Murata Electronics North America 910M Users Manual

Frequency Hopping

Spread Spectrum Transmitter

Modular Approval

Certification Test Report

FCC ID: HSW-910M

ACS Report Number: 04-0132-15C

Manufacturer: Cirronet, Inc.

Equipment Type: Transceiver

Model: WIT910

Manual

Model: WIT910 Advanced Compliance Solutions FCC ID: HSW-910M

WIT910

900MHz Spread Spectrum Wireless Industrial Transceiver

Integration Guide

5375 Oakbrook Parkway

Norcross, Georgia 30093

www.cirronet.com

+1 (678) 684-2000

TABLE OF CONTENTS

1. INTRODUCTION .....................................................................................................................1

1.1. Why Spread Spectrum?.....................................................................................................1

1.2. Frequency Hopping vs. Direct Sequence ..........................................................................2

2. RADIO OPERATION ...............................................................................................................4

2.1. Synchronization and Registration.....................................................................................4

2.2. Data Transmission............................................................................................................5

2.2.1. Point-to-Point...........................................................................................................5

2.2.2. Point-to-Multipoint ...................................................................................................6

2.2.3. Handle Assignment.................................................................................................6

2.2.4. TDMA Operation ......................................................................................................7

2.2.5. Full Duplex Communication ....................................................................................9

2.2.6. Error-free Packet Transmission Using ARQ............................................................9

2.3. Modes of Operation ........................................................................................................10

2.3.1. Control and Data Modes .......................................................................................10

2.3.2. Sleep Mode...........................................................................................................10

2.3.3. Low Power Mode and Duty Cycling ......................................................................11

2.3.4. RF Flow Control Mode ...........................................................................................11

3. PROTOCOL MODES.............................................................................................................12

3.1.1. Data Packet...........................................................................................................14

3.1.3. Connect Packet.....................................................................................................15

3.1.4. Disconnect Packet (base only, receive only)........................................................15

4. MODEM INTERFACE............................................................................................................16

4.1. Interfacing to 5-volt Systems...........................................................................................17

4.2. Evaluation Unit and OEM Module Differences................................................................17

4.3. Three Wire Operation .....................................................................................................17

4.4. Power-On Reset Requirements......................................................................................18

5. MODEM COMMANDS...........................................................................................................19

5.1. Serial Commands ...........................................................................................................19

5.2. Network Commands .......................................................................................................21

5.3. Protocol Commands .......................................................................................................23

5.4. Status Commands ..........................................................................................................26

5.5. Memory Commands........................................................................................................27

5.6. Modem Command Summary..........................................................................................28

6. WIT910 DEVELOPER’S KIT..................................................................................................29

6.1. WinCOM .........................................................................................................................30

6.1.1 WinCOM Tools........................................................................................................32

6.2. Demonstration Procedure...............................................................................................34

6.3. Troubleshooting..............................................................................................................35

7. APPENDICES........................................................................................................................37

7.1. Technical Specifications .................................................................................................37

7.1.1. Ordering Information .............................................................................................37

7.1.2. Power Specifications.............................................................................................37

7.1.3. RF Specifications ..................................................................................................37

7.1.4. Mechanical Specifications.....................................................................................37

7.2. Serial Connector Pinouts................................................................................................38

7.3. Approved Antennas ........................................................................................................38

7.4. Technical Support...........................................................................................................39

7.6.1 Mechanical Drawing – WIT910M4 (Pins Down) ...........................................................41

7.7 Warranty ..........................................................................................................................42

1. INTRODUCTION

The WIT910 radio transceiver provides reliable wireless connectivity for either point-to-point or multipoint applications. Frequency hopping spread spectrum technology ensures maximum resistance to noise and multipath fading and robustness in the presence of interfering signals, while operation in the 900MHz ISM band allows license-free use and worldwide compliance. Standard communication rates between the WIT910 and the host are supported between 1200pbs and 57.6bps. Non-standard rates are supported as well. An on-board buffer and an error-correcting over-the-air protocol provide smooth data flow and simplify the task of integration with existing applications.

- Multipath fading impervious

frequency hopping technology with 54 frequency channels (902 to 927 MHz).

- Supports point-to-point or multipoint applications.

- Meets FCC rules 15.247for license-free operation.

