the channel available before transmitting.

CSMA polling (Mode 0) - is used for point-to-point systems and point-to-multipoint systems where only one remote at a time can receive data to transmit (ModBus, etc.). Since only one remote will attempt to transmits at a time, the CSMA_Predelay parameter is ignored on the first transmission attempt and no predelay is used for minimum latency. This mode provides maximum throughput since there is no contention between remotes and the entire portion of the hop frame following the base station transmission is available for a remote to transmit. The user can set CSMA_MaxBackoff, BaseSlotSize and RemoteSlot- Size parameters when using this mode. Note that a CSMA_Delay parameter setting of 0x00 would lead to collisions if more than one remote tried to transmit. Applications where more than one remote can receive

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serial data to transmit at a time, or where periodic reporting and/or event reporting are enabled should not use this mode.

CSMA Contention (Mode 1) - provides classical CSMA channel access, and gives the user control over both the CSMA_MaxBackoff and CSMA_Predelay parameters. This mode is well-suited for large num- bers of uncoordinated remotes, and/or where periodic/event reporting is used. In addition to CSMA_ MaxBackoff and CSMA_Predelay, the user can set the BaseSlotSize and RemoteSlotSize parameters when using this mode. The following guidelines are suggested for setting CSMA_Predelay:



For lightly loaded CSMA contention networks, decrease CSMA_Predelay

to 0x20 or less to reduce latency.



For heavily loaded CSMA contention networks, increase CSMA_Predelay

to 0x80 or more for better throughput.

As an option, the CSMA modes allow the base station to directly track remotes entering and leaving the network for up to 254 remotes. The base station is operated in protocol mode and is configured to generate a CONNECT message for its host when a remote registers, and a DISCONNECT message when the remote’s registration lease expires.

The base station in a CSMA network can generate CONNECT messages for more than 254 remotes. This allows the host application to track remotes entering and leaving a large CSMA network by creating a table of MAC addresses and periodically sending a GetRemoteRegister command to each remote in the table. Failure to answer a GetRemoteRegister command indicates the remote is no longer active in the network.

CSMA modes work well in many applications, but CSMA has some limitations, as summarized below:



Bandwidth is not guaranteed to any remote.



Marginal RF links to some remotes can create a relatively high chance of

collisions in heavily loaded networks.

2.7.2 TDMA Modes

The TDMA modes provide guaranteed bandwidth to some or all of the remotes in the network. Remotes that register with the base station receive several special parameters, including ranging information and a specific channel access slot assignment. TDMA registrations are always leased and must be renewed every 250 hops. The DNT900 provides three different modes of TDMA access, as discussed below.

TDMA Dynamic Slots (Mode 2) - is used for general-purpose TDMA applications where scaling the capacity per slot to the number of active remotes is automatic. Each remote that registers with the base receives an equal time slice. As new remotes join, the size of the TDMA slots shrink accordingly. The number of slots, individual slot start times, and the RemoteSlotSize are computed automatically by the DNT900 network in this mode. The user should note that the bandwidth to each remote will change immediately as remotes join and leave the network. When running in protocol mode, care must be taken not to format packets too long to be sent in a single hop due to automatic RemoteSlotSize reduction.

TDMA Fixed Slots (Mode 3) - is used for applications that have fixed data throughput requirements, such as isochronous voice or streaming telemetry. The slot start time and the RemoteSlotSize are computed automatically by the DNT900 network in this mode. The user must set the number of slots.

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TDMA with PTT (Mode 4) supports remotes with a "push-to-talk" feature, also referred to as "listenmostly" remotes. This mode uses fixed slot allocations. Remotes can be registered for all but the last slot. The last slot is reserved for the group of remotes that are usually listening, but occasionally need to transmit. In essence, the last slot is a shared channel for this group of remotes. When one of them has data to send it keys its transmitter much like a walkie-talkie, hence the name push-to-talk (PTT).

The slot start time and the RemoteSlotSize are computed automatically by the DNT900 network in this mode. The user must specify the number of slots. The last slot is reserved for the PTT remotes. The user must configure PTT remotes individually to select Mode 4 operation. The network makes no guarantee that PTT remote transmissions will not collide in the shared slot. The user's application must ensure that no more than one PTT remote at a time is using the slot.

