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500 tri-state inputs

Discussion in 'Electronic Design' started by Tony, Nov 13, 2003.

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  1. Tony

    Tony Guest

    Greetings
    I need to monitor 500 inputs "wires" that swing - to + or hiZ and am
    looking for ideas that are efficient. I need to determine which of the 3
    states each of the wires is in. Any input would be appreciated, no pun
    intended.
    Tony
     
  2. You can do the basic function with a single push-pull driver and
    comparator, and a multiplexer input and a resistor per "wire" (say 1/8
    of a SIP network). Drive it low, read the pin, drive it high, read the
    pin and you've got 4 possibilites:

    Driver
    L H
    0 0 Driven low
    0 1 High Z
    1 0 Shouldn't happen, maybe it changed during read
    1 1 Driven high


    The details will depend a lot on what - and + are, how high HiZ is,
    how fast you want to detect it, and if there are any real-world
    considerations (eg. ESD or other transient protection). Those may well
    dominate the component count etc.

    Best regards,
    Spehro Pefhany
     
  3. Luhan Monat

    Luhan Monat Guest

    Tony,

    Looks like you could use 500/8 = 63 CD4051 analog mux chips feeding two
    comparater circuits feeding a pic. Parallel the 3 'select' inputs of
    all of the 4051s, then connect the Enable input of each one to an output
    of 8 shift registers daisey chained on two outputs from the pic.

    Shift in the first bit and run thru the 8 inputs of each 4051, then
    shift the bit to enable the next one and do likewise with the rest.

    chip total: 1 + 63 + 8 = 72.
     
  4. Tony

    Tony Guest

    I like this idea. The ability to program the comparator window on the fly
    would solve the wire length problem. The wires are quite long in some cases
    and float a bit above ground when released. A divider connected to the wire
    would keep the wire that is "broken" in the window of the comparators.
    Spehro's idea was similar but requires active drivers. Definitley worth
    asking you guys for ideas. I'll check back again later before I heat up the
    iron. Thanks a bunch
    Tony
     
  5. Tony

    Tony Guest

    Wait a second. I just realized I misunderstood Spehro. 1 push pull driver
    would reduce the number of resistors on the wires to 1 current limiting
    resistor. Instead of two for a divider. Hum. I need to think about this one.
    Maybe no input resistors at all? How much abuse can a MUX chip stand?
    http://www.speff.com
     
  6. Not very much at all unless you shell out for the special ones with
    built-in resistors and clamping (maybe 10-30 times the cost). It would
    sure knock the component count down. You might even be able to use a
    single resistor to push/pull current *through* the switch instead of
    500 resistors, but you'll have to allow for the fairly significant
    switch resistance (maybe < 1K but some are much higher). The
    unprotected switches have much less resistance (under 100 ohms) and
    some VERY fast ones that are only spec'd for digital switches get down
    to the 20 ohm range, IIRC.

    Try Analog Devices (site is currently down) or Maxim. (www.analog.com
    , www.maxim-ic.com )


    Best regards,
    Spehro Pefhany
     
  7. GPG

    GPG Guest

    http://www.maxwell.com/microelectro...witches_multiplexers/81840RP/description.html
    Times 4?
     
  8. Tony

    Tony Guest

    fascinating part, No doubt. I am afraid to inquire as to its cost though
    "intended for space craft" etc.. I found another part mcp506a Burr Brown
    that is a 16 to 1. With a dual tier setup I would need two sets of 17 chips.
    They are fairly robust and have a R on of ~1k. With the low impedance
    source I think that is ok. Two set would need two 8 bit ports + the sample
    pin / pins. I am thinking just read the input voltage using the onboard ADC
    available on some pics. But the open circuit test needs considering. But I
    am getting there,
    Tony
     
  9. Rodney Josey

    Rodney Josey Guest

    Tony,
    Are you needing to 'monitor' the 500 inputs at the same time, or can
    the 500 be split into smaller chunks (say 8, or 16)? Might make the
    circuitry a lot simpler - cycling through each block of 8 / 16 up to
    the 500. Also - if you are checking 'long' wires, then you may need
    to take into account the 'current' required to insert into each tested
    wire to ensure the required - or + test voltage is received at the
    monitoring end - thus the 4051 analog MUX chips may not be suitable
    for handling the higher 'current' at it's inputs - just a thought.

    If into smaller chunks (8 or 16) - could consider using 'back-to-back'
    opto's for each input (isolation plus direction) - one led for the -,
    one for the +, no led active for the HiZ. Also - consider using an
    EPROM memory chip as the decoder - 16 address lines IN, 8 data lines
    OUT - with each memory byte (8 bits) to reflect which state the
    referenced wire (input address) is in. Could use several EPROM chips
    as decoders!

    Such a circuit could then be easily interfaced via either a serial
    port link or parallel port link to a bog standard PC for monitoring
    purposes.

    IHTH.

    Oz-Rod (sometime electronics tinkerer from down-under)
     
  10. Tony

    Tony Guest

    Hey Oz
    I need not monitor simultaneously. I am able to access only one end of the
    wire and must determine if the other end is connected to a common - or + or
    disconnected all together. I can poll each wire sep[erately and allow as
    much time as needed so capacitive effects are irrelevant. Induced currents
    from other wires is a concern as it limits the resistance of the divider I
    would need to make the wire if broken, float in the middle between the two
    rails. Since the mux's have 1k inputs and I will be using a tiered method
    the input resistance is 2k then the divider then sampled by an adc . The
    samples will be forwarded to a pc via the PIC's serial port. for decisions
    on whether they are hi low or floating. My current dilemma is the divider
    resistance. The lower the resistance the farther it will pull the inputs
    from the rails and the iffier the -,~,+ decision. The higher the resistance
    the weaker the ability to float a disconnected wire between the rails.
    That's where I am at so far. The MUX chips I have in mind at the moment are
    available at DIgi Key for $8 and change in quantities of 25. I would scrap
    it all for a better idea though. Thanks for the input.
    Tony
     
  11. Rodney Josey

    Rodney Josey Guest

    Tony,
    Are you needing to 'monitor' the 500 inputs at the same time, or can
    the 500 be split into smaller chunks (say 8, or 16)? Might make the
    circuitry a lot simpler - cycling through each block of 8 / 16 up to
    the 500. Also - if you are checking 'long' wires, then you may need
    to take into account the 'current' required to insert into each tested
    wire to ensure the required - or + test voltage is received at the
    monitoring end - thus the 4051 analog MUX chips may not be suitable
    for handling the higher 'current' at it's inputs - just a thought.

    If into smaller chunks (8 or 16) - could consider using 'back-to-back'
    opto's for each input (isolation plus direction) - one led for the -,
    one for the +, no led active for the HiZ. Also - consider using an
    EPROM memory chip as the decoder - 16 address lines IN, 8 data lines
    OUT - with each memory byte (8 bits) to reflect which state the
    referenced wire (input address) is in. Could use several EPROM chips
    as decoders!

    Such a circuit could then be easily interfaced via either a serial
    port link or parallel port link to a bog standard PC for monitoring
    purposes.

    IHTH.

    Oz-Rod (sometime electronics tinkerer from down-under)
     
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