DEC bus transceivers

Paul Koning paulkoning at
Tue Oct 25 18:37:19 CDT 2016

> On Oct 25, 2016, at 4:36 PM, Eric Smith <spacewar at> wrote:
> On Tue, Oct 25, 2016 at 2:09 PM, Paul Koning <paulkoning at> wrote:
>> As for the receiver, it seems that a TI 75140 (adjustable threshold line
>> receiver) might do the job.
> The high-level input current spec on that is max 100uA, which exceeds the
> DEC specification.


I guess a better option might be a voltage comparitor with FET input.  The LT1011 would be an example except that it isn't quite fast enough.  Or a voltage follower before the 75140, at the expense of higher part count.

> One thing everyone seems to be ignoring is that the DEC leakage current
> specifications are for Vcc from 0V to 5.25V. In other words, Unibus devices
> can't load the bus more when they're not powered.  Almost all proposed
> alternatives to the DEC-recommended chips do not meet that requirement.

I don't see any such requirement in the spec.  And it would be odd to have one; I can't think of anyone who would expect a PDP11 to work with part of its devices powered down.  For one thing, if the box with the terminator loses power, you don't just have unpowered I/O cards, you have  no power on the terminator.


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