25 Oct
2016
25 Oct
'16
2:58 a.m.
On 10/25/2016 02:35 AM, ben wrote:
Some of the recovered and restored system are big.
Allison
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.