80-Bus News


Summer 1985, Volume 4, Issue 2

Page 22 of 31

with enough software to suit all but the most fastidious user. The board is the usual high quality 8" x 8" card, with a fair sprinkling of big chips and some miniature relays as well. This all comes with the documentation, a lead fitted with the ‘new-type’ BT rectangular plugs to connect it to the phone and a disk with enough ‘comms’ software to keep most users quiet for a long time. In fact everything you need to plug it in and go.

The Line Side

Close examination of the board reveals good adherence to the BT ideas of line isolation using both a line transformer (to the more rigid BT 5KV spec.) for line coupling and an opto-isolator for ringing detection. The whole area of the board associated with the phone line part is ringed by a heavy earth track, and a plate through M3 hole is provided for the isolating earth. Tracking is provided for spark gaps, but these are missing, the manual emphasizing that the system is for connection as a secondary device within the BT ‘new-type’ six pole plug and socket scheme. If the MODEM is connected as a secondary, then it is protected by the spark gaps in the primary socket, but with proliferation of diy telephone wiring, there is always the danger of someone doing a short cut job and not using a primary socket for the phone input. This danger is also present if the MODEM were connected direct to the older type BT wiring and the original phone removed. Three fuses are provided, one each on the A and B pair, and a third on the bell suppress line. I’ve never seen one there before, but useful none-the-less.

Having said that the board appears to adhere to BT specs. with regard to the line side of the board, why

the big red triangle saying ‘Not approved for ... etc."?

Well here lies an interesting story, typical of the muddle headed thinking which tends to go with big bureaucracy. A few years ago, with the idea that BT was to go public, HMG considered the business of BT approval. BT could hardly carry out the approvals itself as this could smack of monopoly, and the only one allowed to run monopolies is HMG. So the approval was farmed off to an independent body, BABT, the British Approvals Board for Telecommunications, along with draft specs. from BT. It appears they also had a brief to tighten the specs. a bit and were to be allowed to charge for approval (expenses you know}. So BABT beavered away, and did what it was told and tightened the specs.

Now what follows is hear-say, but sounds rather likely. When BT heard what BABT had done, and this was before BABT got hold of the approval monopoly, BT hastily rushed through a few projects of their own. Could this batch of approvals have included the ‘new-type’ telephone socket, as

by no means would that socket get BABT approval now. For starters, you can shove the British Standard finger into it, and the ringing voltage is 150 volts!! Oh dear, oh dear.

Another bit of spec. tightening involved the situation of MODEM cards. They don’t actually approve a card any more, they approve the card and the system it will live in. So we have the very odd situation where the Gemini GM870 meets the spec., so the card will actually get (or by the time you see this, maybe have got) approval; BUT, only when fitted in a specified Gemini machine by Gemini themselves. Under any other circumstances the card is not approved, and unless you submit your own system, including the card, and fork out quite a bit of loot, the card will never have approval in your system. Even more wierd, if Gemini made the card stand-alone, with its own PSU and serial interface, and put the lot in a pretty box, then it would likely gain approval, despite the fact that you might have the live side of the mains connected to system ground! So stand-alone MODEMs get approval. Built-in MODEM cards don’t.

So what’s the point, why sell an unapproved MODEM? None really, it’s not illegal to sell thern, and it’s up to the conscience of the purchaser who buys and uses it. The red triangle notice only says ‘... action MAY be taken ... ‘ and BT are fully aware of the situation regarding the use of non-approved phones in this country. If someone were mad enough to try and make a stand on the matter, the courts would be tied up from here to kingdom come with petty flouting of BT regulations, and no-one in authority likes to see the law made a fool of, least of all BT. So provided the card does everything it needs to to ensure it’s safety, and provided you have done everything to ensure that your system is electrically safe, and provided the MODEM doesn’t draw attention to itself by acting in an unapproved fashion (which it won’t}, then the chances of being done for using it are vanishingly small. Not that I would encourage anyone to flout BT regulations, it’s up to you.

To finish off the coupling arrangements the line transformer is coupled to the AMD 7910 ‘World Series’ MODEM chip via a terminating pad for output, and via a 741 op-amp with carefully controlled gain to reduce the input on V.21 operation, thereby improving the integrity of the signal. No in-line filtering is incorporated, as the AMD 7910 includes its own line filtering, although I have a sneeking suspicion that the inclusion of a bandpass filter may improve the performance under marginal conditions. That said, none of the other MODEMs in the same price bracket using the AMD chip include bandpass filtering, nor the gain control, so the performance should be at least as good as the competition, and possibly better on V.21 due to the gain controlled amplifier. The

This is an OCR’d version of the scanned page and likely contains recognition errors.

Page 22 of 31