Wednesday, November 8, 2017

HAL Communications ST-8000A Overview





              This will be the first of a series of blog posts on the HAL ST-8000A.  I have had quite an adventure getting one of these terminal units working in my station and there is very little information on the internet to help with the process.  I hope to provide an overview as well as some hints and tricks I learned, or discovered that helped me along the way.  If someone else wants to use this unit in their station, I hope that they can use this information to make the process more painless than mine was.

              The HAL ST-8000A is a teletype modem that was designed for the military to provide a drop-in replacement to the Frederick Electronic 1280A modem.  It can be used in both radio and hardline applications and converts FSK Teletype at a wide range of baud rates to/from either RS-232 or MIL-188 protocols.  Since it was designed for the US Air Force, it uses US Department of Defense Bendix plugs and is built to military specifications.  This means that it is very well build, EMP hardened, and subsequently very costly.  This was HAL Communication’s last terminal unit that did not have a DSP circuit.

              The flow diagram of the unit is as follows: audio is fed through a balanced line into the unit where it passes through a series of filters.  The filters are automatically selected based on the programed front-end settings.  Filter width varies directly with the selected tones and baud rate.  The unit can handle tones from 300 Hz to 3000 Hz, which will handle most radio SSB bandwidths.  There are a series of low pass filters that cut out noise and then the audio is routed both the I/O port (more on this later), and one of two individual discriminators. The first discriminator is used at baud between 30 and 600 baud, and the second for baud rates from 601 to 1200.  The discriminators decode the audio input and uses a microprocessor to convert it to one of three digital signal outputs: 1) RS-232 data, 2) MIL-188 data, or 3) audio tones (Regeneration).

              On the transmit side, the ST-8000A will take data input (RS-232 or MIL-188) and convert it to AFSK tones which will drive the SSB radio or pass it over hard lines.  This is very similar to how most sound card and many TNC’s are utilized to send RTTY.  The unfortunately part of this is that you cannot use the FSK mode on the radio which is disadvantage on many radios due to the filter limitations while in SSB mode instead of RTTY mode.

              Using the ST-8000A as designed will require the appropriate software to decode the RS-232 or MIL-188 data streams in to text.  Given the age of these units, the terminal programs will be difficult to find, and will need to use older operating system emulators to run the software.  Add to this the lack of DSP and the use of AFSK on the transmit side, you may wonder why you would bother to use an ST-8000A in the first place.  The answer to this is the front-end filters.

              When people still used the ST-8000A, or it’s civilian cousin the ST-8000, the operators were looking for a way to use the superior decoders on the ST-8000’s while using their favorite software, which often did not interface with the terminal units.  The stop gap measure was to use the “regeneration” feature of the 8000’s.  This had the 8000 decode the audio with it’s superior filters and discriminators, and then convert them to pure, high S/N ratio audio signals to be converted to data by a secondary TNC which fed data to the software.  Any TNC was able handle the high-quality signals from the regenerator, and then the secondary TNC could be wired to send FSK signals to the rig, overcoming the second issue of the 8000’s.

              With the utilization of DSP, this stop-gap use of terminal units was rendered obsolete.  While some newer TNC’s use DSP, it was pointless to input regenerated tones into them, as the regenerated audio did not need further processing.  In standard soundcard interfaces, software was able to take audio directly from the radio and DSP algorithms were superior to the discriminators to the 8000’s.  Some software and interfaces could even overcome the FSK keying issue and software took over from the terminal units in most applications.  The use of external TNC’s and expensive terminal units essentially stopped.

              My idea in getting the 8000A was to see if I could use the TU as an audio filter and then feed the processed audio into a DSP TNC to take advantage of the filtering of the ST-8000A and then use a secondary TNC to use DSP on the signals, and decode them.  This also allows the secondary TNC to use FSK keying.  This plan could also have audio sent to a sound card interface and so the same thing, but onboard the computer.  This was the challenge that the internet had not seemed to meet as I rooted around for information on the HAL ST-8000A.

              The 8000A was never as popular as the 8000.  A few reasons for this are the fact that the 8000 used standard 25 and 9 pin serial connectors which were easily and cheaply available.  The 8000A uses Bendix 37-pin plugs that are harder to find, and far more expensive.  The other big issue for hams was the lack of dials and the replacement of an analogue tuning oscilloscope with the less popular MARK/SPACE LED bars.  The fact that the Mil-spec 8000A was also considerably more expensive sealed the deal.  Today you can find 8000A’s inexpensively, but the 8000’s are quite rare.  While the interface issue is a matter of preference, there is a way to pull filter audio out of the 8000A and feed it into an oscilloscope for a similar display.  The plugs, while expensive, can be bypassed and refitted because the jumpers between the plugs and the board go into a Molex plug.  Just be sure if you go this way, to ground all of the unused pins or you will get errors. 

                I was able to find a ST-8000A online for a price that I could justify, and had it shipped to me.  The unit had never been wired up, so all the original accessories including the Bendix plugs and pins were included.  Unfortunately, the instruction manual was not in the box.  Fortunately for me, the customer service at HAL is excellent and I had a pdf version of the manual in my Dropbox account which was worth it’s weight in gold.  Now that I had the ST-8000A, I had to research to see if it could do what I wanted it to do, and if it could, how I needed to set it up.  Once I had the ST-8000A in my shack, the real adventure could begin.

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