[GTALUG] ancient computer history

James Knott james.knott at jknott.net
Tue Sep 18 19:26:46 EDT 2018


On 09/18/2018 07:11 PM, D. Hugh Redelmeier via talk wrote:
> The PDP-8i was meant to be part of a hybrid computer (analogue +
> digital).  I never used the analogue part (what's an op amp?).

As far as I know, the PDP-8i I worked on didn't have any analog
hardware, though it might have been available as an option.  Many years
ago, analog computers were often used in science and engineering, as
digital computers weren't fast enough.  Of course an analog computer was
useless for business use, where you need accurate results and not just
so much precision.  Op amps are high performance amplifiers, with
characteristics primarily controlled by components in the feed back
loop.  For example, it possible to determine exponents or logs, just by
adding a diode to the feedback loop, in the appropriate manner.  It's
also easy to create a summing circuit, by using multiple input
resistors.  Op amps are still commonly used in instrumentation, servos,
etc..

Incidentally, those mini-computers, from DEC, Data General, etc. often
had prototyping boards available, which could be used to build any
custom interface that might be needed in a lab, factory, etc..




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