2.8 Network Configuration Planning

Some planning is necessary for a DNT900 network to coordinate the RF_DataRate, HopDuration, BaseSlotSize, RemoteSlotSize, MinPacketLength, TxTimeout and TDMA_MaxNumSlots parameters to

achieve a practical configuration. This is true even for modes that automatically compute some of these parameters. Each parameter has a limited range of usable values, as shown in the Table 2.8.1 below:

Parameter Useable Range Value

RF_DataRate

HopDuration

TDMA_MaxNumSlots

BaseSlotSize

RemoteSlotSize

MinPacketLength

Tx Timeout

0..3 500, 200, 115.2 and 38.4 kb/s

40..4095 2. .204.75 ms (0.05 ms/count)

1 ..16 max number of TDMA slots (MNS) for remotes

6..233 max number of user data bytes transmitted per hop

3..243 max number of user data bytes transmitted per hop

0..255 0. .255 bytes

0..255 0. .255 ms (1 ms/count)

Table 2.8.1

The highest RF data rate, 500 kb/s, provides the highest throughput and the most flexibility with respect to the other parameters. The maximum RF power that can be used at 500 kb/s is 85 mW. The three lower data rates can run up to 1 W of RF power, and the receiver becomes progressively more sensitive as the data rate is lowered. So for greatest range, one of the three lower RF data rates should be used.

The maximum DNT900 HopDuration setting is about 200 ms regardless of the RF data rate chosen. For a given data rate, FHSS operation tends to become more robust as hop duration is reduced. However, running with a shorter hop duration may require setting the BaseSlotSize and RemoteSlotSize parameters well below their maximum values at the lower RF data rates. The equation below calculates the minimum hop duration needed at a given RF data rate for a specific number of remote slots and Base- SlotSize and RemoteSlotSize parameter settings. Support for optimizing a DNT900 configuration for a specific application is also available from RFM’s Technical Support Group. See Section 10.3. for technical support contact information.

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The minimum required hop duration for a DNT900 configuration is:

THD = T

BRO

NRS*TRO

TRFB*(BBSS

+ N

RSS

)

Where:

THD is the minimum required hop duration in milliseconds

BRO

is the base and registration request overhead time for each hop (RF data rate dependent)

NRS is the number of remote slots

TRO is the remote overhead time for each hop (RF data rate dependent)

RFB

is the transmission time for one user byte (RF data rate dependent)

BSS

is the

RSS

is the

BaseSlotSize

RemoteSlotSize

parameter in bytes

The constants in the equation for each RF data rate are given in the following table:

RF Data Rate kb/s T

38.40 11.620 4.817 0.2080

115.2 4.953 2.039 0.0694 200 3.540 1.450 0.0400 500 2.388 0.970 0.0160

BRO

ms TRO ms T

RFB

Table 2.8.2

For example, consider a point-to-point CSMA Mode 0 system operating at 38.4 kb/s with the BaseSlot- Size parameter set to 133 bytes and the RemoteSlotSize parameter set to 128 bytes. The minimum hop duration needed to support one-hop transmissions of full slot size messages in both directions for this configuration is:

= 11.620 + 1*4.817 + 0.2080*(133 + 1*128)

= 16.437 + 0.2080*261 = 70.725 ms

The closest programmable hop duration is 70.750 ms.

It should be noted that the base station operating system will commandeer 5 bytes from the BaseSlotSize allocation in Mode 0 and up to 13 bytes in Mode 1 to send reception acknowledgements (ACKs) back to the remotes. The BaseSlotSize should be sized accordingly. In the above example, the BaseSlotSize parameter is set five bytes larger than the RemoteSlotSize parameter to accommodate the ACK bytes.

When running a point-to-multipoint network with uncoordinated remotes using CSMA Mode 1, it is useful to set

to a value of 3 or higher in the equation. Although CSMA does not create reserved time slots

for remotes, extending the hop duration this way allows several uncoordinated transmissions of user data and/or periodic/event reports to arrive in the same slot with a relatively few collisions.

The performance of a CSMA Mode 1 system can often be helped by setting the MinPacketLength and TxTimeout parameters on any remotes running transparent mode to non-zero values, especially if host messages only contain a few bytes each and transmission latency is not critical. For starting point values, set the MinPacketLength equal to one-half the RemoteSlotSize and TxTimeout to at least three times the hop duration. This will help avoid excessive transmission collisions due to having many packets transmitted, each carrying only a small amount of user data on top of the relatively large packet overhead

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structure. As an example, consider a TDMA Mode 2 or 3 system operating at 500 kb/s. Up to 10 registered remotes need to be accommodated. A BaseSlotSize of 138 bytes is needed, and each remote needs enough slot time to support a RemoteSlotSize of 64 bytes. The minimum hop duration needed to support this configuration is:

= 2.388 + 10*0.970 + 0.0160*(138 + 10*64) = 12.088 + 0.0160*778 = 24.536 ms

The closest programmable hop duration is 24.550 ms.

