Icom IC-F1721/1821/2721/2821 Serial Snooping

This is not really a complete post, but rather a quick snippet about snooping the serial data from Icom IC-F1721/1821/2721/2821 P25/analog mobile radios. There are a number of serial buses within the radio: one on the DB25 connector for accessories such as GPS, an internal NMEA GPS serial bus, a CLONE IN/OUT serial bus for radio programming, and the FMDA/MFDA bus which goes between the main microprocessor on the main board and the microprocessor on the control head. This snippet of info has to do with the final option, the FMDA/MFDA bus for control head communication. It operates at 5v TTL levels, at a baud rate close to 57600 (as best I could tell… It may not be an even baud rate.) I was impressed with how fast this bus was running, compared to the 9600 baud rate of Motorola’s internal SB9600 serial bus. Unfortunately, the only place to tap into this bus is inside the radio. I found that the data I was receiving from the bus was not error-free, either (probably due to lack of a buffer on the line, or incorrect baud rate), but I could clearly make out some of the channel names being printed on the display, and I could see what I thought were button codes. I believe FMDA stands for the TX from the control head to Main board, and MFDA stands for TX from Main unit to control head,  but I cannot be sure; One would have to check the service manual. The connection points I tapped into were very close to the ribbon cable connector on the main board for the control head ribbon cable:

The white wire is soldered to a plated-through hole which has either FMDA or MFDA on it (I can’t remember which I had at the time…). The other line (either FMDA or MFDA) is on the plated-through hole just below and left of the one that I have the white wire soldered to in the picture. In theory, you could inject your own “packets” into the serial bus to the radio or control head and control what channel the radio is on, emulate button presses, write to the display, etc.  (even PTT) via serial commands.

One Response to “Icom IC-F1721/1821/2721/2821 Serial Snooping”

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