Murata Electronics North America 2492 Users Guide

FCC Part 15.247

Transceiver Certification

Digital Transmission System with Frequency Hopping Spread Spectrum

Test Report

FCC ID: HSW-2492

FCC Rule Part: 15.247

ACS Report Number: 07-0052-15C

Manufacturer: Cirronet

Model: WIT2492

Integration Guide

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

WIT2492

2.4 GHz Spread Spectrum

Wireless Industrial Transceiver

Preliminary

Integration Guide

3079 Premiere Pkwy Ste 140

Duluth, Georgia 30097

www.cirronet.com

+1 (678) 684-2000

Important Regulatory Information

Cirronet Product FCC ID: HSW-2492

IC 4492A-2492

Note: This u nit has been tested and found to comp ly 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 when t he equipment is operat ed in a commercial environment . This equipment generates, uses, and can radiate radio frequency energy and, if not installed and used in accordance with the instruction manual, may cause harmful interference to radio communications. Operation of this equipment in a residential area is likely to cause harmful interference in which case the user will be required to correct the interference at their expense.

FCC s MPE Requirements

Information to user/installer regarding FCC s Maximum Permissible Exposure (MPE) limits.

Notice to users/installers using the following fixed antennas, with Cirronet RF products:

Cushcraft 15dBi Yagi, Mobile Mark 14dBi Corner Reflector, Mobile Mark 9dBi Corner Reflector

Notice to users/installers using the following mobile antennas, with Cirronet RF products:

Mobile Mark 9dBi omni-directional, MaxRad 5dBi whip, Cirronet Patch antenna, Ace 2dBi dipole, Mobile Mark 2dBi Stub

Notice to users/installers using the following Portable antennas, with Cirronet RF products:

Ace 2 dBi dipole Cirronet 2 dBi patch antenna (PA2410) Cirronet 6 dBi patch antenna (PA240 0) Cirronet 5 dBi whip antenna (Omni245RCBL)

The field strength radiated by any one of these antennas, when connected to Cirronet RF products, may exceed FCC mandated RF exposure limits. FCC rules require professional installation of t hese antennas in such a way that the general public will not be closer than 2 m from the radiating aperture of any of these antennas. End users of these systems must also be informed that RF exposure limits may be exceeded if personnel come closer than 2 m to the apertures of any of these antennas.

The field strength radiated by any one of these antennas, when connected to Cirronet RF products, may exceed FCC mandated RF exposure limits. FCC rules require professional installation of these antennas in such a way that the general public will not be closer than 20 cm from the radiating aperture of any of these antennas. End users of these systems must also be informed that RF exposure limits may be exceeded if personnel come closer than 20 cm to the apertures of any of these antennas.

The field strength radiated by any one of these antennas, when connected to Cirronet’s WIT2492 product, is below FCC mandated RF exposure limits for distances less than 2.5 cm or next to the body.

Declaration of Conformity

The WIT2492 to which this declaration relates is in conformity with the essential requirements of the R&TTE directive 1999/5/EC and complies with the following standards and/or other normative documents:

For Interfaces For RLAN Transceiver

EN 55022 EN 55024

Use Within the European Union

The WIT2492 is intended for use within the European Community States and in the following non-European Union States: Norway & Switzerland

Use of the WIT2492 in France

When used in France, the WIT2492 can only be operated with the France hopping pattern selected. This is accomplished by setting the pe parameter to 1. Refer to European Union Settings in this manual for details.

Canadian Department of Communications Industry Canada (IC) Notice

Canadian Department of Communications Industry Canada (IC) Notice This apparatus complies with Health Canada’s Safety Code 6 / IC RSS 102.

"To prevent radio interference to the licensed service, this device is intended to be operated indoors and away from windows to provide maxi m um shi eldi ng. Equipment (or its transmit antenna) that is installed outdoors may be subject to licensing."

The term “IC:” before the radio certification number only signifies that Industry Canada technical specifications were met.

ICES-003

This digital apparatus does not exceed the Class B limits for radio noise emissions from digital apparatus as set out in the radio interference regulations of Industry Canada.

