WIT2420 2.4GHz Spread Spectrum Wireless Industrial Transceiver Integration Guide June 2nd 2021 Note: This unit has been tested and found to comply with the limits for a class A digital device, pursuant to part 15 of the FCC Rules. These limits are designed to provide reasonable protection against harmful interference when the equipment is operated in a commercial environment.
The modular transmitter is only FCC authorized for the specific rule parts listed on the grant; when it is installed in a host device, the host product manufacturer is responsible for compliance to any other FCC rules that apply to the host not covered by the modular transmitter grant of certification. The final host product still requires Part 15 Subpart B compliance testing with the modular transmitter installed.
TABLE OF CONTENTS 1. INTRODUCTION ...................................................................................................................................................1 1.1 Why Spread Spectrum? ...................................................................................................................................1 1.2 Frequency Hopping vs. Direct Sequence ........................................................................................................2 2. RADIO OPERATION ....
1. INTRODUCTION The WIT2420 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.
Spread spectrum reduces the vulnerability of a radio system to both interference from 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.
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.
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.
the base station can transmit per hop is determined by the base slot size parameter. The maximum amount of data sent by a base station per hop is 192 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.
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.
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. When setting up a TDMA 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.
The above calculations are provided as a means of estimating the capacity of a multipoint WIT2420 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 number by the hop duration as above to get the remote's exact capacity. 2.2.4. Full Duplex Communication From an application perspective, the WIT2420 communicates in full duplex.
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 WIT2420 has two modes of operation: Control mode and Data mode.
2.3.3. Low Power Mode and Duty Cycling To conserve power, WIT2420 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 2N frequency hops where 1/2N is the duty cycle.
3. PROTOCOL MODES In point-to-point applications, it is generally desired that the radios operate in a transparent mode. That is, raw unformatted data is sent from the host to the radio and is received as raw data from the receiving end. The addressing and error detection and correction are still performed by the radios, but it is transparent to the user application. To set up a point-topoint network, one radio has to be set up as a base station.
Protocol Modes Definitions mode 00 Transparent mode used for point-to-point networks or multipoint remotes; does not support any packet types. mode 01 This is the simplest protocol mode supporting Data packets only. No CONNECT or DISCONNECT packets are supported and no sequence numbers are provided.
3.1. Packet Formats The byte formats for each packet type are shown in the table below. Packet fields are organized to fall on byte boundaries. In the case of bit-level fields, most-significant bits are on the left.
Handle 63 is reserved for broadcast packets from the base to all remotes. Acknowledgment requests are not supported for broadcasts. For this reason, it is a good idea to send broadcast messages several times to increase the odds of reaching all remotes. 3.1.3.
4. MODEM INTERFACE Electrical connection to the WIT2420 is made through a 16-pin male header on the modem module. The signals are 3.3 volt signals and form an RS-232 style asynchronous serial interface. The table below provides the connector pinout. Pin Signal Type Description 1 GND - 2 TXD Input Transmit data. Input for serial data to be transmitted. In Control Mode also used to transmit modem commands to the modem. 3 RXD Output Receive data. Output for received serial data.
4.1. Interfacing to 5 Volt Systems The modem interface signals on the WIT2420 are 3.3 volt signals. To interface to 5 volt signals, the resistor divider network shown below must be placed between the 5 volt signal outputs and the WIT2420 signal inputs. The output voltage swing of the WIT2420 3.3 volt signals is sufficient to drive 5 volt logic inputs. 10 k From 5v Output To 3.3v Input 20 k 4.
asynchronous into the radio serial port). If the base transmits continuously at a higher rate than this, unless the default settings are changed, the transmit buffer will eventually overflow. To allow a higher base throughput, either increase the base slot size or the hop duration or both. A similar analysis needs to be performed for the remote radios. Refer to Section 2.2.3 TDMA Mode for the remote throughput calculation. 4.
5. MODEM COMMANDS The WIT2420 is configured and controlled through a series of commands. These commands are sent to the modem directly when the modem is in Control Mode when the modem is in Data Mode if the escape sequence is enabled. The command syntax is the same for either method, a one- or two-letter command followed by one or more parameters. The modem will respond with a two-byte message that indicates the new modem parameter value.
Set Data Rate Divisor Sets the serial bit rate between the modem and the host. This command takes effect immediately and will require adjusting the host serial rate to agree. Nonstandard rates may be programmed by entering a data rate divisor computed with the following formula: DIVISOR = (230400/RATE)-1 Round all non-integer values down. Set Protocol Mode Enables the base station to operate in a multipoint network.
Set Default Handle Sets handle number between 1 and 62 inclusive for a remote. This handle will override the automatic handle assignment by the base station. This command can be used in applications where it is desired to have specific modems have specific handles. When specified for the base, the default handle determines which remote it will address when transparent protocol mode is in effect. When 3FH is specified for the base, broadcast mode is entered.
