12 Jun
2003
12 Jun
'03
4:01 a.m.
--- Andreas Freiherr <Andreas.Freiherr(a)Vishay.com> wrote:
:-)
These are certainly harder to get than the 7400s, but it appears to me
that the 7460 is actually used a a kind of delay line: since all inputs
of each gate are tied together, there is no logic function involved, so
maybe you can get away with something completely different.
That is logically consistent with what I recall about the M702 - It
has a delay line similar to what's on the M360 and M310 modules. I
don't know exactly why, but there needs to be some amount of delay
in the memory field detection process... perhaps to let some other
part of the circuit stabilize first.
As far as I can remember from looking at the fiche
yesterday, there are
three SN7400N chips and one SN7460N in the circuit...
Nope... it looks pretty straightforward. All I need are some 7460s. Since I had the fiche looked up already: the only
difference between the
original schematics and Rev. A is C7: it had ".005MFD" originally and
was changed to ".02MFD" in Rev. A. Looks like the one-shot made from E3
needed a longer trigger pulse.
Good to know. Thanks for the tip.
*Now* I need to find a spec sheet for an SN7460 and see what its
propogation delay is. I can probably estimate based on other members
of its family, but in this particular case, the specifics will matter.
Obviously, I won't be substituting a 74HCT part or a 5ns GAL. :-)
-ethan