Le présent appareil numérique n'émet pas de bruits radioélectriques dépassant les limites applicables aux appareils numériques de Classe B prescrites dans le règlement sur le brouillage radioélectrique édicté par Industrie Canada.

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 above, and having a maximum gain of [15] dB. Antennas not included in this list or having a gain greater than [15] dB are strictly prohibited for use with this device. The required antenna impedance is [50] ohms.

Changes or modifications to this device not expressly approved by Cirronet could void the user’s authority to operate the equipment.

Warning! The RLAN transceiver within this device uses a band of frequencies that are not completely harmonized within the European Community. Before using, please read the European Operation Section of the Products User’s Guide for limitations.

EN 300 328

Warning!

TABLE OF CONTENTS

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

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

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

2. RADIO OPERATION ...................................................... ... .................................... ..................5

2.1. Synchronization and Registration.......................................................................................5

2.2. Data Transmission...............................................................................................................6

2.2.1. Point-to-Point............................................................................................................6

2.2.2. Point-to-Multipoint....................................................................................................7

2.2.3. Handle Assignment...................................................................................................7

2.2.4. TDMA Operation......................................................................................................8

2.2.5. Full Duplex Communication..................................... .................................... ... ....... 10

2.2.6. Error-free Packet Transmission Using ARQ........................................................... 10

2.3. Modes of Operation ..........................................................................................................11

2.3.1. Control and Data Modes ......................................................................................... 11

2.3.2. Sleep Mode..............................................................................................................11

2.3.3. Low Power Mode and Duty Cycling....................................................................... 12

2.3.4. RF Flow Control Mode.................................... ... .................................... ................12

2.3.5. Co-Existing with 802.11b Networks.......................................................................13

2.3.6. European Union Settings......................................................................................... 13

3. PROTOCOL MODES..............................................................................................................14

Note on Using Protocol Mode 4........................................ .................................... .............16

3.1.1. Data Packet.............................................................................................................. 17

3.1.3. Connect Packet........................................................................................................18

3.1.4. Disconnect Packet (base only, receive only)............................................. ... ..........18

4. MODEM INTERFACE ...........................................................................................................19

4.1. Interfacing to 5 Volt Systems............................................................................................20

4.2. Evaluation Unit and OEM Module Differences ...............................................................20

4.3. Three Wire Operation............................................. ... .................................... ... ................20

4.4. Power-On Reset Requirements......................................................................................... 21

5. MODEM COMMANDS..........................................................................................................22

5.1. Serial Commands..............................................................................................................22

5.2. Network Commands .............................................................. ... ...... ...... ... ...... ... ...... ...... ... .24

5.3. Protocol Commands..........................................................................................................27

5.4. Status Commands..............................................................................................................30

5.5. Memory Commands.............................. ..................................................................... ....... 31

5.6. Modem Command Summary............................................................................................ 32

6. WIT2492 DEVELOPER’S KIT...............................................................................................33

7. WinCOM.....................................................................................................................

.............34

7.1. Starting the program..........................................................................................................36

7.2. Function Keys....................................... ... ...... ... ...... ...... ... ...... ...... ... ...... ...... ... ...... ...... ....... 39

7.3. WinCom Tools..................................................................................................................40

7.4. Script Commands..............................................................................................................42

7.5. Demonstration Procedure..................................................................................................44

8. Troubleshooting.......................................................................................................................45

9. APPENDICES .........................................................................................................................47

9.1. Technical Specifications...................................................................................................47

9.1.1 Ordering Information .................................................. ...... ... ...... ...... ... ...... ...... ... ...... .47

9.1.2. Power Specifications...............................................................................................47

9.1.3. RF Specifications ................................................ ... .................................... ... ..........47

9.1.4. Mechanical Specifications.......................................................................................48

9.2. Serial Connector Pinouts...................................................................................................48

9.3. Approved Antennas...........................................................................................................49

9.4. Technical Support.............................................................................................................49

9.5. Reference Design..............................................................................................................50

9.6.1. Mechanical Drawing – WIT2492M4 (Pins Down)........................................................51

9.6.2. Mechanical Drawing – WIT2492S4 (Pins Up)................................................. ... .......... 52

10. Warranty.................................................................................................................................53

OEM Installation Compliance Labeling

The WIT2492 module transmitter must be 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 module.

