DEC bus transceivers

David Bridgham dab at
Mon Oct 24 15:18:05 CDT 2016

On 10/24/2016 01:37 PM, allison wrote:

> The voltages are based on TTL levels.  What are the unique voltages?

The QBUS spec from the 1979 Bus Handbook (the Unibus levels are the same):

Input low voltage (maximum): 1.3 V
Input high voltage (minimum): 1.7 V

And from the TI datasheet for the 74LS74:

Vil - low-level input voltage 0.8 V (maximum)
Vih - high-level input voltage 2 V (minimum)

So no, the DEC bus voltage levels are not TTL levels.  Yeah, TTL might
work on a smaller system but you can see that if you push it out to its
limits, TTL could start getting flaky.  That's the kind of bug I'm happy
to have DEC's engineers figure out and not have to track down myself.

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