Need the reverse logic value between a 74HC85 and 75HC238

[laxamar]

I am using a 4 bit comparator (CMOS version) 74HC85. Actually using it to only compare 3 bits and 4th is hard coded to GND (0). I am only interested in Perfect Match (All inputs equal). Signals are coming from another board.

My (stupidly simple) issue is that this chip gives a High Value when things are equal. The next chip using this info has 3 input to be activated (G1 - NOT G2A - NOT G2B) - 2 NOT and 1 TRUE. I Obviously need to attach the output of comparator A=B to G1. But that forces my other control signal (STROBE_R1) to be reversed. I need STROBE_R1 to be ON elsewhere.  STROBE_R1 is software controlled, so I can chose it's state. But it comes from another board and all the connector pins are accounted for. I don't want to add the complexity of 2 STROBE SIGNALS (on/off) at the source. This has to work on this PCB.

My question is this (any of these solutions will do)

• Are there ICs other then 74HC85 or 75HC238 that can give/accept the proper value (a reversed value comparator seems  doubtful, as it needs to cascade)?
• Is there a very simple circuit that can reverse the state of a signal and reverse it in SMD? (an XOR circuit seems sooo much waste)
• Any other solution that will take as input 3 bit count, 3 bit compare and ON/OFF signal to binary

[oPossum]

Is there a very simple circuit that can reverse the state of a signal and reverse it in SMD?

https://www.ti.com/lit/sg/scyt129g/scyt129g.pdf

[laxamar]

Is there a very simple circuit that can reverse the state of a signal and reverse it in SMD?

https://www.ti.com/lit/sg/scyt129g/scyt129g.pdf

The winner seems to be SN74AUP1G04YFPR
https://www.ti.com/general/docs/suppproductinfo.tsp?distId=10&gotoUrl=https%3A%2F%2Fwww.ti.com%2Flit%2Fgpn%2Fsn74aup1g04

Thanks

[Benta]

Aren't the single-gates great? They used to be called "kludge logic", but I use them all the time.

On an aside: why are two of your 'HC85 inputs unconnected? I know they are listed as "don't-cares" in your configuration, but it's an open door to electrical interference problems.

[laxamar]

On an aside: why are two of your 'HC85 inputs unconnected? I know they are listed as "don't-cares" in your configuration, but it's an open door to electrical interference problems.

Because I was lazy and stupid. And your timing could not have been more spectacular!

I've had the PCB for a couple of weeks and have been running tests on them - fixing design issues one by one. The last piece which I did not --- UNTIL NOW --- consider critical was that every once in a while, the comparator would give wrong results. I'm saying after hours and hours of sequencing. I was thinking some sort of connection problems as 4 boards are involved. Literally was going to research it now. This completely explains it and was "caught" before sending the next batch.

I have to buy you some beer or something, right?

[edavid]

If I understand the question correctly, you could use a CD74HC688 comparator instead of the 74HC85 + inverter.  It's in a 20 pin package, so might not save any space.

[pgo]

Hi,

I might be misunderstanding what you are asking but can you use ~(A>B) & ~(A<B) & strobe?

[laxamar]

If I understand the question correctly, you could use a CD74HC688 comparator instead of the 74HC85 + inverter.  It's in a 20 pin package, so might not save any space.

Brilliant!

I do have room for it, simplifies my life as I don't have to deal with anything but equality. Additionally, I can move from 3 bit to 8 bit in the future, once I become rich and famous because my device actually works!

And @Benta, yes, I know. All unused inputs to ground!

[laxamar]

I might be misunderstanding what you are asking but can you use ~(A>B) & ~(A<B) & strobe?

Impeccable logic. No other change necessary and the pins are literally next to each other on my PCB.

I now have 3 completely acceptable solutions!

Probably will not use the inverter, as cool as it is because routing PWR and GND on the current PCB might get complex. But great arrow in my quiver.
Love the idea of a simpler compare chip. But I'd have to order DIP version, test on my breadboard and redesign. Damn I like it though.
Just routing 1 extra line on a validated design sounds great and I can test on the breadboard NOW. I'll see if I can route STROBE around on the PCB.

Wow. Thanks everybody!

[metertech58761]

74xx688? Haven't seen that part number in ages. I know it by an older part number - the 25LS2521.

One of my employer's past customers used those on a number of their boards for addressing purposes.

Just putting that part number out there as a possible alternate if needed.

[edavid]

74xx688? Haven't seen that part number in ages. I know it by an older part number - the 25LS2521.

One of my employer's past customers used those on a number of their boards for addressing purposes.

Just putting that part number out there as a possible alternate if needed.

1. OP wants a CMOS part
2. 74HC688 is actually in production, 25LS2521 is long obsolete

[laxamar]

ADDENDUM - JUST FUN STUFF

I added small inverter to my circuit where I needed the signal reverse. I'm using KiCad. I downloaded a symbol and footprint from the manufacturer.

The Symbol for this smallest component of my design is *literally* the biggest element on there. It only uses 4 pins, but I had to use the 6 pin X2SON footprint which has 6 pins. 2 NC. The symbol dwarfs everything else. The foot print is minuscule... I honestly know how to create my own symbol that's not obnoxious, but it's funny (to me!)

[Benta]

1: The 74AUP1G04 is already in the KiCAD library, if not, use the 74VHC1G04 instead.
2: The 7406/74AUP1G06 has open-collector/open-drain output and will not work here.