DEC bus transceivers
bfranchuk at jetnet.ab.ca
Tue Oct 25 01:35:39 CDT 2016
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.
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