6 Jan
2012
6 Jan
'12
11:59 a.m.
Your description's a little vague, but I suspect
something like a
74156 with a pullup on the (combined) outputs. If that's not right,
a better explanation of the problem might help.
OK, her;'es the problem specified more completely :
A large, complex PCB. It had soem high-speed analogue stuff on it, and
thus it had to be a PCB. In fact it had to be a 16 layer PCB with ground
planes either side of the analogue tracks. Said PCB could not be made
in-house, the turnaroudn time was significant. And it was expesnive,
meaning we wanted it to work first time.
The deisgn was almost complete. Oen bit that hadn't been done -- I think
it was the last bit -- was a digital signal which was a function -- an
unknown (at hthis time) function -- of 3 other signals. We didn't want to
haev a wirewrap area or anything like that to add ICs later.
The otuptu signal was an enable to another device,it wasn't a clock, so
glitches didn't matter too much.
my solution, as others have suggested, was to use a 74F151 multiplexer.
Select inputs fA, B, C) to the 3 input signals, multiplexer output to the
signal I was tryign to produce. The inputs to the mux were, IIRC, pulled
high by 1k reissotors (one of those SIL packs) with links to ground to
configure it. I think the prototype had a pin header and push-on links,
the 'production ones had links soldered i nthe right positions.
-tony