80-Bus News


Summer 1985 · Volume 4 · Issue 2

Page 23 of 31

transformer is sampled via a high impedance pad and fed to an LM386, which is a small audio amplifier. An 8R speaker output is thus provided to monitor the line. Useful if you keep dialing engaged numbers or keep finding noisy lines. A software on/off switch is provided for the audio output, and the audio level is adjustable from zero to full volume using a preset pot. Maximum audio output was not measured, but was certainly loud enough!

The Computer Hardware

So on to the system side of the hardware, the bit the computer side of the coupling transformer and isolator. It’s an 8″ x 8″ bus card like all the rest, and the height of the coupling transformer and line inductor are such that the card will fit in to a normal Vero frame or the reduced pitch (0.95″) Gemini frame used in the Galaxy series computers. The major interface devices are a CTC, a PIO and a DART. The CTC and the PIO were familiar, the DART was a new one on me, and unfortunately little detail was provided about any of these devices in the manual. Not a problem as we shall see, but unusual, as Gemini manuals tend to go into detail describing unfamiliar devices. The usual I/O buffers were provided, handing the usual bus signals and providing NASIO. More typical of present technology, I/O decoding was provided by a PAL rather than discrete logic, with no options for alternative addressing. The board uses 12 ports, from 80h to 8bh, which overlaps the Belectra Arithmetic card. This I understand is due to a Gemini ‘internal communications failure’ as the decoding was intended to be 84h to 8fh. Not that it matters as the Belectra card may be re-addressed and the dedicated software for the Belectra card may also be re-addressed.

Three of the four counters of the CTC are used for internal clock signals, two for the DART Rx and Tx clocks, and one to control the dialling. The PIO device does most of the card control, and all 16 bits are used. Port A produces a 3 x 4 matrix which is directed to the dialling decoded (effectively it pretends it’s a numeric key pad of a telephone). Port B pulls in the relays as required and selects the MODEM chip mode. The DART I don’t know a lot about, except that it is a dual UART of some kind. Only one side is used, the same register decode is used for status in either direction and another port for data either direction.

Dialling is quite sneaky, it uses a Mostek telephone dialling chip, which is normally used with ‘touchdialling’ phones. Give it a matrix of numbers and it just gets on with it. Board tracking is included for tone dialling to CCITT standards using a second Mostek chip, but this chip and crystal are not provided. This means the MODEM may be equipped for use with UK tone dialling PABX switch boards as well as the normal direct line connect.

The guts of the matter revolves around the MODEM chip, I haven’t got the full words for this device (I borrowed and read the words some time ago) and information in the MODEM manual is sparse (like the CTC, PIO and DART). This chip is the all laughing dancing singing thingy capable of CCITT and BELL protocols. This is all controlled by five address lines, the first two being connected to the PIO, the remainder being returned to patch plugs which are sealed with paint. Now the words say that if you remove and change the links, the card will be forced to work outside BT regulations, which translated means it will start working in the BELL mode. What the manual does not say is what exactly happens when you do swop the plugs about. So for that you need the AMD manual. What’s on offer with the standard setup as supplied:

V.21 300/300originatefull duplex
V.21 300/300answer full duplex
V.23 75/1200originateasymmetrical duplex
V.23 1200/75answer asymmetrical duplex

This is standard for most things found in the UK, and the likelihood of BELL protocols being encountered is remote.

On test a strange anomoly has been observed which must be something to do with the hardware. I wonder if it has occurred elsewhere? I haven’t looked into it yet, but it goes as follows:

Computer mains off but not unplugged from mains
MODEM connected to line (plug in socket) Video recorder on play back in another room Incoming call, phone ringing

Ringing ‘burr’ heard quite loudly from the TV set when an incoming call occurs??? Now, as I say I haven’t looked for it, I just unplug the MODEM when not in use, but I haven’t the foggiest idea what causes it!!

The Manual

A pretty comprehensive manual is supplied which runs to 52 pages, and covers all the major aspects of the card and its software. Details are missing about the I/O devices used, referring you to the respective device manuals. Instead of blow by blow accounts of the devices used, source listings of the setup and I/O procedures are provided. As can be imagined, with a PIO, a CTC and DART to set up, the routines are complicated and tedious. With the source listings provided, they can all be seen and analysed. These are much more useful than accounts of the workings of the chips. The

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