Mixing NMOS and CMOS

Question

I have an old system board for a pinball machine that I'm fixing up. One of the ROMs is failing post. The reason is because the 27S256 EPROM never had the sticker applied to the window, and due to a series of circumstances the EPROM wasn't touched for several years so it became altered via incidental lighting.

My question: Is it safe to replace this part with a CMOS part of the same Pinout and type? Is it safe to mix Schottky and CMOS?

Please correct any factual errors in the above. A little new to this.

EDIT: it's not a 27S256, it's a HN27256G-25. I couldn't read correctly. According to some googling it is an NMOS part. Can I mix NMOS and CMOS

Answer 1

You can replace your HN27256G-25 (250 ns) NMOS EPROM with for instance a M27C256B-10F1 (100 ns) CMOS EPROM. They even have, in stock.

Most important things to check:

They are pin-to-pin compatible.
Supply(CMOS)=5 V = Supply(NMOS)=5 V.
Access time(CMOS)=100 ns \$\le\$ Access time(NMOS)=250 ns.
Operating current(CMOS)=30 mA @ 5 MHz \$\le\$ Operating current(NMOS)=100 mA max (!).
Standby current(CMOS)=100 \$\mu\$A \$\le\$ Standby current(NMOS)=40 mA (!).
\$V_{OLmax}(NMOS)=0.45\;V \le V_{ILmax}(CMOS)=0.8\;V\$.
\$V_{OHmin}(NMOS)=2.4\;V \ge V_{IHmin}(CMOS)=2\;V\$.
\$V_{OLmax}(CMOS)=0.4\;V \le V_{ILmax}(NMOS)=0.8\;V\$.
\$V_{OHmin}(CMOS)=3.6\;V \ge V_{IHmin}(NMOS)=2\;V\$.

All that is fine, so the CMOS EPROM should work.

Russel is right in that there are several other things to check, related to timings but, being the CMOS a 2.5x faster device, odds are that the new device will work with no problems. And the reason for this is that memory interfacing protocols are designed precisely with that in mind: so that devices with shorter delays will not pose problems. Manufacturers specify maximum delays, not minimum delays. They also specify minimum setup times, but that's another thing, which doesn't pose a problem here.

Answer 2

No, since they have different logic (voltage) levels (TTL is lower that CMOS). You may need to convert the logic levels. This can be done using ICs like the TI CD4504B. Check the datasheets (EPROM and level shifter) for details.

Answer 3

Can you mix types? = Maybe.

I'd go over the data sheets in detail to look for compatability issues - of which there are potentially many. But I'd guesstimate that you've got a fighting chance of it working as long as they both use the same supply rails. ie hopefully both are 5V single supply.

HOWEVER, if you can get the original code or read the failing part reliably enough sometimes to recover the code, then you can probably get the original part regrogrammed. as you are willing to replace it you lose little by trying this - EXCEPT if this is your sole code source.