Given the little literature I've seen on the
subject, and also the
amount of real-time control computers that I've seen in labs being
dismantled... I'd say that it was probably a late IBM response to PDP-11
and HP 21xx and 1xxx process control computers.
That would be correct - and is even shown in the period advertisements
from IBM. In keeping with the DEC philosophy, the S/1
was a very open
system,and in the mid 1970s, IBM tried to cultivate a sea of third
party vendors, as there was for other minicomputer makers at the time.
Some ventured in, but the third party market really never grew much.
The S/1 was not IBMs first minicomputer by any metric. In process
control, for example, the System/7 predated the Series/I, and the 1800
family before that, and the 1710 before that. A System/360 could also
do realtime process control using something called a 1070, but very
few were used this way.
--
Will