80-Bus News


November–December 1983, Volume 2, Issue 6

Page 52 of 67

Let’s have a look at some of the things BT want, as I understand it. I’ll only deal with a couple of the sensible things, I’m not sure about some of the things I’ve read, they seem daft, and I’m not sure whether they are serious or just ‘hearsay’.

Firstly, they don’t allow anything to be connected directly across the telephone line unless it’s been approved. Now this is sound sense. Despite all the precautions a home constructor could take, a few are careless. I know, I’ve seen power supplies with no earths, I’ve seen power supplies with the neutral connected to the system ground (I admit I haven’t seen one with the live connected to system ground yet). Now BT aren’t interested in the ways in which you set out to kill yourself, but despite your personal opinions of BT engineers, imagine what would happen to an unsuspecting guy checking the wiring down at the exchange when he finds a very live wire. Live, not because of the red hot conversation taking place, but live because it’s connected to the mains at your end. That’s why any gear approved for direct connection is equipped with an isolating transformer.

Now a second thing that seems a bit odd at first sight is that you’re not allowed to indirectly connect anything to the phone unless it’s been approved. Indirect connection means amongst other things, acoustically. Now I was under the impression that acoustic couplers (home brew or otherwise) were Ok. Not so. The answer is signal levels. In this case BT are worried about excessive signals causing crosstalk between cables and so annoying other subscribers. (They’re probably also a bit leary about excessive levels upsetting some of their tone controlled switching networks, but they don’t say that.) Anyway that’s my guess at the reason they don’t like the relatively cheap kit modem supplied by a large company in Southend.

So it seems that legally you can’t connect anything directly or indirectly to your telephone unless it’s been approved; approval costs a bomb and takes a long time, and in the case of a kit modem, they’d want to approve each one individually before it’s used.

Now about Micronet, it is a Prestel utility and contains a billboard and lots of useful software. But for Micronet you need access to Prestel. Prestel itself is odd in computer terms in that its data rates are peculiar, being 75 BAUD for transmit and 1200 BAUD for receive. Prestel modems are all directly connected, I don’t know of any acoustic modems capable of going at those speeds. Most acoustic modems run bidirectionally at 300 BAUD. However, what is not widely publicised is that Prestel do have a 300 BAUD service on the Kipling computer, although only admittedly in the London area, try __-___-____. This service is primarily for business use, where business computers already have the usual 300 BAUD I/O.

So it seems from the guy at Micronet that Dr. Dark’s Marvin couldn’t be connected to the public telephone network, or could he? Well a new approved acoustic modem could be purchased for about twice the price of a Spectrum and its approved modem; or --- not that I would condone such a thing --- something else could be connected to the telephone network which satisfied the rules. In this instance, if BT actually detected it (indiscriminate phone tapping is still illegal, and a warrant is needed for discriminate phone tapping), it would appear to be an approved device and therefore wouldn’t give rise to questions.

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