Logic family question

Archiver

Thanks to all for the help on the SMT soldering..

Another question..my circuit uses a 7474 as a pulse train
conditioner, feeding into a stamp input pin. The prototype used
74HCT74, and worked fine. I built some more boards with 74AHC74
(supposedly a direct replacement per TI) and the outut pulse train
was erratic (dropping random pulses). The circuit uses the flipflop
to the divide incoming pulse train by two. The incoming pulse train
is clean (at least on my Optascope). In investigationg this I tried
to find info on the input characteristics of the Stamp and was unable
to locate any specifics. So...

What logic families are appropriate to use as inputs to Stamps

and

Anybody have any idea why AHC won't work and HCT will?

Thanks in advance again

Archiver

These two chips do not appear to be an exact replacement although
their function is the same. Compare the following links to see what
I mean:

http://focus.ti.com/docs/prod/productfolder.jhtml?
genericPartNumber=SN74HCT74

http://focus.ti.com/docs/prod/productfolder.jhtml?
genericPartNumber=SN74AHC74

The HCT accepts a TTL level input signal while the AHC accepts a CMOS
level input signal. There are also some other difference in Vcc
ranges and so forth. As far as the Stamp is concerned, I believe it
would be considered a TTL level input.

Ron

--- In basicstamps@yahoogroups.com, "diacon1208" <dpotter@d...> wrote:
> Thanks to all for the help on the SMT soldering..
>
> Another question..my circuit uses a 7474 as a pulse train
> conditioner, feeding into a stamp input pin. The prototype used
> 74HCT74, and worked fine. I built some more boards with 74AHC74
> (supposedly a direct replacement per TI) and the outut pulse train
> was erratic (dropping random pulses). The circuit uses the flipflop
> to the divide incoming pulse train by two. The incoming pulse train
> is clean (at least on my Optascope). In investigationg this I tried
> to find info on the input characteristics of the Stamp and was
unable
> to locate any specifics. So...
>
> What logic families are appropriate to use as inputs to Stamps
>
> and
>
> Anybody have any idea why AHC won't work and HCT will?
>
> Thanks in advance again

Archiver

Yup, 7474, 74LS74, and 74HCT74 should be compatible
with each other. They provide 'TTL' logic levels,
which are 0 to 0.4 V is a '0', 2.4 to 5V is a '1',
space in-between is a 'transition area'
(don't sit there). TTL also has some current
driving capability, which means you can connect
multiple receivers to one transmitter.
The BS2 is TTL compatible.

'Pure' CMOS is much more flexible about the
voltages it uses, and uses much less current,
and much less power. However, it can't
reliably drive 'pure' TTL circuits.

The 'HCT' is internally CMOS, but has a
TTL interface, which gives the benefits of
both worlds.

--- In basicstamps@yahoogroups.com, "Ron Jeremy" <ronjeremy912@y...>
wrote:
> These two chips do not appear to be an exact replacement although
> their function is the same. Compare the following links to see
what
> I mean:
>
> http://focus.ti.com/docs/prod/productfolder.jhtml?
> genericPartNumber=SN74HCT74
>
> http://focus.ti.com/docs/prod/productfolder.jhtml?
> genericPartNumber=SN74AHC74
>
> The HCT accepts a TTL level input signal while the AHC accepts a
CMOS
> level input signal. There are also some other difference in Vcc
> ranges and so forth. As far as the Stamp is concerned, I believe
it
> would be considered a TTL level input.
>
> Ron
>
> --- In basicstamps@yahoogroups.com, "diacon1208" <dpotter@d...>
wrote:
> > Thanks to all for the help on the SMT soldering..
> >
> > Another question..my circuit uses a 7474 as a pulse train
> > conditioner, feeding into a stamp input pin. The prototype used
> > 74HCT74, and worked fine. I built some more boards with 74AHC74
> > (supposedly a direct replacement per TI) and the outut pulse
train
> > was erratic (dropping random pulses). The circuit uses the
flipflop
> > to the divide incoming pulse train by two. The incoming pulse
train
> > is clean (at least on my Optascope). In investigationg this I
tried
> > to find info on the input characteristics of the Stamp and was
> unable
> > to locate any specifics. So...
> >
> > What logic families are appropriate to use as inputs to Stamps
> >
> > and
> >
> > Anybody have any idea why AHC won't work and HCT will?
> >
> > Thanks in advance again