25 Oct
2016
25 Oct
'16
4:37 p.m.
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:
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.
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. 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