TELECOM Digest OnLine - Sorted: Re: Very Early Modems

Re: Very Early Modems

Scott Dorsey (
24 May 2005 11:46:07 -0400

<> wrote:

> In the IBM history series by Pugh et al, they said IBM converted
> punched cards to paper tape for transmission in the 1940s. My guess
> is that that particular transmission used telegraph TTY lines (not
> voice) of either AT&T or Western Union. Recall that AT&T maintained
> telegraph long distance lines as part of carrier long distance
> circuits. Because of the low bandwidth, a telegraph channel could be
> carried on the low end of a carrier channel. Accordingly, no
> modulation was required and thus no modem needed.

> It was also said IBM limited development in this area to avoid
> annoying AT&T who was IBM's best customer.

This makes sense. The Western union systems were basically designed to
take baudot paper tape, 5 bits across, with certain headers at the
beginning and ends of messages. Messages would often be punched on
paper tape at a switching office and batched up for later transmission,
basically the first store-and-forward systems.

> However, in the 1950s, IBM developed card-to-card directly without
> paper tape and "over AT&T lines". Modems were developed to take good
> advtg of the available bandwidth (about 1200 baud). Undoubtedly the
> equipment and implementation was developed in close cooperation with
> AT&T.

This was the IBM "Card-to-card" transceiver. I don't know when they
first came out, but the Army started implementing them in a nationwide
network in September of 1956.

> I was wondering if the modems in that application were supplied by IBM
> (who appears to have developed the technology) or by AT&T. My
> understanding that AT&T's "Dataset" modem-telephones didn't come out
> until the 1960s.

> Comments by anyone familiar with pre-1960 data communications would be
> greatly appreciated.

I believe they used 4-wire leased lines, with data access arrangement
boxes provided by Ma Bell. So the signals going into the big grey box
next to the reader/punch were analogue. I don't recall what the
transmission rate was, but they sent EBCDIC directly without any
translation to a 5-channel code and no added headers.


"C'est un Nagra. C'est suisse, et tres, tres precis."

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