In all TDMA modes, the base station operating system will commandeer one byte from the BaseSlotSize allocation for each registered remote to send ACKs to the remotes. The BaseSlotSize and MinPacket- Length should be sized accordingly.

2.9 Serial Port Operation

DNT900 networks are often used for wireless communication of serial data. The DNT900 supports serial baud rates from 1.2 to 460.8 kb/s. Listed in the table below are the supported data rates and their related byte data rates and byte transmission times for an 8N1 serial port configuration:

Baud Rate kb/s Byte Data Rate kB/s Byte Transmission Time ms

1.2 0.12 8.3333

2.4 0.24 4.1667

4.8 0.48 2.0833

9.6 0.96 1.0417

19.2 1.92 0.5208

38.4 3.84 0.2604

115.2 11.52 0.0868

230.4 23.04 0.0434

460.8 46.08 0.0217

Table 2.9.1

To support continuous full-duplex serial port data flow, an RF data rate much higher than the serial port baud rate is required for FHSS. Radios transmissions are half duplex, and there are overheads related to hopping frequencies, assembling packets from the serial port data stream, transmitting them, sending ACK’s to confirm error-free reception, and occasional transmission retries when errors occur.

For example, consider a CSMA Mode 0 transparent data system operating at 500 kb/s with the Bas- eSlotSize parameter set to 133 bytes (128 bytes net after the five byte allocation for sending ACKs) and the RemoteSlotSize parameters set to 128 bytes. The minimum hop duration needed to efficiently support this configuration is:

= 2.388 + 1*0.970 + 0.0160*(133 +1*128)

= 3.358 + 0.01 60*261 = 7.534 ms

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Setting the hop duration to 7.55 ms, the average full-duplex serial port byte rate that can be supported under error free conditions is:

128 Bytes /7.55 ms = 16.942 kB/s, or 169.42 kb/s for 8N1

Continuous full-duplex serial port data streams at a baud rate of 115.2 k/bs can be supported by this configuration, provided only occasional RF transmission errors occur. Plan on an average serial port data flow of 75% of the calculated error-free capacity for general-purpose applications, and 50% of the calculated error-free capacity for RF challenging applications such as vehicle telemetry and heavy industrial process environments.

Most applications do not require continuous serial port data flow. The DNT900 transmit and receive buffers hold at least 1024 bytes and will accept brief bursts of data at high baud rates, provided the average serial port data flow such as shown in the example above is not exceeded. It is strongly recommended that the DNT900 host use hardware flow control. The host must send no more than 32 bytes additional bytes to the DNT900 when the DNT900 de-asserts the host’s CTS line. In turn, the DNT900 will send no more than one byte following the host de-asserting its RTS line. Three-wire serial port operation is allowed by connecting the DNT900 CTS output to its RTS input. However, three-wire operation should be limited to applications that send small bursts of data occasionally at an average serial port data flow less than 50% of the calculated error-free capacity. Data loss is possible under adverse RF channel conditions when using three-wire serial operation.

2.10 Sleep Modes

To save power in applications where a remote transmits infrequently, the DNT900 supports a hardware sleep mode. Hardware sleep mode is entered by switching DTR Pin 11 on the DNT900 from logic low to high. While in hardware sleep mode, the DNT900 consumes less than 0.5 mA. This mode allows a DNT900 to be powered off while its host device remains powered. After leaving hardware sleep mode (Pin 11 low to high), the radio must re-synchronize with the base station and re-register.

In addition to the sleep mode controlled by the DTR pin, in CSMA mode the DNT900 remotes support an additional sleep mode to support battery-powered applications. When this mode is enabled, the DNT900 is in a low-power state and only wakes up in response to the I/O report triggers. The following list explains the rules that sleeping remotes follow:



The DNT900 will wake up when any of the enabled I/O report trigger conditions fire. When any of

the ADC triggers are enabled, the radio will also wake up every ADC_SampleIntvl long enough to sample the ADCs, and then go back to sleep.



When a sleeping radio wakes up, it must acquire and synchronize to its base before it can send

or receive any data. To prevent excessive battery use, if the remote is unable to acquire before the WakeLinkTimeout elapses, it will cancel any pending event trigger(s) and go back to sleep.



If a remote is linking for the first time or if its last attempt to acquire and synchronize was unsuc-

cessful, it will scan and record the entire broadcast system parameter list before it goes back to sleep. Otherwise, in order to conserve battery life, a sleeping remote will update any values that it may hear while it is awake, but is not required to listen to the entire list.

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Murata Electronics North America DNT900 User Manual

Specifications and Main Features

Frequently Asked Questions

User Manual