This exterior label can use wording such as the following:

“Contains Transmitter Module FCC ID: HSW-2492” or

“Contains FCC ID: HSW-2492”

Any similar wording that expresses the same meaning may be used. The Grantee may eith er 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.

1. INTRODUCTION

The WIT2492 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 2.4GHz ISM band allows license-free use and worldwide compliance. A simple serial interface supports asynchronous data up to 230400 bps. An on-board 3 KB buffer and an error-correcting over-the-air protocol provide smooth data flow and simplify the task of integration with existing applications.

- Frequency hopping technology

with 43 frequency channels (2400-2483.5 MHz).

- Supports point-to-point or multipoint applications.

- Meets FCC rules DTS and ETS

300.328 for worldwide licensefree operation.

- Superior range to 802.11 wireless LAN devices.

- Transparent ARQ protocol w/3KB buffer ensures data integrity.

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

- Low power 3.3v CMOS signals

1.1. Why Spread Spect r um?

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

- Simple serial interface handles both data and control at up to 230400 bps.

- Fast acquisition typically locks to hopping pattern in 2 seconds or less.

- Selectable 10 mW or 100 mW transmit power.

- Built-in data scrambling reduces possibility of eavesdropping.

- Nonvolatile memory stores configuration when powered off.

- Smart power management features for low curren t consumption.

- Dynamic TDMA slot assignment that maximizes throughput.

shifts in location and

WIT2492

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

2.4 GHz 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.

Spread spectrum reduces the vulnerability of a radio system to interference from both jammers and multipath fading 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 transit.

Figure 1

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.

WIT2492

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 direc t 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.

WIT2492

As an additional benefit, RF spectrum has been set aside at 2.4 GHz in most countries (including the U.S.) for the purpose of allowing compliant spread spectrum systems to operate freely without the requirement of a site license. This regulatory convenience alone has been a large motivation for the industry-wide move toward spread spectrum.

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 43 frequencies and the remote is scanning 43 frequencies, it can take several seconds for a remote to synch up with the base station.

Once a remote has synchronized with th e 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 assign ed 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 automatically 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 0). The amount of data that the base station can transmit per hop is determined by the base slot

WIT2492

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 WIT2492 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 betw ee n the base station an d a n y 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 bu ffer 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 data transmit delay can

WIT2492

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 00 (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 WIT2492 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 mod e.

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 ho st 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 se tting.

2.2.3. Handle Assi gnment

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.

WIT2492

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 remo te, 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 a 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, th e 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 10msec per hop, this could be as along as 2.55 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 b ase 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 WIT2492 can meet this requirement. In the WIT2492 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.

WIT2492

7.36

When setting up a network, keep in mind that ti me 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 1.7 ms overhead for tuning, the synchronization signal and parameter updating, as well as a guard time of 500µs between each remote slot. Thus the amount of time allocated per remote slot is roughly:

hop duration – base slot – 1.7ms - ( # of registered remotes-1)·500µs

( # of registered remotes)

Take for example a network comprised of a base station and 10 remotes. A hop duration of 10 ms is chosen. We decide that the base station needs t o be able to send up to 32 bytes each hop (equivalent to a capacity for the base of ~ 32 kbps). Counting the 1.7 ms overhead for the base packet and making use of the fact that our RF rate is 460.8 kbps, we determine that the base slot requires approximately:

32·8

460.8kbps

+ 1.7 ms = 2.3 ms

Each remote time slot will be:

10 ms – 2.3 ms – (9)·0.5 ms

= 0.32 ms

From our RF data rate of 460.8kbps we see that it takes 17.36 µs to send a byte of data, so each remote will be able to send up to

= 18 bytes of data per hop.

0.32 ms 1

Note that the 18 bytes is the actual number of data bytes that can be sent. If the WIT2492 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:

18 bytes

10 ms

= 18 kbps

If we figure a minimum margin of safety for lost packets and retransmissions of about 20%, we see that this would be more than sufficient to support 14.4 kbps of continuous data per remote. It is also useful to remember that the asynchronous data input to the WIT2492 is stripped of its start and stop bits during transmission by the radio, yielding a "bonus" of 10/8 or 25% in additional capacity.

