2019-02-11 The College IT Datacomm System

from FB group Bell Telephone…


After the early ’80s when the RBOCs created the MPOE, all their wiring on campus became ours, and we never got any records from them of what was in the ground. On top of that we had a contractor come and install gel filled telephone cable along with ITV RG-6 coax in conduits separate from the telecom. This was just for datacomm. We ran RS-232 signals over 1200 feet of hundreds of twisted pairs, up to 2400bps, and 9600 bps with line driver boxes, multidropped. I was responsible for maintaining and upgrading all that, while AT&T took care of telecom and Pac Bell did the KSUs and keysets. I didn’t inherit the phone system until later, 1991. 😎

3 Responses

  1. Nick says:


    I am quite curious about these RS-232 transmission lines. It would be super helpful for long sensor data transmission lines.

    Anyway, do I understand correctly that you built this system? And that the specs were RS-232 over 1200 feet with 0.3 KBps (unpowered) and 1.2 KBps (powered, with multiple signal boosters across the line)?

    Would you be willing to explain how or what kind of theory you need to apply to make the “line driver boxes” and how often you need to place them per foot or whatever? I am considering running lines like this over CAT cable or POE cable. Still doing research, and knowledge you could impart would be greatly appreciated! Thank you for your time!

    Best regards,

    • admin says:

      The lines were in operation during the time of mainframe computers, before there was datacomm cabling. The speeds in RS-232 serial data are given in kb/s (kilobits per second) not kB/s (kiloBytes per second) because the length of the byte doesn’t include start and 1 or 2 stop bits, which thus can be 9 or 10 bits per byte. So I used bits per second.

      The wiring was just regular telephone wiring. The 2400 bps system was connected directly to the RS-232 connectors on the back of the Lear Siegler ADM-3A terminals, which used the 1488 and 1489 RS-232 chips. (cont’d)

    • admin says:

      The other mainframe system used line driver boxes, the master on the mainframe and multiple slaves, one line driver boxes for each of the terminals at different locations. The speed was 9600 bps and the line driver boxes used line driver and receiver chips that are no longer available and were directly connected to the lines. That made them susceptible to damage by lightning storms and power glitches. I replaced the chips in many of them after a storm. That’s why they are no longer used.

      Today the networking cabling uses transformers at the ends to isolate the lines from the circuitry. If you intend to use long lengths of cable, you can use RS-232 to RS-422 converters/adapters, which can handle hundreds of feet of cable, full duplex. You can use RS-232 to RS-485 adapters if you want to use a single pair, master/slave protocol, single or multiple slaves per line.

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