The motivation behind 4-input NAND in old differential line driver ICs - Page 1
Forum section: electronics / projects

author: lfs2K
author_rank: Newbie
timestamp: June 18, 2025, 09:35:26 pm
content: Hi, I recently needed to come up with a replacement for one of these old ICs, that itself is not the problem, it just got me wondering why these were originally designed like this?The ICs in question are MM88C30, DS8830, MC662 and SN75183 for example, the last one still being produced by TIThe structure inside is not complex nor hard to replicate, it's just odd to me, why would you want 4 inputs NAND'd for driving a differential line?Is this just a case of saving a few logic gates from a few designs back in the 70s and 80s? If so then why the popularity, these were available from multiple manufacturers and still are produced to this day?

author: David Hess
author_rank: Super Contributor
timestamp: June 18, 2025, 11:48:07 pm
content: Original TTL has the same thing; the only 50/75 ohm line drivers are the 74S140 dual 4-input NAND and slightly lower power 74128 quad 2-input NOR.If there was some special significance to the 4-input NAND for a transmission line driver, I never found it. Maybe the power requirements are high enough than only 2 drivers could be included in one package, so they had extra pins for more inputs?

author: floobydust
author_rank: Super Contributor
timestamp: June 19, 2025, 12:57:08 am
content: The two or quad input was common, everybody had it, goes back to before 1970. Everyone wanted their IC have an edge, something others didn't have, and to use up the DIP-14 pins. I can't really remember, it might have been for driving core memory clk, strobe, data etc.For the party line: "with types MM78C29 and MM78C30 CMOS line drivers, the inputs and outputs both go to high-impedance states when powered down."

author: pqass
author_rank: Super Contributor
timestamp: June 19, 2025, 01:02:10 am
content: WAG: Given that extra inputs are almost free (just another emitter),to fit in a 14-pin DIP the only possibilities are either: dual 4-input or triple 2-input AND+NAND gate.I can see having a data+enable, 2-input, gate as useful but more than two is just redundant.Circuit designers relying on old, trusted patterns keeping old chips alive. That's why the 555 will never die.It's also why multiple suppliers want to get in on the $$$ action.

author: ejeffrey
author_rank: Super Contributor
timestamp: June 19, 2025, 02:03:32 am
content: There are plenty of reasons to want multi input AND/OR gates, so the question is why pair that with a line driver. My total guess would be address decoders and interrupt requests for mainframes and mini computers. They certainly made extensive use of differential drivers for communication between modules.You can also assume any integrated circuits from this era had military customers. Weapons systems, radar, and vehicular systems were all build with distributed modules communicating over differential interconnects. If a circuit needs to detect a coincidence between multiple inputs and send a signal to a central controller this is the type of chip they would use.And yeah in the days of DIP logic anything that would reduce the number of packages was helpful.