The above calculations are provided as a means of estimating the capacity of a multipoint WIT2492 network. To determine the precise amount of capacity, you can actually set up

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 nu mber by the hop duration as above to get the remote's exact capacity.

2.2.5. Full Duplex Communication

From an application perspective, the WIT2492 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 tran smissions 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 tra nsmission. In a typical office or factory environment, 1% - 2% of the 2.4GHz frequency b a n d may be unusable at a ny 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 WIT2492 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 WIT2492 simply tries again on the next channel. Even if two thirds of the band is unusable, the WIT2492 can still communicate reliably.

Data input to the WIT2492 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 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

WIT2492

00H

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

2.3. Modes of Operat ion

2.3.1. Control and Data Modes

The WIT2492 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 inte rpreted as command data. Once the CFG pin is deasserted, 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 deassertion of the Sleep pin on the modem. The modem will respond in the same way with a Mode, enter the Exit Modem Cont rol Mode command,

> prompt. To return to Data

z>, or assert and deassert the

Sleep pin. There are three modes for the escape sequence, controlled by the Set Escape Sequence Mode command,

zc:

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. 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 WIT2492 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 50µA. 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.

WIT2492

:WIT2492

2.3.3. Low Power Mode and Dut y Cycling

To conserve power, WIT2492 remotes power down the receiver and transmitter between hops when not in use. Base stations must remain active all the time to handle any transmission from any remote. Remotes can save even more power by enabling the duty cycle feature. This feature causes a remote to power down for 2

1/2

is the duty cycle. Rather than attempting to transmit on every frequency hop when

frequency hops where

data is in the transmit buffer, a remote will attempt to transmit only every 2 Roughly speaking, this will proportionately reduce the average power consumption while increasing average latency. When there are more than 16 remotes being operated, duty cycling must be enabled since a maximum of 16 time slots is available per hop.

When a remote radio is powered up but is out of range of a base station, it will continuous scan the frequency bands for the presence of a base radio. During this scanning the radio can consume up to 80mA of current. A low power seek mode is available in which the remote radios seek base stations only 50% of the time. Th is will reduce current consumption by about 50% but will double the time it can take a remote to link with a base up to 4 seconds.

2.3.4. RF Flow Control Mode

Because of slight differences in baud rates between transmitting and receiving hosts, when sending large amounts of data (100’s of KB) in one direction in a point-to-point application, it is possible to overrun the receive buffer of the receiving radio. For example a nominal 115.2Kbaud at the transmitting radio’s host might really be 115,201 and at the receiving radio’s host it might be 115,199. This is similar to a situation where the transmitting radio is sent data at a higher baud rate than the baud rate at which data is received by the receiving host. To compensate for the variations in nominal baud rates, the WIT2492 supports an RF flow control mode for point-to-point operation. In this mode, when the receive buffer of the receiving WIT2492 is close to full, the receiving WIT2492 stops acknowledging transmissions. The transmitting radio is set to infinite retries which invokes the RF flow control mode (See Set Packet Attempts Limit in Section

5.3). The receiving radio will not begin acknowledging transmissions from the transmitting radio until more room in the receive buffer has become available. This will cause data in the transmit buffer of the transmitting radio to back up. If it backs up to the point where the transmit buffer fills up, the transmitting radio will deassert CTS stopping data from the transmitting radio’s host device. Once room is available in the receiving radio’s buffer, the receiving radio will begin acknowledging transmissions from the transmitting radio allowing the transmitting radio’s buffer to begin to empty which will cause the transmitting radio to reassert CTS. Either one or both of the radios in a point-topoint installation can be configured for the RF flow control. If this mode is invoked in a point-to-multipoint installation, communications with all radios will be stopped when any one radio’s receive buffer becomes full.

hops.

WIT2492

+ 41 hidden pages

Murata Electronics North America 2492 Users Guide

Specifications and Main Features

Frequently Asked Questions

User Manual