- 20+ mile range with omni antenna.

- Transparent ARQ protocol w/512byte buffer ensures data integrity.

- Digital addressing supports up to 64 networks, with 62 remotes per network.

1.1. Why Spread Spectrum?

The radio transmission channel is very hostile, corrupted by noise, path loss and interfering transmissions from other radios. Even in a pure 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 phase, thereby partially or completely canceling the desired signal. This is a problem particularly prevalent in indoor installations. In the frequency domain, a multipath fade can be described as a frequency-selective notch that 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. This means that from a probabilistic viewpoint, a conventional radio system faces a 1% - 2% chance of signal impairment at any given time due to multipath.

- Low power 3.3v CMOS signals

- Selectable 10mW, 100mW or

- Built-in data scrambling reduces

- Nonvolatile memory stores

- Smart power management features

- Dynamic TDMA slot assignment

- Simple serial interface handles both

500mW transmit power.

possibility of eavesdropping.

configuration when powered off.

for low current consumption.

that maximizes throughput.

data and control at up to 115.2 bps.

shifts in location and intensity over time as

WIT910

Spread spectrum reduces the vulnerability of a radio system to interference from both jammers and multipath fading by distributing the transmitted signal over a la rger 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 transit.

Narrowband vs. spread spectrum in the presence of interference

1.2. Frequency Hopping vs. Direct Sequence

The two primary approaches to spread spectrum are direct sequence (DS) and frequency hopping (FH), 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 baseband data is called the processing gain, and is equal to the amount of rejection the system affords against narrowband 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.

WIT910

Figure 1

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Figure 2

Forms of spread spectrum

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 DS systems to overcome fading and in-band jammers is relatively weak. By contrast, FH systems are capable of probing the entire band if necessary to find a channel free of interference. Essentially, this means that a FH system will degrade gracefully as the channel gets noisier while a DS 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, FH 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.

2. RADIO OPERATION

2.1. Synchronization and Registration

As discussed above, frequency hopping radios 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 over which the current transmission is being sent. To do this, all the radios in the net must be synchronized and must be set to the same hopping pattern.

In point-to-point or point-to-multipoint arrangements, one radio module is designated as the base station. All other radios are designated 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 over 54 frequencies and the remote is scanning 54 frequencies, it can take several seconds for a remote to synch up with the base station.

Once a remote has synchronized with the base station, it must request registration from the base station. The registration process identifies to the base station the remotes from which transmissions will be received and not discarded. Registration also allows tracking of remotes entering and leaving the network. The base station builds a table of serial numbers of registered remotes. To improve efficiency, the 24-bit remote serial number is assigned a 6-bit “handle” number. Two of these are reserved for system use, thus each base station can register 62 separate remotes. This handle is how user applications will know the remotes. Note that if a remote leaves the coverage area and then re-enters, it may be assigned a different handle.

To detect if a remote has gone offline or out of range, the registration must be “renewed” once every 256 hops. Registration is completely automatic and requires no user application intervention. When the remote is registered, it will receive several network parameters from the base. This allows the base to automatically update these network parameters in the remotes over the air. Once a parameter has been changed in the base, it is automatically changed in the remotes. The parameters auto matically changed are hop duration and the duty cycle.

At the beginning of each hop, the base station transmits a synchronizing signal. After the synchronizing signal has been sent, the base will transmit any data in its buffer unless data transmit delay has been set. The data transmit delay parameter allows for the transmission of groups of continuous data in transparent mode (protocol mode amount of data that the base station can transmit per hop is determined by the base slot

WIT910

00H). The

size parameter. The maximum amount of data sent by a base station per hop is 208 bytes. If there is no data to be sent, the base station will not transmit until the next frequency.

The operation for remotes is similar to the base station without the synchronizing signal. The amount of data a remote can send on one hop is dependent upon the hop duration, the base slot size and the number of registered remotes. 212 bytes per hop is the maximum data length a remote can transmit per hop, subject to limitations imposed by the hop duration, the base slot size and the number of registered remotes. A detailed explanation of this relationship is provided in Section 2.2.3. Minimum data length and data transmit delay operate the same as with the base station.

