Curious ex-BT modems

From: Tony Duell <ard_at_p850ug1.demon.co.uk>
Date: Thu Dec 25 13:24:03 2003

>
> Merry Christmas to all!
>
> Obligatory pre-feast Christmas posting...
>
> Hopefully there's some UK telecoms people on the list who can tell me

Well, I'm from the UK, and I fiddle with telecoms. Does that count?

> some more about these. I unearthed a pair of BT modems (look to be from
> the mid 1980s), badged as Datel 4961x's.

RIght...

>
> They're in nice finned metal cases (with plastic end caps; Datel unicase
> 4001X's) - I think I hung onto them intending on using the shells to

The Unicase is, IIRC, a case + power transformer for a single modem
board. You could put various boards into the case, and there were also
racks to hold several boards at once.

> house a pair of amplifiers, having never had any luck getting the modems
> to work.

You may well find they're for private line working. I have something
similar that originated from the CUDN (Cambridge University Data Network)
-- I got them at a radio rally along with some statistical multiplexers
and a load of mains leads and RS232 leads. Alas the modems were the least
interesting part of that pile.

>
> I was about to scrap them, and opened them up to strip and useful parts
> first. Now I'm curious.
>
> There's a *lot* of silicon inside these things. Most of the larger chips
> appear to be custom, badged in a range from MS2006 to MS2011. One of the
> 40 pin chips carries the marking "BTRL / 403 / 5113". British Telecom
> Research Labs maybe? There are three OKI 40 pin chips too, several ROMs,
> and no less than 42 DIP switches internally.
>
> There are six fuses inside - they didn't hold back there.

At one time (before these modems), you had to have a 'line isolator'
between your terminal/computer and the (rented (from the GPO)) modem.
This was a box containing 48 zener diodes (inverse series pairs from each
RS232 pin to protective ground on pin 1) and 24 low-value fuses. The idea
was to protect the momen and/or the telephone network if something failed
in your terminal,

For some reason these were 'required' on other connections. I have one
that was cabled up in the GPIB link from a PERQ to the Versatec V80
printer. I have no idea why it's needed -- the manuals make no mention of
it, but it came with the printer...

>
> The rear of the case has a 25 pin DTE socket, a telephone pass-through
> socket, PSTN socket for connection to the network - and also a
> mysterious "PC" socket (same type as used for the phone network
> connection). Any idea what that's for? ("Private Circuit" maybe)
>
> The front has push-switches for 4800 baud operation, plus AL, ST, RDL
> and DL. I assume ST is self-test, but I don't know what the others are.

AL = Analogue Loopback
DL = Digital Loopback
RDL = Remote Digital Loopback

I can't remember the exact definitions of each, but they're all various
test modes.

> There are status LEDs for 4800 operation, DSR, RFS, TD, CD, RD and test.
> Those all make sense.
>
> I remember I never got one of these to talk to a remote modem of any
> type. I'm wondering if they're supposed to be used in pairs (or talk to

Yes, IIRC, they were designed to talk in pairs. I don't think they used
any standard protocol.

-tony
Received on Thu Dec 25 2003 - 13:24:03 GMT