DEC bus transceivers

Jon Elson elson at
Tue Oct 25 20:58:47 CDT 2016

On 10/25/2016 03:36 PM, Eric Smith 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.
> 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 believe this is a holdover from the PDP-8 days.  I do not 
know of any PDP-11 system that would actually tolerate a 
Unibus peripheral being powered down in the middle of the 
bus.  I am pretty sure that due to the bus grant logic 
present on nearly every peripheral controller, that powering 
it down would jam the bus.
So, I suspect that for a PDP-11 or VAX Unibus system, this 
requirement does not really make much sense.


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