7 Jan
2012
7 Jan
'12
8:21 p.m.
On 6 Jan 2012 at 19:59, Tony Duell wrote:
There is nothing wrong with obvious solutions (if they work).
OI can think of several disadvantages to the '156 method :
The output line, driving other circuitry, is an open-colelctoer signal
with a psasive pull-up reissotr. It may not rise as sharply as a
totem-pole output, particualrly if it's driving a capactivie load
The jumper links are carrying high-seped logic signals. If you use the
'151, the configuration links are 'static'
Does the 74F156 exist? I've neer come across it. There aren't that many
open-colelctor functions i nthe mroe recent families. Yes, the 74S156
existed, and would hae been fast ewnough, but I think it was discontinued
by the time we were doing this design. And I certainly wouldn't recomend
a device that was out-of-production. The 74F151 ws in all the catalogues
at the time.
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.
Same idea, but inside-out. Uses an OC 3-to-8 demux rather than a
multiplexer. One pullup instead of a bunch. I thought the mux
solution too obvious.
AAR, not very challenging. Can you do a similar thing when the
I will reserve judgement on that....
inputs were of dissimilar logic families (you
mentioned ECL) with a
single IC?
Not that I am aware of. I suppose you're goign to tell me that there;s an
FPGA with configurable I/O cells that does TTL, 3.3V CMOS, ECL, etc.
Actually, I've never seen one, but,,..
But that wasn't part of the problem Had one of the signals been ECL, I'd
have specified adding a 10124 or 10125 as appropriate. The problem wasn't
to get it down to one chip. It was to come up with somethign that could
be put on the PCB, so that the artwork could be finished and the boards
made. If it has been 2 ICs, that would have been fine.
-tony