Except for the registration process which occurs only when a remote logs onto the network, the whole procedure is repeated on every frequency hop. Refer to the section on Modem Commands for complete details on parameters affecting the transmission of data.

2.2. Data Transmission

The WIT910 supports two network configurations: point-to-point and point-tomultipoint. In a point-to-point network, one radio is set up as the base station and the other radio is set up as a remote. In a point-to-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, all communications take place between the base station and any one of the remotes. Remotes cannot communicate directly with each other. It should be noted that point-to-point mode is a subset of point-to-multipoint mode and therefore there is no need to specify one mode or the other.

2.2.1. Point-to-Point

In point-to-point mode, unless data transmit delay or minimum data length have been set, the base station will transmit whatever data is in its buffer limited to 208 bytes or as limited by the base slot size. If the base station has more data than can be sent on one hop, the remaining data will be sent on subsequent hops. In addition to the data, the base station adds some information to the transmission over the RF link. It adds the address of the remote to which it is transmitting, even though in a point-to-point mode there is only one remote. It also adds a sequence number to identify the transmission to the remote. This is needed in the case of acknowledging successful transmissions and retransmitting unsuccessful transmissions. Also added is a 24-bit CRC to allow the base to check the received transmission for errors. When the remote receives the transmission, it will acknowledge the transmission if it was received without errors. If no acknowledgment is received, the base station will retransmit the same data on the next frequency hop.

In point-to-point mode, a remote will transmit whatever data is in its buffer up to the limit of its maximum data length. If desired, minimum data length and

WIT910

data transmit delay can

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also be set, which force the remote to wait until a certain amount of data is available or the specified delay is exceeded before transmitting. If the remote has more data than can be sent on one hop, it will send as much data as possible as a packet, adding its own address, a packet sequence number and 24-bit CRC. These additional bytes are transparent to the user application if the protocol mode is

00H (which is the default). In

the event a remote has more data to send, the data will be sent on subsequent hops. If the transmission is received by the base station without errors, the base station will acknowledge the transmission. If the remote does not receive an acknowledgment, it will retransmit the data on the next frequency hop. To the user application, acknowledgments and retransmissions all take place behind the scenes without the need for user intervention.

The WIT910 has a point-to-point direct mode which fixes the remote radio’s handle at 30H. This mode is recommended for point-to-point applications, especially if the remote is likely to periodically leave and re-enter the coverage area of the base. See the section on Network Commands for details of this mode.

2.2.2. Point-to-Multipoint

In point-to-multipoint mode, data sent from the user application to the base station must be packetized by the user application unless the remote device can distinguish between transmissions intended for it and transmissions intended for other remote devices. This is necessary to identify the remote to which the base station should send data. When the user packet is received by the remote, if the remote is in transparent mode (protocol mode

0), the packetization bytes are stripped by the remote. In this instance the remote host

receives just data. If the remote is not in transparent mode, the remote host will receive the appropriate packet header as specified by the remote’s protocol mode. Refer to the section Protocol Modes for details on the various packet formats.

When a remote sends data to a base station in point-to-multipoint mode, the remote host does not need to perform any packetization of the data. Remotes can operate in transparent mode even though the base is operating in a packet mode. The remote will add address, sequence and CRC bytes as in the point-to-point mode. When the base station receives the data, the base station will add packetization header bytes according to its protocol mode setting.

2.2.3. Handle Assignment

Handles are used to reduce overhead by not sending the unique 24-bit serial number ID of a remote when sending or receiving data. The use of the various protocol modes causes the base radio to issue CONNECT packets when a new remote registers with the base. In addition to indicating the presence of a new remote, the CONNECT packets provide the current relationship between remote serial numbers and handles.

WIT910

When a remote links to a base and requests registration, it requests by default that it be assigned handle

30H. This default request can be changed by the Set Default Handle

command. If that handle is not currently in use by another remote, the base will assign that handle to the remote. If the requested handle is already in use by another remote, the base will assign the next higher handle that is available. Thus, if remote requests handle

30H and that handle is already assigned, the base will assign the remote handle 31H if that

is available. If

31H is already assigned, the base will assign handle 32H is that is available

and so on.

