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Old 1st Oct 2019, 2:56 pm   #1
JohnBHanson
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Default GPO Modems

I remember a long while ago the GPO supplied (rented?) modems. One was a filing top unit and another was attached to the base of a telephone much like the planset.

Does anyone have any data on there, circuit/connection diagrams etc or even photos. I believe one may have the name datel on it.

technical information would be most welcome?

Many thanks.
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Old 1st Oct 2019, 3:27 pm   #2
Dave Moll
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Default Re: GPO Modems

I believe the Modem 13B is an example of the "planset" type of modem, and is indeed marketed under the name of Datel.
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Old 1st Oct 2019, 6:13 pm   #3
emeritus
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Default Re: GPO Modems

That reminds me that the modem we had at Plessey in 1969 to access a timeshare computer (GEIS) via a Teletype terminal was one where you had to put the phone handset in a cradle. I don't recall if it was a special phone or how it was used. Because we had had problems with the clicks that were generated when the site operator listened in corrupting our data, it was given a direct line, making it popular with engineers who wanted private conversations with girlfriends.
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Old 1st Oct 2019, 6:45 pm   #4
Dave Moll
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Default Re: GPO Modems

That sounds like a description of an acoustic coupler, where the audio tones are transmitted through the handset. I remember using one when I worked for British Leyland, and recall that 300 baud was about the limit of its capability. It was designed to fit any 700 series telephone handset.

I have one in my collection, and will post a picture of it in due course.
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Old 1st Oct 2019, 7:13 pm   #5
TonyDuell
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Default Re: GPO Modems

Ones I need to get round to figuring out (I have the units -- too many projects I guess)

Modem 2B (large box with plug-in modules -- power supply, modulator, demodulator, control, all discrete transistors and relays) 300 baud originate and answere

Modem 13B (Plinth under a 700 telephone, also 300 baud originate/answer)

D1200A. May not be GPO, but in the same case as the Modem 2B with similar modules This is the host-end modem for a prestel connections (75 baud receive, 1200 baud transmit).

And one I have done, in that I have reverse-engineered the circuit diagram and understand how it should work :

Modem 13A : Plinth under a Telephone 740, 300 baud originate only.

I would be happy to send said hand-drawn reverse-engineered diagram to anyone who wants it.
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Old 1st Oct 2019, 7:23 pm   #6
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Default Re: GPO Modems

Yes please that would be wonderful if you could post the diagrams. Well done.

Thanks.
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Old 1st Oct 2019, 7:24 pm   #7
Dave Moll
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Default Re: GPO Modems

And here is my acoustic modem:
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Old 1st Oct 2019, 7:39 pm   #8
TonyDuell
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Default Re: GPO Modems

The diagrams of the Modem13A are too large to post here, if you PM me with your e-mail address I can sent them that way.

Dave Moll : I obtained an acoustic modem looking like that a few months back. Probably the same model. There's a backplane inside with 5 (IIRC) PCBs plugged into it. Again I reverse-engineered it, so if you want the circuit PM me with your e-mail address. There are some photos of mine (in bits) in my flickr account (tony_duell)

The attached photo is some rather more modern modems (but still on-topic here I think) and is proof that some people will collect just about anything...
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Old 1st Oct 2019, 10:29 pm   #9
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Default Re: GPO Modems

AFAIR the acoustic coupler used at Plessey had a plastic case in that sort of beige colour used for ordinary office phones. After having dialled the number, you listened for the presence of the noise that cane back when a connection was being established and then put the handset in the cradle so you could log on. No cover or anything to hold it there apart form gravity. It was till in use when I left in 1976. No photos unfortunately, cameras not allowed on site due to nature of work. During the paper shortage of the early 1970's, rolls of paper for the teletype's printer became unobtainable, so we used rolls of paper towelling.
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Old 1st Oct 2019, 10:35 pm   #10
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Default Re: GPO Modems

At my college in the mid 1990s there was a modem about the size of a small laptop in a cupboard, with a 4 way jack connector to connect to a phone line
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Old 2nd Oct 2019, 2:24 am   #11
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Default Re: GPO Modems

Acoustic couplers are back...
...In the NHS (I kid you not).

