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Is there no 4 bit identity comparator in TTL or CMOS? It could fit into a DIP-12 package as all it would have is 4 XOR gates for selectable invesion and 1 4-way AND gate.

OUT := (A1 xor B1) and (A2 xor B2) and (A3 xor B3) and (A4 xor B4)

Just 11 pins with Vcc and GND. Could even have both OUT and (not OUT) for easy (not CS) or (nor LOAD) application.

The comparators I know, 74LS85 being the smallest, comes in a DIP-16 package and has A < B and A > B outputs which I don't need.

I am building a TTL or CMOS mini-computer from scratch. I have a bus and various modules can be addressed on that bus using 3 or 4 module address lines. Now each module listens to these 3 or 4 lines with a comparator to find out if their address is called. I have seen this principle on ISA bus cards where they use 74LS688, but that's for 8 bit.

I'm just trying to save space. I went down the entire 7400 list and 4000 list, so I know it's not there.

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  • \$\begingroup\$ Sounds like you want to hail back to GAL or PAL devices. Or a CPLD. I don't know if they still exist, today. But if they do, great! Other than that, why are you bothered by using two packages? (You have already assured yourself the 74xx part doesn't already exist.) \$\endgroup\$
    – jonk
    Nov 9 '19 at 5:54
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No, it seems pretty definitive that while there are 4-bit identity comparator ICs (eg. the 54LS85 and the 74LS85 you mention) the lowest pin count packages they come in is 16, giving you a few unnecessary pins.

DIP-12's seem very rare in general. Even the PIC microcontrollers only come in DIP-8 or DIP-14, and CPLDs/FPGAs don't seem to be available in packages that small. If saving the 4 pins is that important, the only other thing I would suggest is to design your own - you could use small SMD discrete logic ICs and build them into a DIP-12 sized custom PCB.

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  • \$\begingroup\$ I don't think "JM38510" uniquely identifies a logic device, it seems to be a prefix for a large family of parts built to a military reliability standard. Did you mean the "JM38510/31101"? \$\endgroup\$ Nov 9 '19 at 9:22
  • \$\begingroup\$ Apologies, I didn't see a family listed when I came across it and guessed (wrongly). Part number is JM38510/08201BEA but now that I look I see it's just another SN54LS85 so I'll remove it from the answer. \$\endgroup\$ Nov 9 '19 at 10:26

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