When a remote leaves the coverage area of the base or otherwise loses link, e.g. the remote was turned off or put into sleep mode, the base detects this event when the remote does not renew its registration within 255 hops. With the default setting of 25msec per hop, this could be as along as 6.38 seconds. If within this time the remote re-establishes link with the base, the previous handle assigned to this remote will still be marked active in the base radio. Thus the remote will be assigned a new handle. If the base radio is in one of the protocol modes, a new CONNECT packet will be issued indicating the current handle assigned to the remote. The remote is identified by the serial number that is contained in the CONNECT packet.

If the radio is to be used in a point-to-point mode where there is only one base and one remote, using the point-to-point mode command of the radios will override this handle mechanism and always assign the remote the same handle.

2.2.4. TDMA Operation

For applications needing guaranteed bandwidth availability, the TDMA operation of the WIT910 can meet this requirement. In the WIT910 TDMA scheme, each remote has an assigned time slot during which it can transmit. The base station time slot is set independently of the remote time slots through the Set Base Slot Size command. The base station assigns each remote a time slot and informs the remotes of the size of the time slot. All remote time slots are the same size that is determined by the number of remotes registered with the base station. The slot size is a dynamic variable that changes as the number of registered remotes changes. The remotes are continually updated with the time slot size. This approach continually maximizes the data throughput. The base station divides the amount of time available per hop by the number of registered remotes up to a maximum of 16 times slots per hop. If the number of registered remotes is greater than 16, the time slots will be spread across the required number of hops. For networks with more than 16 possible remotes, the Set Duty Cycle command must be used to specify a duty cycle -- the number of hops over which the time slots must be spread. For 1 to 16 remotes, no duty cycle is required; for 17 to 32 remotes a duty cycle of at least ½ is required; and for 33 to 62 remotes a duty cycle of ¼ or more is necessary. An added benefit of using the power save mode to set a duty cycle is improved average current consumption efficiency. Refer to the Status Commands section for details of this command.

WIT910

6.3

s us

When setting up a network, keep in mind that time slot length, maximum packet size and hop duration are all interrelated. The hop duration parameter will determine the time slot size and the maximum amount of data that can be transmitted per hop by the remotes. There is a hard limit of the absolute maximum amount of data that can be sent on any given hop of 212 bytes regardless of any parameters. (Note that this is different than the 208 byte maximum for the base station.) The base station requires 7.04 ms overhead for tuning, the synchronization signal and parameter updating, as well as 1.11 ms overhead for each remote. Thus the amount of time allocated per remote slot is roughly:

hop duration – base slot – 7.04ms - ( # of registered remotes)·1.11ms

( # of registered remotes)

Take for example a network comprised of a base station and 5 remotes. A hop duration of 25 ms is chosen. We decide that the base station needs to be able to send up to 32 bytes each hop (equivalent to a capacity for the base of 19.2 kbps asynchronous). Counting the 7.04 ms overhead for the base packet and making use of the fact that our RF rate is 172.8 kbps, we determine that the base slot requires approximately:

32·8

172.8kbps

+ 7.04 ms = 8.52 ms

Each remote time slot will be:

25 ms – 8.52 ms – (5)·1.11 ms

= 2.18 ms

From our RF data rate of 172.8kbps we see that it takes 46.3 µs to send a byte of data, so each remote will be able to send up to

= 47 bytes of data per hop.

2.18 ms

However, the WIT910 sends data in groups of 4 bytes. Thus, each remote will be able to send 44 bytes of data. Note that the 44 bytes is the actual number of data bytes that can be sent. If the WIT910 is using a protocol mode, the packet overhead does not need to be considered. So in this example, the total capacity per remote would be:

44 bytes

25 ms

= 14.08 kbps

It is also useful to remember that the asynchronous data input to the WIT910 is stripped of its start and stop bits during transmission by the radio, yielding a "bonus" of 10/8 or 25% in additional capacity. Thus, 1.25 x 14.08 kbps = 17.6 kbps asynchronous. In actual deployments, some allowance must be made for retransmissions of data, yielding a throughput somewhat less than the calculated value.

The above calculations are provided as a means of estimating the capacity of a multipoint WIT910 network. To determine the precise amount of capacity, you can actually set up

WIT910

the radio system and then query the maximum data length from one of the remotes in control mode to discover its exact setting. Divide this number by the hop duration as above to get the remote's exact capacity.

