DEC bus transceivers

Don North north at
Tue Oct 25 00:23:24 CDT 2016

On 10/24/2016 1:18 PM, David Bridgham wrote:
> 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.

FYI here is an I/O plot I did of the bus input receiver of an 8641 device (the 
blue line). A threshold of about 1.5V.
Also included is a plot of a 26S10 transceiver (which has a higher input voltage 
threshold) which I think is used
in later designs (SBI? IIRC).

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