ARC RF Modem

Low cost RF Transceiver (902-928 Mhz) modem

FH 900 Point-to-Point RS232 up to 57,600bps Wireless  Spread Spectrum - Frequency Hopping Access Solution.

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301-924-7400 EXT 25 x 17

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The Frequency Hopping (FH 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. 

Wireless connections tend to be noisy and error prone, the FH 900 has built in firmware and protocol to implement error control and automatic retransmission's if required. Because this function is transparent to the user application, the FH 900 is compatible with popular terminal programs e.g. Hyper-terminal, ProComm, etc.


Communications Technique  Spread Spectrum (Frequency                     Hopping) 902-928 MHz  band   
Number of Channels 76 (seventy six)
Modulation Frequency Shift Keying (FSK)
DTE Rate   1200, 2400, 9600, 19,200, 57,600 
switch selectable)
Flow Control  Hardware or Software (Switch Selectable)
Transmit Power  50 mw
Protocol Half Duplex
Power Supply 9 Volts DC @ 500 ma
Data interface    RS 232 Asynchronous
Signals supported TXD, RXD, DSR, RTS, CTS
Range (line of sight) typical 2500 feet

Optional through wall installations use RG-58 cable

Custom Configurations available 

Small order price $325.00


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RF Transceiver FH 900 Manual


  Your package includes:


Custom Configurations and Options Yes


The FH 900 is being designed in the following applications:


 ID Management

Just like every home has a distinct phone number, each FH 900 radio has distinct ID number. The ID is set by the user.

The ID for each radio is set using the AT+ID=nnnn <CR> command. Once entered, the ID is stored in the radio’s non-volatile memory and does NOT have to be entered again.

The ID consists of 4 digits. The first digit is the Group number and only radios of the same group will talk to each other. If ID of 3 digits or less is entered, the Group number is assumed to be 0 (zero)

The last 3 digits of the ID uniquely identify each radio within the group.

There are 2 commands to put the radio in the data transfer mode.

These are 1) ATDT xxxx

                  2) ATO


If radio A is issued ATDT B, then it will communicate with unit B only. Although if the data packet is addressed to it, A will receive data from any radio, but outgoing data will only be to B.

Therefore, if the application is a point to point communication with A talking to B, then ATDT B <CR> has to be issued to radio A and ATDT A <CR> has to be issued to B.

The radios will remember this configuration on power up.

In the case of Point to Multipoint configuration (A is the master and B & C the slaves) then units B & C must be issued the ATO command.

When A talks to B, it will have the ATDT B <CR> command. When B gets the message, it will then send all data to A. This will be till another radio with a different ID calls B.

Similarly if at A the command ATDT C <CR> is issued, unit C will respond only to A.


Please note that with the ATO commands, the slaves will never initiate communications unless they are addressed first.

Installation & quick start using Windows Hyper-terminal:

For most applications, the FH 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 FH 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 FH. The DSR light will come on and the unit is ready for operation.


Pin out diagram:

The pin-outs on the DB 9 (F) connector are a below:


Pin #    Signal Name               Description                              Type                Remarks

1          DCD                            Carrier Detect                         Output             Not used

2          RXD                            Received data                         Output

                                                (from the airwaves)

3          TXD                             Transmit Data                         Input

4          DTR                            Data Terminal Ready              Input

5                    Ground

6          DSR                            Data Set Ready                      Output

7          RTS                             Request to send                     Input

8          CTS                             Clear to Send                          Output

9          RI                                 Ring Indicator                          Output             Not used


In case, the FH 900 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 9600 bps, 8N1, Hardware flow control.

On the FH 900, select switch settings for 9600 bps (switch position 3 ON i.e. down), Hardware Flow control (switch position 6 ON i.e. down).

Connect the RS 232 cable provided. Connect the Antenna.

1)                  Connect the power supply. After approximately 10 seconds, the MR light should turn ON. The unit is now ready for operation.

2)                  Connect the FH 900 to the PC with the RS 232 cable provided.

3)                  Click the Hyper Terminal icon on the PC. Or select Start>Programs>Accessories>Hyper Terminal>Hyperterm.exe

The Connection Description Dialog Box opens.

4)                  In the name field, enter FH 900 and click OK.

The Connect To dialog box is now active.

5)                  Select Connect Using and then select the COM Port the CDL 900 is connected to. For example, if the FH 900 is on COM 2, choose “Direct to Com2” and click OK.

6)                  The COM2 Properties dialog box now opens. Select the following:

a)                  Bits per second to match the DIP switch settings on the FH 900

b)                  Data Bits: 8

c)                  Parity: None

d)                  Stop Bits: 1

e)                  Flow Control based on the FH 900 switch settings.

f)                    Click OK.

7)                  Select Call>Disconnect

8)                  Select Call>Connect

The TR (Terminal Ready) light on the FH 900 should turn ON. If it does NOT turn ON, please check that the FH 900 is connected to the right serial port on the PC.

