21 Dec
2006
21 Dec
'06
11:12 a.m.
Tony Duell wrote:
A word of warning. Screw down the front panel before moving the thing. If
you don't, and you tilt it the right (wrong?) way, the PSU module will
push the panel open and fall out, If it hits you on the foot it's
painful. If it doesn't, it can land in such a way that the frame bends
and cracks the SRBP PCB. Please don't ask how I found that out!
I can't help with dates, but I do have a few
old ex-GPO modems in the
collection.
The Modem 2B is a large box (about 16" square and 7" high from memory.
It's 300 baud, original and answer. The front panel opens by undoing 2
captice screws at the bottom and then hinging it up (there's a little
'stay' to keep it open). Insider are 4 plug-in modules -- PSU,
Demodulator, Control, Modulator, with testpoints, fuses, and adjustments
on the front panels.
Hmm yes, we have one of those at the museum; it's a wonderful bit of
construction! I've been unable to work out if the 2B was the UK's first foray
into modems or not though - was there ever an official
2A, or maybe a 1B or 1A
that were even earlier? (I'm off over to the museum tomorrow, but I don't
think any of the ex-GPO lot will be in to ask)
There are some really good UK telecomms websites out there, but unfortunately
whilst they cover phones and exchanges and test equipment, none of them seem
to provide any details on the GPO modems...
I was a member of the THG for a short time, but I gave up when it was
obvious that I was the only person interested in datacomms.
One thing I forgot to mention last night was the 'barrier'. This was a
box that connected between the terminal's RS232 port and that on the
modem. It contained a pair of invese-series zener diodes between each of
the 24 pins (2 ... 25) and frame ground (pin 1) and a low-value fuse
(50mA) in series with each line (the zeners being on the modem side of
the fuses). The idea was to protect the modem against any electrical
breakdown in the terminal, and AFAIK was a GPO requiremnt if you rented a
modem.
For some odd reason you're also supposed to use one in the UK if you link
ccertain periperhals, including the Versatec V80, to a PERQ. Yes, I have
some oddly wired cables with GPIB connectors at one end and DB25s at the
other to put one of these barries in that GPIB link.
That barrier is a fairly small metal box, I also have a much larger one
(called a 'Model 7 Modem Selector'), built in a sloping-panel box about
12" * 9" * 9".. Inside it's mostly air, there's a little PCB with
the
fues and zneers on it. and 3 (IIRC) swithcs on the panel to toggle varios
status llines on the DB25 to select original or answer mode.
-tony