Unknown TI logic series
hilpert at cs.ubc.ca
Sat Dec 18 15:59:25 CST 2010
On 2010 Dec 18, at 9:29 AM, Shoppa, Tim wrote:
>> but the speed, temperature range, etc. requirements for a desktop
>> calculator are not that high.
> I think you (or maybe someone else) may have hit it earlier with the
> suggestion of fan-out/fan-in requirements and maybe some
> specializations such as open-collector wired-or with built-in pullups
> at the receiving end.
Christian was noting the use of presumed OC output devices
(SN1401->7401) with no apparent pull-up R in view. I noted that I have
seen the use of OC with no pull-up R in logic design using another
series of ICs, so it's not unprecedented, and the 1401 may well, as
presumed, be equivalent to the 7401.
We might like to think of no pull-up R as poor design, but one can also
see that a design could get away with it. TTL (and DTL) are
current-sinking logic and the OC output cuts off current to an input
even without pull-up R. The R is there to discharge the stray C of the
junctions and connection, or more accurately it speeds up the discharge
of the stray C. If you don't need the speed the design may get away
without pull-up R.
> Later 7400 series chips for specialized functions (e.g. verge of SSI
> and MSI) had not-really-TTL-compliant inputs and outputs for
> specialized daisy-chain or low-fanin-load purposes. Look at the BI/RBO
> pin of a 7447 etc. (It's an input AND an output! Wow!) It's easy to
> see how this could be applied to some of the "stock" standard logic
> functions to greatly cut pin counts for a desktop calculator, and I
> suspect that's what the unique numbers are for.
That was quite an anomaly in the 7447, but off-hand it is the only IC I
can think of that being done in.
I did some more researching on the other noted oddball-numbered series,
the DTL SN3900/4500. Found the internal schematic for the very common
900/800 series DTL (Tom Jennings' WPS web site) and compared it to that
of the SN3900/4500. They are identical (including a "1.75K" resistor)
except for the value of one resistor (3K/5K) for an internal transistor
base pull-down (the discrepancy could conceivably even be a typo
somewhere along the way).
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