DEC bus transceivers

[Paul Koning]

> On Oct 25, 2016, at 4:36 PM, Eric Smith <spacewar at gmail.com> wrote:
> 
> On Tue, Oct 25, 2016 at 2:09 PM, Paul Koning <paulkoning at comcast.net> 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.

True.

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.

	paul