EEVblog Electronics Community Forum
Electronics => Projects, Designs, and Technical Stuff => Topic started by: technix on May 09, 2017, 10:11:09 am
-
I finally found myself two D8087 NMOS FPU to go with my 80C88 processor. It is very hard to find the elusive CMOS variant of 8087 (I know that 80C287 are fairly common but those required a 16-bit bus, which my 80C88 certainly don't have.)
Can I let them live on the same bus?
-
Probably, although it would be best to check the datasheets of the particular chips that you have, and there's a chance I've overlooked something.
The usual problem with interfacing NMOS with CMOS is that NMOS's outputs aren't guaranteed to go above 2.4V, and CMOS usually likes a bit more. But the random 80C88 datasheet I just checked says VIH minimum is 2.2V, so it looks like they've given it TTL compatible inputs. It should be OK.
-
Probably, although it would be best to check the datasheets of the particular chips that you have, and there's a chance I've overlooked something.
The usual problem with interfacing NMOS with CMOS is that NMOS's outputs aren't guaranteed to go above 2.4V, and CMOS usually likes a bit more. But the random 80C88 datasheet I just checked says VIH minimum is 2.2V, so it looks like they've given it TTL compatible inputs. It should be OK.
What if the specific 80C88 (or other members of the bus for that matter, especially my two AS7C4096 SRAM chips and the EPM7128S bus matrix chip) does not support TTL levels?
-
Pullup resistors on the buss & controls would bridge that little extra. It's not as if you need to worry about signals in the 100s of MHz. I would use SIP resistor packs on sip sockets so you may adjust the value if needed, but, you need so little a boost, I would say 1k or 470ohm would give you a clean reliable clearance. Go for the higher resistance, 1k if it works, to prevent excess current consumption unless you really need the 470 ohm pull. Reserve the 470 ohm for open-collector/open-drain outputs only if truly needed.