6 March 2018

some tests with Harris RF-5710A modem

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.

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 asynchronous (unknown) 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 the 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

Fig. 1b

The modem provides (un-framed) 8-bit ASCII output so to get ITA2 or 6-7 bits coded data you will need to run a terminal software that provides a binary stream, since dumb terminals as Putty will only display ASCII characters, and process it with a bit editor. 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, as said above, 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)

Fig. 2
Fig. 3
I tested the modem using several waveforms, here I report only the tests of STANAG-4285, 188-110A Serial and FSK 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)
* 188-110A Serial 1200bps L, ASCII Async transmission 8N1 (test file from DMT) 
* FSK 50Bd/450, ITA2 Async transmission 5N1 (DDH9)

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.

Fig. 4 - decoding STANAG-4285 in Async-Async mode
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 also done using 188-110A Serial getting the same results.

Fig. 6 - decoding STANAG-4285 in Sync-Async mode
Fig. 7 - 8-bit stream as received from the 5710A
Note the 5-bit ITA2 stream after the extra-bits removal

FSK test is shown in Figure 8, since the low on-air rate (50Bd)  you must set the DTE Port to Async: for this test I used 600bps while framings is the same as the one on-air data.

Fig. 8 - decoding FSK in Async-Async mode

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/