On Fri, Feb 28, 2020 at 5:12 PM Dennis Boone <drb@msu.edu> wrote:

> using the secondary pins as well.

Best I could tell, the major intended purpose of the secondary circuits
was to implement autodialer, especially when those handshaking signals
on the primary were down because there was no connection, so you
couldn't do anything through them yet.

The autodialer is a separate spec. It's ECMA RS-4xx something IIRC, I forget the number and I'm not near my books. That was the DN-11 from DEC which talked to an AT&T 801 dialer. It used RS-232C electrical signals, but it's actually different than RS-232C (IIRC, Able called them a 'QuadraCall').

The way auto-dialing is spec'ed, is that a single dialer supports N modems. The Out-Band dialing stuff was an invention of Hayes who clearly had not read the AT&T (later ECMA) spec.

Was there anything else in mind during the spec process?

RJE stations.