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

Discussion in 'Electronic Design' started by Johan W, Jul 7, 2007.

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  1. Johan W

    Johan W Guest

    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.
     
  2. Fred Bloggs

    Fred Bloggs Guest

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

    MooseFET Guest


    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.
     
  4. Tim Wescott

    Tim Wescott Guest

    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
     
  5. krw

    krw Guest

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

    Johan W Guest

    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.
     
  7. 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
    rss: http://www.tinaja.com/whtnu.xml email:

    Please visit my GURU's LAIR web site at http://www.tinaja.com
     
  8. 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
     
  9. bogax

    bogax Guest

    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
     
  10. ehsjr

    ehsjr Guest

    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
     
  11. 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
     
  12. Fred Bloggs

    Fred Bloggs Guest

    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.
     
  13. MooseFET

    MooseFET Guest

    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 ----+----------+----------+--------------+
     
  14. Tim Wescott

    Tim Wescott Guest

    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
     
  15. Tim Wescott

    Tim Wescott Guest

    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
     
  16. Fred Bloggs

    Fred Bloggs Guest

    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."
     
  17. MooseFET

    MooseFET Guest

    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
     
  18. ehsjr

    ehsjr Guest

    Yup. And your solution is neat.

    Ed
     
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