WA4DSY 56 KB Modem User Guide


This guide is intended to explain how to hook up and use the WA4DSY 56KB RF modem. There is no theory of operation in this document. The reader is refered to the ARRL publication Proceedings of the 6th Computer Networking Conference, page 68 for details on the original design, and page 4 of the proceedings of the 14th ARRL Digital Communications Conference for technical details on this version.

Click here to see an html version of the technical paper on this modem

Accessories You Will Need

The modem requires 12 to 14 volts DC at 350 MA. The transverter is powered from the modem. At 7 watts output, a typical Microwave Modules brand transverter requires about 1.8 amps. Any ripple free DC source capable of providing 12 to 14 volts at 2.5 amps should operate both modem and transverter satisfactorily. It may also be possible to run it from your computers disk drive power. The center pin on the modems power connector is the positive 12 volt input. Do not reverse the polarity!


The modem requires a linear transverter to convert its 29 MHz IF frequencies to the band you want to operate on. It does not interface to an FM voice radio like the K9NG and G3RUH 9600 baud modems. It does not produce any kind of audio like signal. The modem outputs a modulated RF signal in the 10 meter band (28 - 30 MHz) at a nominal 1 milliwatt power level. This signal must be converted (not multiplied) to the desired operating frequency. The modems receiver also operates in the 10 meter band and requires a down-converter to receive UHF signals.

A transverter both up-converts and amplifies the transmit signal and down-converts the receive signal. The modems 29 MHz transmitter power level is adjustable -10dbm to +5dbm. Most transverters should find this acceptable. The transverters receive conversion gain should be at least 18 db. Many go up to 30 db. Avoid transverters with use relays to do T/R switching. Most transverters have a timer which delays the switch from transmit to receive mode when PTT is released. This timer must be disabled. This is accomplished in Microwave Modules brand transverters by clipping a lead on a 33uf capacitor in the timing circuit.

TNCs and Packet Interface Cards

Many TNCs used for packet radio are too slow and will not operate at 56KB. Check with the manufacture before you buy a TNC for this modem.

The best way to interface an IBM PC or clone to the modem is with a synchronous serial interface card. The modem was specifically designed to interface directly to the Ottawa PI-2 card. This card has 2 channels, A and B. The A channel uses DMA (direct memory access) for data transfer. The B channel uses interrupts only. Since 56 kilobaud is too fast for the interrupt driven channel, only channel A can be used. The Gracillis PackeTwin card can also be used since it has a DMA channel. Avoid interrupt driven cards.

It is also possible to use a PacComm Spirit-2 Pad with the modem. Click here to view the hookup diagram.

Cable Hookup Guide

Modem to Transverter

Three cables connect the modem to the transverter.

Note: Pins 2 and 4 (Relay contacts) do not connect to the transverter . They are for controlling other things such as reseting the computer if you want.

Modem to serial card DB-15 connector

If you are using an Ottawa PI-2 card and have installed the RS-422 option, you may use a straight through ribbon cable with a male DB15 connector on each end. If you wish to connect the modem to another card with different pin assignments or connector type, you will need to build a custom cable. Also, note that the original Ottawa PI card didn't have RS422 capability and can't be used with a straight through ribbon cable because of the ground connections on pins 9-15. See the CMOS pin assignment table for details.

RS422 is the preferred interface. The RS422 standard claims cable runs up to 4000 feet are possible.

RS 422 Pin assignments.

Select this interface mode by setting dip switch S4, switch 1 OFF.

CMOS Pin assignments.

Use this only if your serial card or TNC doesn't support RS422. Keep the cable length less than 3 feet. If possible, use twisted pair for the data and clock signals with one wire in each pair connected to ground.

Select this interface mode by setting dip switch S4, switch 1 ON.

Power connector

Power is supplied to the modem through the 2.5mm coaxial jack. The center pin is +12 to 14 volts at up to 2.5 amps. If you use a high amperage power source such as a car battery, please use a 5 amp fuse in series with the source.

Switches, Controls and Indicators

Internal Dip switch S4

1 RS422 CMOS Set on if you need to interface to a non RS422 device
2 Normal Full Duplex Set on if you don't want the RX muted when transmitting
3 Normal Repeater Enable Set on if you want RX data to be retransmitted
4Normal Scrambler Disable Set on to disable TX and RX scramblers. See note 2
5Normal Key Transmitter Set on to key transmitter for tune-up and testing
6 Normal Tune up Set on to activate sweep generator mode.
7Normal Watchdog Disable Set on to disable 2 second watch dog timer
8 ** Frequency Select 2 Bit 2 of 3 bit code ON=1 See note 1.
9 ** Frequency Select 1 Bit 1 of 3 bit code ON=1 See note 1.
10 ** Frequency Select 0 Bit 0 of 3 bit code ON=1 See note 1.

Note 1: These 3 switches are set according to a 3 bit code which defines 1 of 8 frequency memories. The code and corresponding TX and RX frequencies are determined by the EPROM contents. A program called EDIT56K.EXE can be used to edit the EPROM file and change the frequencies. A new EPROM must be programmed. See the "readme" file which came with the EDIT56K program for details.

Note 2: Use this setting only if you are randomizing the data external to the modem. The modems DCD and clock recovery logic doesn't work well with non-random data.

** Some modem models may use a front panel rotary switch to select the frequency memories.

R20 - TX Level

This control adjusts the transmitter output drive level. Midrange adjustment is about 1 milliwatt.

Rear panel push buttons

Remote Control

Pushing this makes the modem send the "Remote Control" code to a distant modem. The relay K1 in the distant modem will close in response. The 2 second watch dog timer limits the maximum transmission time. Both modems must be programmed with the same "code". See the EDIT56K program documentation for details on setting the code.

Key TX

Pushing this makes the modem transmit scrambled one bits continuously. The watchdog timer is disabled while the button is pressed. This is used for testing.

Front Panel LEDs

READY Green/Red Green ndicates the modem has initialized and is ready for use
Red indicates a "Remote Control" code is being received from a distant modem and relay K1 is closed.
PTT Red The modem is transmitting a signal.
DCD Yellow The modem is receiving a valid data signal.

Signal Strength LEDs

These LEDs provide a relative indication of received signal strength. If the same frequency is used for both TX and RX they will also indicate the transmitter signal. (TX is looped back to RX in transverter)

LED Signal level chart

Here is a chart of the signal levels at the modems' 29 mhz IF input required to light each LED. These figures were obtained from a prototype modem. Your modem may vary. Add the transverter receive gain to figure the input level at the transverter RX antenna input. You will need about 1.5 microvolts (-104 dbm) at the antenna input for solid error free performance.

Internal Test Points

TP1 TX VCO Transmitter VCO control voltage
TP2 RX VCO Receiver VCO control voltage
TP3 EYE Receiver "EYE" pattern. (FM Quad detector output)
TP4 RSSI DC voltage indicates RX signal strength. (range is about .5 to 1.2 volts)
TP5 IF 448 KHZ receiver IF signal. (before limiter)
TP6 -LOCK Goes low when the TX VCO is properly phase locked
TP7 RX CLK Clock for received data
TP8 TX FILTER Output of 448 KHZ TX IF filter
TP9 TX OUT 29 MHZ TX output at coax connector
TP10 TX CLK Clock for transmitted data
TP11 RX DATA Received data

End of document