> I received a pdf from one of the others on the list. The pdf shows
>that the 861 is not exactly like the 7420, in that the unused pins
>of the 7420 are for gate expansion on the 861. The circuit that uses the
>part, I think is failing, doesn't use these pins. The biggest potential
>problem I see is that the original designer used two outputs tied
>together. This means that the other output has to pull it down
>as well. I tried a 74LS20 and it didn't work but I think it is
>just that the 74LS20's have a strong pull-up. I'll try to locate
>a 7420 and give it a try. I've also found a part supplier that
No No NO! TTL aka 7420 has an active pullup and one output
will fight another. Those parts are RTL/DTL and the ouputs
are either open collector (is there a pullup resistor?) or have
an internal Pullup. So if you going to try TTL use parts that
are open collector like 7401 or 7403...
Allison
>stocks old DTL and even RTL parts. I haven't contacted them
>yet but if the 7420 doesn't work I'll give them a try. They
>have these in the Motorola part numbers. For others looking for old
>parts, they are:
>
>http://lansdale.com/~lansdale/index.html
>
> I'm sure these will be expensive but then what the heck.
>Thanks for the reply
>Dwight
>
>
Received on Tue Sep 05 2000 - 17:47:29 BST