There are several boxes in the chain. PDP-11 to IMP adapter connection was a ribbon cable, and IMP adapter to IMP was bit-serial but multi-conductor.
Everything required a lot of boxes back then. An IMP adapter was about 4U of rack space.
Bit-serial but multi-conductor cable. The 6 pairs, ground and if I count correctly two single-ended status lines make more sense than 30-pair Cisco serial cables or 34-pair HSSI of the 90's…
Re:How the Arpanet Protocols Worked
by DonHopkins on Thursday December 05, 2024 @01:25PM
(#42330921)
You mean with luxurious word sizes! ;)
The physical layer as described in report 1822 is bit-serial, but uses synchronous clocking scheme without fixed baud-rate reminiscent of parallel SCSI's REQ/ACK. It is a weird scheme for serial links, but it has the clear advantage that it adapts to cable length automatically (but runs at significantly smaller bitrate that could be supported by the cable, which probably was not exactly an issue in 70's).
In fact the whole report 1822 deals mostly in bits (the messages on the IMP-host interface can have arbitrary bit lengths). Obviously this was done in order to support architectures with weird word sizes (eg. 36bit PDP-10).