2.2.5. Full Duplex Communication

From an application perspective, the WIT910 communicates in full duplex. That is, both the user application and the remote terminal can be transmitting data without waiting for the other to finish. 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 a packet of data. After the base station transmission, the remotes will transmit. Each base station and remote transmission may be just part of a complete transmission from the user application or the remote terminal. Thus, from an application perspective, the radios are communicating in full duplex mode since the base station will receive data from a remote before completing a transmission to the remote.

2.2.6. Error-free Packet Transmission Using ARQ

The radio medium is a hostile environment for data transmission. In a typical office or factory environment, 1% - 2% of the 900MHz frequency band may be unusable at any given time at any given station due to noise, interference or multipath fading. For narrowband radio systems (and also many spread spectrum radio systems which use direct sequence spreading), this would imply a loss of contact on average of over 30 seconds per hour per station. The WIT910 overcomes this problem by hopping rapidly throughout the band in a pseudo-random pattern. If a message fails to get through on a particular channel, the WIT910 simply tries again on the next channel. Even if two thirds of the band are unusable, the WIT910 can still communicate reliably.

Data input to the WIT910 is broken up by the radio into packets. A 24-bit checksum is attached to each packet to verify that it was correctly received. If the packet is received correctly, the receiving station sends an acknowledgment, or station. If the transmitter doesn't receive an

ACK, at the next frequency hop it will attempt

ACK, back to the transmitting

to send the packet again. When ARQ is enabled, the transmitting radio will attempt to send a packet packet attempts limit times before discarding the packet. A value of

00H

disables ARQ. When it is disabled, any transmission received with errors is discarded. It is the responsibility of the user application to track missing packets. A second parameter, ARQ Mode, allows the choice between using ARQ to resend unsuccessful transmissions or always sending a transmission packet attempts limit times regardless of the success or failure of any given transmission.

All of this error detection and correction is transparent to the user application. All the user application sees is error-free data from the modem. However, if the ARQ mode is disabled, transmissions with errors are discarded, and missing data detection will be the

responsibility of the user application. Refer to the Protocol Commands section for complete details.

2.3. Modes of Operation

2.3.1. Control and Data Modes

The WIT910 has two modes of operation: Control mode and Data mode. When in Control Mode, the various radio and modem parameters can be modified. When in Data Mode, only data can be transmitted. The default mode is Data Mode. There are two ways to enter Control Mode. The first way is to assert the Configure (CFG) pin on the modem. Upon entering Control Mode, the modem will respond with a > prompt. After

each command is entered, the modem will again respond with a > prompt. As long as the CFG pin is asserted, data sent to the modem will be interpreted as command data. Once the CFG pin is de-asserted, the modem will return to Data Mode.

The second method for entering Control Mode is to send the escape sequence (all lower case) followed by a carriage return. In the default mode, the escape sequence is only valid immediately after power up or after de-assertion of the Sleep pin on the modem. The modem will respond in the same way with a > prompt. To return to Data Mode, enter the Exit Modem Control Mode command, Sleep pin. There are three modes for the escape sequence, controlled by the Set Escape Sequence Mode command,

zc = 0 Escape sequence disabled zc = 1 Escape sequence available once at startup (default setting) zc = 2 Escape sequence available at any time

zc2 mode setting is useful if the user application has a need to change the modem

The settings "on the fly". In this mode the escape sequence is always enabled and may be sent at any time after a pause of at least 20ms. The modem will respond in the same way as when in the default mode. It is necessary to issue the Exit Modem Control Mode command,

z>, before resuming data transmission.

Note: The escape sequence must be interpreted as data until the last character is received

and as such may be transmitted by the modem to any listening modems.

2.3.2. Sleep Mode

To save power consumption for intermittent transmit applications, the WIT910 supports a Sleep Mode. Sleep Mode is entered by asserting the Sleep pin on the modem interface. While in Sleep Mode, the modem consumes less than 250 µA. This mode allows the radio to be powered off while the terminal device remains powered. After leaving Sleep Mode, the radio must re-synchronize with the base station and re-register.

WIT910

:wit2410

z>, or assert and de-assert the

zc:

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Murata Electronics North America 910M Users Manual

Specifications and Main Features

Frequently Asked Questions

User Manual