[COFF] DDP-516 hardware (was: 516-TSS Documents)

[Greg 'groggy' Lehey]

[Redirecting to COFF]

On Monday, 23 November 2020 at  8:42:34 -0500, Noel Chiappa wrote:
>> On Mon, Nov 23, 2020 at 12:28 PM Erik E. Fair <fair-tuhs at netbsd.org> wrote:
>
>> The Honeywell DDP-516 was the computer (running specialized software
>> written by Bolt, Bernanek & Newman (BBN)) which was the initial model of
>> the ARPANET Interface Message Processors (IMP).
>
> The IMPs had a lot of custom interface hardware; sui generis serial
> interlocked host interfaces (so-called 1822), and also the high-speed modem
> interfaces. I think there was also a watchdog time, IIRC (this is all from
> memory, but the ARPANET papers from JCC cover it all).

I worked with a DDP-516 at DFVLR 46 years ago.  My understanding was
that the standard equipment included two different channel interfaces.
One, the DMC (Direct Multiplexer Control, I think) proved to be just
what I needed for my program, a relatively simple tape copy program.
The input tape was analogue, unbuffered, and couldn't be stopped, so
it was imperative to accept all data as it came in from the ADC.

But the program didn't work.  According to the docco, the DMC should
have reset when the transfer was complete (maybe depending on
configuration parameters), but it didn't.  We called in Honeywell
support, who scratched their heads and went away, only to come back
later and say that it couldn't be fixed.

I worked around the problem in software by continually checking the
transfer count and restarting when the count reached 0.  So the
program worked, but I was left wondering whether this was a design
problem or a support failure.  Has anybody else worked with this
feature?

Greg
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