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

I have a system which have three (output) monitor signals. If the system
runs as normal, the three signals are zero. If there is a fault, one (or
more) of the monitoring signals will become one. Also, the system has a
shutdown intput. Put a zero here, and the system runs; put a one here
and the system stops.

Now, to the problem: I want to build a device (e.g. with 74... ICs) with
three inputs, one output and three LEDs. The three inputs connects to
the monitor signals, and the output connects to the shutdown output.
When one (or more) of the inputs becomes one (indicating an error), the
output should become one (to shut the system down). The three LEDs
should indicate witch of the input(s) that caused the shutdown.

Note, when the system shuts down, the montoring signals may, or may not
reset to zero.

Whats the simplest way to do this (least amount of ICs) ?

Best Regards Johan.

 

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.

--
Tim Wescott
Control systems and communications consulting
http://www.wescottdesign.com

Need to learn how to apply control theory in your embedded system?
"Applied Control Theory for Embedded Systems" by Tim Wescott
Elsevier/Newnes, http://www.wescottdesign.com/actfes/actfes.html

 

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?


--
Many thanks,

Don Lancaster voice phone: (928)428-4073
Synergetics 3860 West First Street Box 809 Thatcher, AZ 85552
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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: http://www.tinaja.com/whtnu.xml email:

Please visit my GURU's LAIR web site at http://www.tinaja.com

 

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

 

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

PB
+ -o--o-+----------+----------+------+-----+
| | | | |
[1K] [1K] [1K] | |
| | | [10K] |
[LED1] [LED2] [LED3] | |
| | | | /e
+---|<------------+----------+---| PNP
| D1 | | | | \c
| +--|<--+ +--|<--+ |
|a | D2 | D3 |
--- --- --- +--- Shutdown
\ /SCR1 \ /SCR2 \ /SCR3 |
--- --- --- |
g/ |k / | / | [1K]
1 --+ | 2 --+ | 3 --+ | |
| | | |
Gnd ----+----------+----------+------------+

A + at input 1, 2 or 3 turns on the corresponding SCR
and LED and puts a + on the shutdown line. Pressing the
normally closed pushbutton resets the circuit, the SCR
and LED turn off and the shutdown line goes -
Use sensitive gates SCRs and appropriate gate resistors.

Ed

 

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.

--
Tim Wescott
Control systems and communications consulting
http://www.wescottdesign.com

Need to learn how to apply control theory in your embedded system?
"Applied Control Theory for Embedded Systems" by Tim Wescott
Elsevier/Newnes, http://www.wescottdesign.com/actfes/actfes.html

 

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.

--
Tim Wescott
Control systems and communications consulting
http://www.wescottdesign.com

Need to learn how to apply control theory in your embedded system?
"Applied Control Theory for Embedded Systems" by Tim Wescott
Elsevier/Newnes, http://www.wescottdesign.com/actfes/actfes.html

 

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