5.3. Protocol Commands These commands can be used to tune the transceiver for optimum transmission of data across the RF link. For most applications, the default values are adequate. Command pe[?|0-4] Description Set Alternative Frequency Band 0 = FCC/ETSI operation.
decrease packet latency. A larger value increases network capacity, due to decreased overhead in channel switching. The hop duration is specified in 69.4µs increments. The default value of 90H corresponds to a duration of 10ms. The maximum value of FEH is 17.627ms. For best results, do not specify a duration of less than 3 ms. This value only needs to be set in the base which broadcasts the parameter to all remotes.
length of data to be sent together is longer than the time slot can send, the data will not be sent together but will be broken up over multiple hops. The length of time the radio will wait is equal to the specified value times the hop duration.
5.4. Status Commands These commands deal with general interface aspects of the operation of the WIT2420. Command Description zb[?|0|1] Banner Display Disable 0 = disabled 1 = enabled (default) zc[?|0..2] Set Escape Sequence Mode 0 = disabled 1 = once after reset (default) 2 = unlimited times zh? Read factory serial number high byte. zm? Read factory serial number middle byte. zl? Read factory serial number low byte.
roughly ¼ the consumption as when N=0. This parameter must be set to the appropriate value when more than 16 remotes are in use. Enable Low Power Acquisition Mode. When a remote is searching for a base to acquire and register with, it scans the frequency band very rapidly. This mode consumes about 80mA of current during this mode. To reduce the frequency consumption when a remote is in acquisition mode, a low power acquisition mode is provided.
5.6. Modem Command Summary Serial Commands sd[?|00..ff] sp[?|00..14] Set Data Rate Divisor Set Protocol Mode Network Commands wb[?|0|1] wd[?|1..3f] wn[?|00..3f] wg[?|0|1|2] wp[?|0|1] wr? dx[?|0..62] Set Transceiver Mode Set Default Handle Set Hopping Pattern Enable Global Network Modes Set Transmit Power Read Receive Signal Strength Set Range Optimization (remote only) Protocol Commands pe[?|0..4] ph[?|00..fe] pl? pn[?|01..3e] pk[?|00..d4] pr[?|00..ff] pt[?|00..ff] pv[?|0|1] pw[?|00..
6. WIT2420 DEVELOPER’S KIT The WIT2420 Developer’s Kit contains two self-contained wireless modems (HN-510s) built around the WIT2420M OEM module. In addition, two WIT2420M OEM modules are included in the kit. The self-contained units allow developers to get up and running quickly using standard RS-232 interfaces without having to build a CMOS level serial interface. In addition, the self-contained modems include status LEDs to provide modem status information visually.
COM24 defaults to com1: and 9600 bps. The port and baud rate can be changed through the invocation of the program. The invocation syntax is shown below: COM24 Note that the data rate applies to the serial port of the computer. This parameter has no effect on the modem. If the modem is set for 9600bps but COM24 is invoked to run at 19.2kbps, the computer and the modem will be unable to communicate.
3. Turn the radios on and use the function keys to set DTR and RTS to 1 (if you are using a terminal program other than COM24, these are typically set automatically). The radio should respond by setting both DSR and CTS to 1, and transmit a short sign-on message including the firmware version and whether the unit is configured as a base or remote. Watch the states of the hardware control lines on the status bar as you do this. The DCD indicator should be lit on the base station.
as the remote's. In a multipoint application, check that the remote is not configured for protocol mode and that the base is using the correct protocol format and destination handle. Radio is interfering with other nearby circuits. It is possible for the RF energy envelope to be rectified by nearby circuits that are not shielded for RFI, manifesting as a lower frequency noise signal.
7. APPENDICES 7.1. Technical Specifications 7.1.1 Ordering Information WIT2420M WIT2420S OEM Module, Serial connector pins down - Standard OEM Module, Serial connector pins up 7.1.2. Power Specifications Vcc Input Range: Operating Temperature Range: 3.3v to 10.0v -40C to +70C Current Consumption (Max transmit power, 230.4Kbps I/O) Mode Sleep Standby Typical Average Peak (Tx) Remote 50µA 20mA 50mA 200mA Base Station N/A N/A 120mA 200mA 7.1.3.
7.2. Serial Connector Pinouts Signal GND TXD RXD CFG RTS SLEEP DCD CTS WIT2420M/S OEM Pinout 1 2 3 4 5 6 7 8 HN-510 DB9 Pinout 5 3 2 7 4 1 8 Note: The WIT2420M is the standard part number and has the serial connector pins pointing down allowing connection to a mother board without using a cable. WIT2420S has the serial connector pins pointing up. The HN-510 is wired as a DTE device and as such can be connected to DTE devices such as PCs with a straight-through cable.
7.5. Reference Design Optional pullups to keep RTS and DTR asserted when left unconnected VCC 3.3V MBR0520L C1 0.22uF C1+ V+ 2 2 25 1 12 10 7 6 5 RXD DCD CTS 16 2 1 2 RS232 Interface C5 0.1uF C2+ C4 1 uF + 3 R2 6.8k C3 1 uF 4 + 1 R1 6.8k + C1V- C2 1 uF 27 1 28 1 VCC D1 26 2 1 U5 21 20 18 TXD DTR RTS C2T5OUT T4OUT T3OUT T2OUT T1OUT T5IN T4IN T3IN T2IN T1IN 17 19 22 23 24 RXD_3.3V DCD_3.3V CTS_3.
Murata Electronics Corporation 34 6/2/2021
