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Interfacing a Rockwell MDM-2001 HF modem via the Black Cat Systems MIL-STD-188C synchronous serial interface

USB Sync Interface

Black Cat Systems MIL-STD-188C synchronous serial interface

The Rockwell MDM-2001 is a programmable HF modem available on the
surplus market which can do a variety of advanced HF modes, including
MIL-STD 188-110A 39/16/single tone. This application covers using the
MDM-2001 in a single-channel receive-only configuration. Dual channel
and transmit operation are also possible, but the cables used would
change from the configuration described below.
Our interface does not support transmitting.

The modem used to develop this app note was model number 622-9653-009.

The modem has three connectors on the rear panel which must be
connected to the computer and radio used to operate the unit.

J1 - Synchronous serial data.
J2 - RS-232 (asynchronous) serial control port.
J3 - Radio interface (audio).

J1 is a DD-50 connected to a Black Cat Systems MIL-188C synchronous
serial adaptor with the following cable:

Rockwell MDM-2001    Black Cat Systems
J1                   MIL-188C adaptor
-----------------    -----------------
pin  name            pin name
---  -----           --- -----
2    RXD1            4   Data      
18   RX1 clkout      20  Clock
21   RX1 common      7   Ground

J2 is a standard 9-pin to 25-pin async cable:

Rockwell MDM-2001    IBM-PC
J2                   RS-574
-----------------    ------
pin name             pin name
--- -----            --- -----
2   txd              2   rxd
3   rxd              3   txd
7   gnd              7   gnd

J3 is an audio cable that will vary based on output from your
radio. Assuming you want to convert single-ended audio out of a
receiver into the differential audio the modem expects, a audio
frequency transformer with a center tap is required, and the following
cable will perform the job:

Receiver audio                  Rockwell MDM-2001
1/8" phono or RCA               J3
-----------------               -----------------
name                            pin name
----            transformer     --- ----
tip --------------+    +------- 1   RCV1_AUDIOA
                  | || |
                  | || +------- 14  RCV1_AUDIOC
                  | || |
ring -------------+    +------- 2   RCV1_AUDIOB

The MDM-2001 allows multiple modems to be addressed by a single serial
port attached to J2. Attempts to configure the MDM-2001 to run in
non-addressed mode were unsuccessful. The following are the
recommended jumper settings inside front panel door:

(9600 8/N/1, single modem addressed as unit 1):
S1: 1001111110 S2: 0001001000 (1 = ON, indent towards number)
Email your comments and questions to

Last modified May 31, 2005