--- Andreas Freiherr <Andreas.Freiherr_at_Vishay.com> wrote:
> >>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.
> :-)
>
> 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.
> 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
Received on Thu Jun 12 2003 - 11:01:03 BST