RTL Logic

the 100ua only applies to the 74L family and it only represents the high logic level current that it will supply into other logic inputs. One of the key design specs for TTL was the input current to guarantee a specific logic level. For normal TTL, TI specs call for -1.6ma for a logic low and 40ua for a logic high (these are input currents). In other words, ten standard TTL loads for both low and high logic levels. Ten was the usual output capability of TTL outputs. Short circuit current is spec'd too and at a minimum, is 18ma for a TTL output that is high. You can find the standard logic currents on page 1076 of the 2nd edition on bitsavers and the short circuit current on the same page. Of note, the 18ma was only the minimum short circuit current, TI spec's 55ma for the maximum. It also states that you should not short more than one output at a time. Frankly, as I said before, shorting anything for any amount of time is a great way to create noise transients. I have tried to never recall anything from memory anymore on this list because there is always someone who disagrees... best regards, Steve Thatcher -----Original Message----- From: "Dwight K. Elvey" <dwight.elvey(a)amd.com> Sent: Jan 10, 2005 3:46 PM To: cctalk(a)classiccmp.org Subject: Re: RTL Logic Hi The 16ma is for sink of the NPN to ground. The pullup is limited to around 100ua as I recall. Dwight current for 74xx series is 18ma which is also not within specs for the general output current capability of 16 ma. Transient shorts like this are a great way to generate power supply noise. I guess I never applied any type of "preliminary hackery" to systems I have developed.