[ih] Arpanet physical connectors

[Steve Crocker]

Much of this has been covered already in responses to your note, but we can
add a little more.  The connectors were definitely NOT RS-232.  The BBN
team designed the connector.  I believe they wanted it to be as simple as
possible.  It was bit serial, operating at 100 kilobits per second.  The
lines connecting the IMPs operated at 50 kilobits per second, so 100 kb/s
was a reasonable fit.  Making it run much faster wouldn't have made a
noticeable difference in the overall performance.  The interface was
designed to operate up to 50 feet from the host.  A different interface was
designed later to operate up to, I think, 1000 feet.  These were referred
to as the Local Host (LH) and Distant Host (DH)  interface.  A third
version was designed yet later to operate over unlimited distance.  It was
called the Very Distant Host (CDH) interface.

I believe Severo Ornstein and Ben Barker were the key hardware people at
BBN.  Mike Wingfield at UCLA built the first host interface for our Sigma
7.  All are copied on this message.  BBN Report 1822 has the details, as
reported in other messages.

As noted, each site had to figure out how to connect the IMP into its
host.  This required a separate design and implementation at each of the
initial sites, and thus it was indeed important to have competent EE people
involved.  After a while, ACC and DEC and perhaps others started to make
interfaces available for various hosts.

Steve


On Thu, Jul 23, 2020 at 8:02 AM Stephane Bortzmeyer via Internet-history <
internet-history at elists.isoc.org> wrote:

> https://twitter.com/nielstenoever/status/1286254151874293760
>
> Dear Internet History nerds, what did the connectors and cables of the
> ARPAnet look like?
>
> Were these serial cables? Seems likely because the RS-232 standard dates
> back to 1960.
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