I saw the practice nurse at our surgery and she did my ECG.
The machine produced an audio tone that was frequency modulated with the ECG waveform, which was digitally recorded by the machine, she than carried it over to her desk and plugged in an acoustic coupler, dialled a number and spoke to someone at a cardiology centre, then she (and I presume the person at the other end), placed their handsets into their respective couplers and she replayed the modulated tone, which sounded like SSTV or weather FAX.

My friend had the same a few weeks earlier at a different surgery, I didn't fully believe such quaint technology would be in use today until I experienced it myself.

If it's "KISS", and it works, perhaps I shouldn't knock it!
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Old 2nd Oct 2019, 8:40 am   #12
Dave Moll
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Default Re: GPO Modems

Quote:
Originally Posted by emeritus View Post
No cover or anything to hold it there apart form gravity.
... along with some friction offered by the rubber lining of the cups - which also acts as an acoustic seal to prevent (or at least reduce) interference from outside noises.
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Old 2nd Oct 2019, 7:35 pm   #13
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Default Re: GPO Modems

I remember the "Modem 2B" from a few sites I had to deal with in the very-early-80s. Horrible things that often needed you to make 10 calls before it synced with the remote-end, and then only 300 Baud! Which when you're online for 10 hours a day paying long-distance phone charges ran up some horrid bills!

I was happy to get them slung out and replaced by EPS8/EPS25 circuits with EMI/SE-Labs '9620" modems - giving us permanent 9600-Baud intersite links.

On dialup we also had a bunch of the wood-cased acoustic couplers as mentioned above - from memory they were made by "Andersen Jacobson" - and at one time I was happy to sign-off 2500-a-time for Texas Instruments "Silent-700" data-terminals [Keyboard, thermal printer, optional 1 or 2 cassette drives] - these were used for offline data-preparation, recording the keystrokes on to one of two cassette-drives [the second one allowed for local editing] before you did the dialup-and-acoustic-coupler shimmy to upload your afternoon's efforts to Head Office]

http://www.computinghistory.org.uk/u...duct-90649.jpg
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Old 2nd Oct 2019, 8:16 pm   #14
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Default Re: GPO Modems

Quote:
Originally Posted by JohnBHanson View Post
I remember a long while ago the GPO supplied (rented?) modems. One was a filing top unit and another was attached to the base of a telephone much like the planset.

Does anyone have any data on there, circuit/connection diagrams etc or even photos. I believe one may have the name datel on it.

technical information would be most welcome?

Many thanks.
I've just found a pile of old PO/BT Educational Pamphlets on the early data services and equipment which covers the era and equipment you are looking at. Notes from a long-forgotten training course, I think, during my PO/BT engineering days. The first is "Introduction to data communications", and the other is "BPO 48Kbits/s Datel service". If you want them, PM your snail mail address and I'll post them off.
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Old 2nd Oct 2019, 11:19 pm   #15
TowerRadio
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Default Re: GPO Modems

I found 2 pics Googling " Datel Modem 1979 to 1982" Les
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Old 4th Oct 2019, 11:24 am   #16
TowerRadio
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Default Re: GPO Modems

After a lot of searching I found a shot of the old async modems that every bank was using from the late 60's onwards. I can't remember the model (7b?) but on a dedicated line it ran at 1200 Baud and dial-up 600 Baud. (Bletchley museum treasures vintage tech.) Les
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Old 5th Oct 2019, 11:29 am   #17
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Default Re: GPO Modems

Quote:
Originally Posted by Dave Moll View Post
And here is my acoustic modem:
This is the type that we had at school in the computer lab (we didn't have a computer, just a terminal and teletype machine linked to the local college. We learned to program in CESIL (Computer Education in Schools Instructional Language)
Oh what fun.
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Old 16th Oct 2019, 10:59 am   #18
Dave Moll
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Default Re: GPO Modems

Here, by the way, is a Modem 13A that just happened to turn up for sale online at the time the current thread was started - the purchase of which prompted me to start this thread.
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Old 16th Oct 2019, 11:02 am   #19
Dave Moll
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Default Re: GPO Modems

By the way, in post no. 7, I made an error when setting up the acoustic modem to photograph it. Why wouldn't it work like that?
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Old 16th Oct 2019, 3:48 pm   #20
TonyDuell
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Default Re: GPO Modems

Handset the wrong way round? (so the earpiece is over the modem's speaker and the mouthpiece over the modem's microphone)
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