DEC bus transceivers

[ben]

On 10/24/2016 2:18 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.
>
But who has the big systems now days? The days of 4K core is long gone.
Use TTL and try to keep the systems small.
Ben.