On 2010 Dec 18, at 9:29 AM, Shoppa, Tim wrote: 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. 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).