Whats the simplest way to do the following (digital design) ...

Latch the occurrence of the fault signals- that would be a combinatorial S-R latch.

The OP didn't say whether the reset input needs to be held true until the fault signal is cleared or whether there is a manual clear input or what. A lot is left unstated.

This sounds a lot like a homework problem. The question is really asking for a circuit the performs a certain function and dresses it up with needless details.

BTW: For the OP. Lighting a LED is best done by putting a resistor and the LED in series and hooking it to the Vcc and pulling the other end down with a gate. TTL and HCT parts pull down better than up.

The least-IC way to do this is to use a PIC or other single-chip microcontroller. You'll need a 14- or 16-pin version, as you need one more pin than you can get in an 8-pin version.

If you're constrained to doing this with all 7400 series logic, then you're doing homework and I won't help you much. I will point out that you can latch the signals with S-R latches. 7400 series logic doesn't have SR latches to the best of my knowledge, but some of the other flip flops have asynchronous preset and clear inputs that do the same thing. If you can't figure out what to do with the output of the latches then you don't deserve to be in the class.

Sure it does. 74xx279 is a quad SR-FF. Two have dual sets (or resets, can't remember).

I agree that "least amount of ICs" are not really what I'm after - a uC however will NOT work to my satisfiction. I don't want any kind of software (read: Software bugs) in this - I want it to be hard-wired.

This is NOT a homewok assignment (I already have an M.Sc in electronic engieering) - it's work related problem :-). And I know the Latch/flip-flop + an OR-gate solution (it's just obvious). I was hoping to get one or two "clever" ways on how to deal with this problem. The current plan is to use a single 74x33 (quad 2-input NOR with open collector) and a resistor, configured as a S/R with 3 Set inputs, but that won't give me what triggered the event - which is not that critical, but would be nice feature (since the circuit only comes into work when somethings not working properly and the LEDs would give some extra information on where to start looking for what's wrong).

Furthermore, I'm not restricted to 74... but they are simple, cheap and can drive LEDs directly (HC/HCT), therefore I mentioned them.

As someone pointed out, I have left many things out, and I have done so delibirately. This is just a small part of a much bigger design. And even if I have specified a problem, I'm not really looking for a specific solution, but rather concepts on how to solve it. By "concepts" I mean: "Use S/Rs and an OR" or "use a PIC".

Best regards Johan.

What part of "Use a PIC" are you having difficulty comprehending?

I think I found your problem.

-- Many thanks,

Don Lancaster voice phone: (928)428-4073 Synergetics 3860 West First Street Box 809 Thatcher, AZ 85552 rss:

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heh

use four latches latch the fault signals diode OR the outputs of the fault latches buffer the diode OR gate with the fourth latch use LEDs for the diodes

will that work? (damn if I know, I'd definitely use CMOS, 74HC75 maybe)

One obvious problem the LEDs share a resistor

Johan W wrote:

Here's a way that uses the least amount (ie 0) of ICs:

PB + -o--o-+----------+----------+------+-----+ | | | | | [1K] [1K] [1K] | | | | | [10K] | [LED1] [LED2] [LED3] | | | | | | /e +---|

You should think about how this system starts up. Any potential problems there? An if it's gone into fault condition, how do you reset it? Maybe you need another input or some other kind of circuitry inside your black box.

Best regards, Spehro Pefhany

It's very aggravating to see blatant misinformation like this. There most certainly *are* MSI quad s-r latches in the 7400 series...There is no disconnect between functionality and knowing all the available parts.

That has more parts than needed and it will burn out the PNP when the SCRs fire.

! ! [1K] ! ! !/e +---------+----------+--------------! PNP | | | !\\c [1K] [1K] [1K] | | | | | [LED1] [LED2] [LED3] ! | | | ! |a | D2 | D3 | --- --- --- +--- Shutdown \\ /SCR1 \\ /SCR2 \\ /SCR3 | --- --- --- | g/ |k / | / | [1K] 1 --+ | 2 --+ | 3 --+ | | | | | | Gnd ----+----------+----------+--------------+

It's not misinformation, it was exactly accurate. To the best of my knowledge, at the time that I wrote it, that's what I thought -- and I made sure to qualify that it was to the best of my knowledge so that no one would go and wager their first-born on the 7400 series having no S-R flip flops.

You really should learn to read all of a post before you comment.

A tiny bit of software will give you a tiny chance of a bug -- I've written more than one tiny piece of software that has been released to the field with only one or no revisions. It was possible to do precisely because the software was tiny.

Your software for this would be tiny, too. I suspect that if you don't give in to the temptation to have more than 50 lines of assembly code (and that seems excessive to me) you'll have a smaller chance of a software bug than you will of having a hardware bug if you do it with combinatorial logic.

So why not an S-R latch on each line, and a big OR gate combining their outputs? It does everything you need -- it latches the fault indication, it lets you use one "clear" button, and any one fault will shut the equipment down.

Sorry about mistaking it for homework -- something about the way you worded it sure made it sound like professor Stuckintheeighties had assigned it for a beginning logic class.

If you had preceded the "7400 family has no ff's" by the "to the best of my knowledge" *then* it would have been a qualifier, but when you switch the ordering of that phrasing around, you make it an escape clause. It's like writing a long winded paragraph containing a bunch of definitive sounding assertions and then adding a disclaimer in the last sentence "I may be wrong about the above in whole or in part."

I suggest that the software be in something like a 22V10.

Q1 = (Q1 AND NOT Clear) OR D1 Q2 = (Q2 AND NOT Clear) OR D2 Q3 = (Q3 AND NOT Clear) OR D3 RESET = D1 OR D2 OR D3

Yup. And your solution is neat.

Ed