10 Feb
2003
10 Feb
'03
9:56 a.m.
I am looking at a chip that is causing me confusing
with regards to its
actual date code.
The chip is labeled thusly:
(logo) /718
DM74157N
9322
(logo) looks something like this: /\/
/\/
...which probably makes it National Semiconductor.
Yes.
My first instinct is to guess that its date code is 9322. However, it
My first guess to... However, 9322 is also a Fairchild TTL number, IIRC,
a quad 2 input mux (which is what the '157 is).
does not fit within the context of the rest of the
board, which is 1976.
They are definitely not original, as the board calls for a 74257 but the
74157 has been put in its place with the addition of a capacitor across +5
and ground.
Hmm.. the '157 and '257 are both quad 2 input mux chips with the same
pinout. The difference is that the '257 has 3-state outputs, the '157
doesn't. The pin (15) that handles output enable on the '257 (forces all
outputs to the high impedance state), just forces all outputs low on the
'157.
If this pin is tied to ground then the 2 chips are equivalent. What
worries me is that in 1976 thw '257 was a lot less common than the '157,
so it wouldn't have been specified unless the 3-state capability was
used.
Of course it was common to find '157s with the enable tied low (asserted)
(most of the time being able to force the outputs low was not that
useful), in which case a '257 would replace them. But in my experience if
a '257 was specified, there was a reason for it.
Is it possible that the date code is actually "/718", and what I am seeing
as a slash is actually what remains of a '7' that wasn't printed
correctly?
May be just 718, meaning 1977, week 18 or something.
-tony