9)                  Select File>Properties>Settings>ASCII setup

10)              Check the following:

a)                  In the ASCII Sending field, check Echo typed characters locally

b)                  In the ASCII receiving field, check Append line feeds to incoming line ends and Wrap lines that exceed terminal width.

11)              Click OK twice.

12)              Select Call>disconnect

13)              Select Call>connect

14)              Select File>Save

  Let us say that radio with ID 10 is to communicate to a radio with ID 15.

  Unit 1:

1)          Enter AT<CR> The unit should respond OK

3)                  Type AT+ID=10<CR> The ID is now set to 10.

4)                  Type AT&V<CR>. The unit will display ID 10

5)                  Type ATDT 15<CR>


Unit 2:

1)          Enter AT<CR> The unit should respond OK

2)          Type AT+ID=15<CR> The ID is now set to 15.

3)          Type AT&V<CR>. The unit will display ID 15

4)          Type ATDT 10<CR>

The radios are on line and ready to communicate with each other.

Please note that radio does NOT have to configured every-time. The above information is stored in the Radio’s non-volatile memory.


15)       Type in “ Hello how are you ” and the called party should see the message on their terminal.

16)       Please ask the called party to type in any message and you should see that on your terminal.

17)       To get back to command mode, enter the Escape sequence +++

18)       The modem should respond OK.


At the time of power down, the unit remembers the baud rate and the state. If the unit is powered down when in Command State, on power up, it will be in command state.

If, however, the unit is powered down in the on-line state, on power-up, the unit will go into on-line state.

Type escape sequence +++ (it is important that there be a time gap of 500 msecs between the + signs) to go back to command state.

Theory Of Operation:

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 FH radio can be configured for ID (source ID)  and the destination ID.

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

            b)         On-line state

When the FH 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.


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

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. Upon first time power up, the Radio enters the Command State and the default transmit & receive frequencies are 915 MHz.

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

When on-line, the FH 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>                                Software modem reset                       OK      

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

3)         ATE0                                       Disable echo in Command Mode

                                                            This is default

4)         ATE1                                       Enable echo in Command Mode

5)         ATO<CR>                               Go on line. Unit is now

ready to receive and

transmit data. The unit will respond only to the last

ID which addressed it.

e.g if the unit with ID 10 is issued ATO and then

it receives data from a unit with ID 15 , all further

communication will be with ID 15.

If later another unit with ID 21 addresses ID

10, then all subsequent conversation will be with

ID 21.

6)         ATDT XXXX <CR>                  Similar to ATO command, but the unit

communicates with RF unit whose ID is XXXX.

In a single master and multiple slave units

situation, the ATDT command would be issued to

the master. This will allow the master to talk to

various units.

7)         AT&V<CR>                             Displays unit ID                                               OK

8)         ATI3<CR>                               Displays Software Revision #             OK

9)         AT&F&W<CR>                       Does nothing, for compatibility reasons.        OK

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


11)       ATQ0                                       Normal mode. The FH responds

with appropriate messages e.g OK etc.

12)       ATQ1                                       Quiet Mode. The FH 900 only gives (to DTE)

received data.

Escape Sequence:

When the modem is on-line state, it is possible to break the data transmission/reception and enter into command mode.

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.


            +++                 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 FH will respond with OK.


Note: All commands marked with * are stored in the NVRAM of the FH 900. On power up, the FH 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, packet 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.


Frequently Asked Questions (FAQs)

1)         Question:        What is the flow control method recommended?

            Answer:           If the host is a PC, Hardware flow control is recommended.

However, in most control instrumentation, either software (XON-XOFF) or no flow control is called for.

2)         Question:        I need to connect the FH 900 to a machine that does not

understand your messages. All it needs is data. Which

configuration is recommended?

            Answer:           In the command state, please use the ATQ1<CR> command.

                                    The ATO<CR>

                                    There will be no response from the modem but the unit is now

ready to receive data. The escape sequence will get you back to

the command state.

3)         Question:        The ID of my unit is ID1, but I want to send a message to unit

ID2. How can I do that?

Answer:           To communicate with another unit, enter the command


All further communication will be with ID2 until escape sequence

is entered.

Federal Communications Commission Radio Frequency Interference Statements

This equipment generates, uses and can radiate radio frequency energy and if not installed and used properly, that is, in strict accordance with manufacturers instructions, may cause interference to radio communication. It has been tested and found to comply with limits of class B computing device in accordance with specifications in Subpart J of part 15 of FCC rules, which are designed to provide reasonable protection against such interference when the equipment is operated in a commercial environment. Operation of this equipment in residential area is likely to cause interference, in which case the user, at his own expense will be required to take whatever measures may be necessary to correct the interference.

Changes or modifications not expressly approved by the party responsible for compliance could void the user’s authority to operate the equipment.


a DCE Company

301-924-7400 EXT 25 x 17

e-mail Sales at

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