DEC bus transceivers

allison ajp166 at
Tue Oct 25 04:58:59 CDT 2016

On 10/25/2016 02:35 AM, ben wrote:
> 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)

True, you run the bus at 1.3/1.7 and see how far you go without errors.
Those are limits.  Most systems I've played with if you get over 1V/low
and below 2V/high things tend to be a bit flakey.  

Also TTL switches at 1.7ish and anyone using a 74ls74 on the bus should
be shot!
Look at 74LS240 or 241 as a better example for an bus to board receiver.
For driving the bus look at 74ls38 those are more typical.

Look at a machine that's running well and tends to stay that way and
you see more like .6-.8/low and over 2.4 high.

>> 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.
Some of the recovered and restored system are big.



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