24 December 2017

a MS-110A modem running in ASYNC mode

This is a sample of MIL-STD 188-110A Serial 75bps/L modem working in ASCII ASYNC mode and transporting a Citadel encrypted file. The transmission was heard on 7413.0 KHz/USB following a 188-141A handshake between two Algerian Air Force nodes: CM2 (Algerian Air Force Base - Oran, 2nd Regional Command Centre) and COF (Algerian Air Force HQ - Cheraga). 

In ASCII Asynchronous mode the bitstream consists of a 8N1 structure: one start-bit (0), 8 data-bits and at least one stop-bit (1). Each character is transmitted using a total of 10 bits and the 8 data bits are transmitted with the LSB first.

Fig. 1
Working in the ASYNC part, after removed both the start and the stop bits we get the clean 8-bit data where the characterstic pattern of the Harris "Citadel" encryption is easy to identify (Fig. 2).

Fig. 2
A similar example but related to Asynchronous STANAG-4285 can be read here:

You may use the MIL-STD Data Modem Terminal (MS-DMT) [1] to verify how the MS-110A works in ASYNC mode (Figs. 3,4)

Fig. 3 - MS-DMT settings for ASYNC mode
Fig. 4
The latest MS-DMT test build is available at:  

Thanks to  Steve Hajducek for the update, I suggest to subscribe his group at

(MS-110 Async from Algerian AF)

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