All that passes through the output from the HARRIS RF-5710A modem [1] is the decoded "unknown" data content and none of the raw octets (the overhead added by the HF waveform) will pass; unknown data may be synchronous or synchronous, ITA2 or ITA5 coded, clear-text or encryptyed blocks. Moreover, this is not a piece of software running on a PC but rather "hardware" that shall be connected through its serial interface port (DTE Port): the use of the synchronous or asynchronous port configuration is dictated by the type of equipment that is interfaced to the modem and the mode of operation.
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| Fig. 1a |
At the other side, modern day PCs employ 9 pin RS-232C and do not support external receive (RX Clk) and transmit (TX Clk) clocks which are required for synchronous modes of operation. It is therefore impossible to set a synchronous mode of operation on these PCs and it means that you can only handle synchronous data using two possible configurations:
a) DTE Port Async - PC Port Async (Async-Async)
b) DTE Port Sync - PC Port Async (Sync-Async), only if the on-air speed is constant, ie no autobaud waveforms
Since the Async configuration, Pc Port (and DTE Port in Async-Async) must be set to same framing of the on-air data or you get gibberish. For what concerns the data rates,
Async-Async: ports speed can be equal or greater than the on-air speed
Sync-Async: PC port speed must match the on-air speed
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| Fig. 1b |
For what concerns the data, the Harris modem provide (un-framed) 8-bit ASCII output, to get ITA2 or 6-7 bits coded data you will need to run a terminal software that provides binary streams, since dumb terminals as Putty will only display ASCII characters. I tried several softwares (Termite, Realterm, Aspmon,...) and in my opinion the best solution is the "Br@y Terminal" [2].
In Figures 2,3 you may see the same transmission as appear after a software modem (Sorcerer) and as sent by the 5710A modem: note the presence of the start/stop bits in the Sorcerer output. In case of non-ASCII data, the original binary stream can be obtained by processing the modem output with a bit editor and removing the extra-bits added (3 bits in case of ITA2 data, as in Figure 7)
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| Fig. 2 |
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| Fig. 3 |
I tested the modem using several waveforms, here I report only the tests of STANAG-4285 and 188-110A Serial to confirm what said above about the settings of the serial ports, more precisely:
STANAG-4285 600bps L, ITA2 Async transmission 5N1 (NSS CARBs, NATO from S.Rosa-Rome)
STANAG-4285 600bps L, ITA2 Async transmission 5N1 (NSS CARBs, NATO from S.Rosa-Rome)
188-110A Serial 1200bps L, ASCII Async transmission 8N1 (test file from DMT)
Figure 4 shows the case depicted in a): DTE Port and PC Port set to Async, data and ports use the same 5N1 framing, on-air data rate is 600 bps while both the ports are set to 2400 bps (four times the on-air speed). Figure 5 shows the same configuration (Async-Async) but in case of a 188-110A Serial waveform.
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| Fig. 4 - decoding STANAG-4285 in Async-Async mode |
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| Fig. 5 - decoding 188-110A Serial in Async-Async mode |
Figure 6 shows the case depicted in b): DTE Port set to Sync and PC Port set to Async. The framing settings are the same but in this case the PC Port speed must match the on-air speed (DTE Port is set to Sync!). This is why this configuration (Sync-Async) can handle only fixed data rate waveforms. Fig. 6 is related to 4285 tests, the same Sync-Async test was made using 188-110A Serial getting the same results.
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| Fig. 6 - decoding STANAG-4285 in Sync-Async mode |
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| Fig. 7 - 8-bit stream as received from the 5710A Note the 5-bit ITA2 stream after the extra-bits removal |
For Sync intercepts, an hardware SYNC <=> ASYNC adapter or a synchrounus DB-25 RS-232 interface are needed (both costly). The adapter needs to support all the one-air Sync mode data rates as does the PC Port which must be set to the same. On my side, I'm waiting for a synchronous interface to complete the Sync-Sync tests.
HW modems are NOT friendly as are software solutions, you will need to know how to deal with what is coming out as to its meaning.
[1] https://tsc-60.cellmail.com/tsc-60/modem/Harris.modem.5710a.pdf
[2] https://sites.google.com/site/terminalbpp/
[1] https://tsc-60.cellmail.com/tsc-60/modem/Harris.modem.5710a.pdf
[2] https://sites.google.com/site/terminalbpp/