[ih] Arpanet physical connectors

Carsten Bormann cabo at tzi.org
Thu Jul 23 05:47:28 PDT 2020

On 2020-07-23, at 14:02, Stephane Bortzmeyer via Internet-history <internet-history at elists.isoc.org> wrote:
> Dear Internet History nerds, what did the connectors and cables of the ARPAnet look like?

I wasn’t there, but BBN 1822 says (local host):

"The Host cable supplied with the 516 IMP and the Pluribus IMP is 30 feet long and contains 12 RG 174/U coaxial conductors with grounded shields.
Host personnel must provide an appropriate connector for the Host end of the cable."

"The Host cable supplied with the 316 IMP is 30 feet long and contains 32 twisted pairs. The cable is terminated at the IMP end with a paddle card which plugs directly into the 316 Host interface. Each pair of the cable consists of a colored wire and a black wire numbered with the pin number of the paddle card to which the colored wire is connected. All black wires connect to the paddle card signal ground. Host personnel must provide an appropriate connector for the Host end of the cable."

The interface had a four-way handshake per bit (3 lines + a separate end of message signal), and was based on 0V/5V signals (*).  Distant host was differential -6V/6V.

Fortunately, at the time, most people working in this space had the EE degree to be able to work with this :-)

Grüße, Carsten

(*) "All Host-IMP logic signals (Data, Ready-For-Next-Bit, There's-Your-Bit, Last Bit) are unbalanced, source-terminated lines with a nominal characteristic impedance of 68 ohms.  The line is terminated at the driving end with the characteristic impedance. The receiver is ideally an open circuit; in practice,
TTL gate. In this scheme a voltage step of half the nominal level is propagated from source to receiver.  At the receiver, it is reflected by the high impedance termination, resulting in a full level step at the receiver and another half level step propagating back to the source, where it is absorbed by the termination.”

"The IMP will properly receive 5-volt logic signals; however, signals from the IMP may go to 6 volts. Therefore, the Host must provide a voltage divider, if these signals are to be received by normal 5-volt logic, to prevent destruction of the receiving circuit."

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