NMOS Z80A Electrical characteristics? Compatible with 74HC? - Page 1
Forum section: electronics / projects

author: pacmann
author_rank: Contributor
timestamp: May 15, 2024, 09:56:55 am
content: Is NMOS logic like Z80A compatible with CMOS logic levels? Is it still considered TTL?Im wonder if ok to use 74HC chips with a Z80A?

author: Benta
author_rank: Super Contributor
timestamp: May 15, 2024, 10:27:01 am
content: Generally, no.You need to use 74HCT, that's what they were invented for.

author: RoGeorge
author_rank: Super Contributor
timestamp: May 15, 2024, 11:21:41 am
content: In theory you should use 74HCT, but I guess it will work with 74HC, too.T in HCT stands for TTL logic levels, but in practice the HC series will have the 0 logic close to 0V, so HC should work, too, but I didn't try.

author: S. Petrukhin
author_rank: Super Contributor
timestamp: May 15, 2024, 11:26:56 am
content: Quote from: RoGeorge on May 15, 2024, 11:21:41 amIt is no question of 0, but of 1.The tolerance of 74HC to the TTL level of 5V depends on the manufacturer.There is a possibility that there will be a protective diode at the input, through which the 5V will rest against the power bus.

author: retiredfeline
author_rank: Frequent Contributor
timestamp: May 15, 2024, 11:29:11 am
content: Depends on the direction. Z80 was in the era of 74LS so it will have no problems driving 74HC. Going the other way, I seem to remember reading a datasheet that Z80 inputs were designed to be drivable by 74LS so 74HC should be ok. The issue was the high logic level which 74LS can only guarantee 2.4V worst case. But don't take my word for that, check.

author: Gyro
author_rank: Super Contributor
timestamp: May 15, 2024, 11:41:36 am
content: NMOS outputs should swing to 74HC input high threshold, the question is when?NMOS has active pull-down but passive pull-up, that means that the edge will slow down as it nears the high threshold, the actual rate being determined by the load capacitance - particularly important if the Z80 is driving the capacitance of a bus and multiple devices. Using TTL or TTL threshold 74HCT gives a more predictable timing and frees up timing budget that might be needed elsewhere - not such a problem these days with faster memory and peripherals, compared to, say, the 450ns UVEPROMS and SRAMS in the old days, but still good practice.

author: Gerhard_dk4xp
author_rank: Frequent Contributor
timestamp: May 15, 2024, 04:43:23 pm
content: Z80A IO levels were harmless.Only the clock input (Pin 6 iirc) needed at least Vcc - 0.6V high level.We solved that with a PNP active PullUp. A strong resistive pullupwould do, but require more power.Gerhard

author: Benta
author_rank: Super Contributor
timestamp: May 15, 2024, 09:36:27 pm
content: Quote from: retiredfeline on May 15, 2024, 11:29:11 amGaga.The "high" output voltage of the Z80A is defined as 2.4 V min.Far too low for a 74HC part. 74HCT will work.Is it too difficult reading a datasheet?

author: Gerhard_dk4xp
author_rank: Frequent Contributor
timestamp: May 15, 2024, 11:19:38 pm
content: Methinks there was no 74HC or HCT in Z80A times, just74LS, 74S and maybe 74AS. 74F came with 74AC /ACT,later according to my Fairchild data books.

author: rsjsouza
author_rank: Super Contributor
timestamp: May 15, 2024, 11:54:26 pm
content: SN74C, SN74HC and even SN74F came around 1980 (give or take 2 years), a time where the Z80s were quite mainstream.The SN74AC, ACT came later IIRC.

author: retiredfeline
author_rank: Frequent Contributor
timestamp: May 16, 2024, 12:28:01 am
content: Quote from: Benta on May 15, 2024, 09:36:27 pmAh, my bad, I was remembering the Z80C Voh which will rise to Vcc-0.8.Happy?

author: pacmann
author_rank: Contributor
timestamp: May 17, 2024, 04:09:31 am
content: Thx for the replies.If NMOS logic has pull up resistor to Vcc then why is Voh so small, for my understanding? Is 2.4V measured for high frequencies and it will eventually get to 5v as Gyro suggests?Quote from: Gyro on May 15, 2024, 11:41:36 am

author: ArdWar
author_rank: Frequent Contributor
timestamp: May 17, 2024, 05:51:04 am
content: Quote from: pacmann on May 17, 2024, 04:09:31 amIt's related to output current sourcing capability. Say a 5VCCNMOS logic with 10K pullup connected to 10K load will drop its VOHto 2.5V with 250uA load current since it's basically a voltage divider. Put a little derating and you'll see that's probably how these datasheet values are derived.Most modern MOS-based logic (anything newer than those old power hogging original 7400, S, F, maybe LS, and probably some other) should be fine with that numbers, assuming the outputs aren't shared with something else in a bus. Their input current are low enough not to load down NMOS output. If you're that concerned you can always use TTL-compatible families like HCT/ACT/AHCT/etc like recommended above.