ARC RF Modem

Low cost RF Transceiver (902-928 Mhz) modem

HF 900 Point-to-Point RS232 up to 1,920bps Wireless Access Solution.

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RF Transceiver SR 900 Manual



The Short-Range Link (SR 900) radio modem is designed for ease of use. The commands to control the units are a modified subset of the popular AT commands used in modems. The unit is FCC approved.

Since wireless connections tend to be noisy and error prone, the SR has built in firmware and HDLC protocol to implement error control and automatic re-transmission(s) if required. Because this function is transparent to the user application, the SR is compatible with popular terminal programs e.g. ProComm, Bitcom etc.


Your package includes:



The SR 900 is being designed in the following applications:

Typical configuration:



For most applications, the SR 900 is configured as a DCE (e.g. modem). The host it talks to is a PC and they communicate through the RS 232 cable.

The installation requires that the SR be connected to the host through the serial cable provided. Then simply connect the wall transformer to the outlet, plug the 9 volts DC plug into the SR. The DSR light will come on and the unit is ready for operation.

Pin out diagram:

The pin-outs on the DB25 (F) connector are a below:

Pin Signal Name Description Type

2   TXD          Transmit data from the host Input.

3   RXD           Received data from the airwaves Output

4   RTS            Request to send Input

5   CTS           Clear to Send Output

6   DSR           Data Set Ready Output

7                       Ground

8   DCD          Carrier Detect Output

20 DTR          Data Terminal Ready Input

NOTE:        If  the SR is to be configured as a host ( DTE) a null cable must be used.

Quick Start

For a typical point to point communication between two PCs, the following may help.
Connect the two units to the serial ports.
Set the terminal programs to 19,200 bps, 8N1, Hardware flow control.

Unit 1

Power up the unit, wait 5 seconds. This is the time required for the unit to perform a self calibration.

  1. Enter AT<CR> The unit should respond OK
  2. Type AT&F<CR> This will set frequency to 915 Mhz.
  3. Type AT+ID=10<CR> The ID is now set to 10.
  4. Type ATO <CR> The unit is now in on-line mode and ready for communication.

Repeat the above for unit 2.
The units should now talk to each other.

Note: Approximately 20 seconds after power-up, the unit goes into receive mode. Type escape sequence +++ (500 msec between the + signs) to go back to command mode.


The radio has two states of operation:

a) Command state

In this state, all data received from the PC is interpreted as a command.
Commands are not sent over the air- waves.
Each command must start with AT.
When in this mode, the SR radio can be configured for ID, baud rate, etc,

Serial data sent to the radio in this mode is NOT transmitted over the air.

On power up, the radio is always in Command State. In this state, CD (Carrier Detect) line is inactive.

b) On-line state

When the SR is in on-line state, all serial data presented to the radio is transmitted over the air.

Do NOT change communication parameters (e.g. baud rate) in this mode.

To go back to Command State, Escape Sequence (described in this section) is required.

In this state, CD (Carrier Detect) line is active.

The basic AT (AT-tention) commands used to configure the SR radio modem operation are defined in this section. The commands implemented are a subset of the commands used in typical high-speed modems.

When in Command State, under AT operation, the Radio Module performs autobaud on each AT header. The unit supports 8N1 format (8 bit characters no parity with one stop bit) only.

A command line is a string of characters sent from the DTE (e.g. a PC) while the modem is in command state. A command line has a prefix, a body and a terminator. Each command line must begin with a character sequence AT and must terminate with a carriage return.

Both upper case and lower case characters are supported.

The Radio starts the processing of commands only after AT string sequence is received.

Commands entered with out of range parameters will result in ERROR message.

Correct commands will result in OK message.

Upon first time power up, the Radio enters the Command State. When a valid AT command is received, the RF Module will do the following:

1) Execute AUTO BAUDING routine to match the PC serial port (DTE) speed. The air speed is fixed at 19,200 BPS.

The default transmit & receive frequencies are 915 MHz.

The user can then instruct the SR Radio Modem to go ON LINE by using ATO command.

When on-line, the SR captures only those messages where the destination address matches its ID. All other messages are rejected. This allows multiple radios to operate in the same zone.

AT Commands supported.

Following AT commands are supported:

Upon successful implementation of each command, the modem will return either a value or OK.       

Command                              Action                                                    Response            Note

1) ATZ<CR>                                             Resets the modem.                                        OK

2) AT+ ID=[nnnnn] <CR>                          Stores new ID                                              OK                      *

3)      ATZ<CR>                                         Resets the modem.                                       OK
                                                               transmit data. The source
                                                                   and destination IDs are
                                                                  assumed to be the same.
                                                                 When on-line, CD is active

4) ATDT XXXX <CR>                             The unit communicates with
                                                                  RF unit whose ID is XXXX.

5) AT+FR = yyy<CR>                             Sets transmit / receive frequencies                                              *
                                                                 to yyy, Only 905, 910, 915, 920, 925 MHz
                                                                 supported. Default is 915 MHz.

6) AT&V<CR>                                        Displays ID, frequencies etc.                         OK

7) ATI3<CR>                                          Displays Software Revision #                        OK

8) AT&F& W<CR>                                Clears everything except ID                           OK

9) AT+M0<CR>                                      The SR implements HDLC                            OK                        *
                                                                 protocol. In this mode, the unit
                                                                buffers the data, adds error
                                                                 checking and automatic retransmission.
                                                                 This is the default mode.

10) AT+M1<CR>                                     In this mode, the SR transmits                      OK                       *
                                                                data without any HDLC buffering.
                                                                The user application is expected to
                                                                 Do the data packetizing.
                                                                On the receive side, the SR does
                                                                 The filtering to receive only
                                                                 the valid data. This will require
                                                                 that the OEM have access to our
                                                                 protocol.This is possible only
                                                                after receipt of a non-disclosure

11) AT+M2<CR>                                    In this mode, the SR receives                       OK                       *
                                                                and transmits data without error
                                                                control. This mode is the pass
                                                                through mode And used in time
                                                                critical situations. This command
                                                               must be entered before The ATO
                                                                or the ATDT command.

12) AT+ANI=0<CR>                            Receiving unit does NOT inform                    OK                        *   
                                                               the host of the ID of the
                                                               calling unit. Caller ID is
                                                              disabled. This is the default mode.

13) AT+ANI=1<CR>                           Receiving unit informs the host of                    OK                        *
                                                              the ID of the calling unit. This
                                                             command must be entered before
                                                            ATO or ATDT command. Please note
                                                            that ANI commands will not work
                                                            with AT+M2 mode. 

14) AT+HLP<CR>                             Displays all supported AT                                 OK

Escape Sequence:
When the modem is on-line state, it is possible to break the data transmission/reception.
This is achieved by sending a sequence of three ASCII characters. For ease of use, the default character is +.
The Radio will respond with OK on detection of valid escape sequence.

15) +++                         Escape sequence                 OK

The timing interval between the + signs must be between 250 msecs and 750 msecs. On receipt of a valid escape sequence, the SR will respond with OK. 

Note: All commands marked with * are stored in the NVRAM of the SR 900. On power up, the SR 900 configures itself based on the stored values.

Therefore reconfiguration is NOT required every-time on power up.

Data Packetizing & Depacketizing:

Airwaves tend to be noisy and therefore error prone. To send data reliably, HDLC protocol is implemented. Data from the host is received and bundled into packets. To each packet, header for source and destination address and other information is added. The receiving side verifies all the information and then sends a positive acknowledgment. Else, the sender sends the packet again.