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. Allison > >
