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How to wire output on one device to input on another device

Discussion in 'Electronic Basics' started by PaulKraemer, May 19, 2005.

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  1. PaulKraemer

    PaulKraemer Guest


    I have a device (#1) that has a digital input. In order to put thi
    input in the "ON" state, you have to connect the input pin the th
    ground pin on the device

    I have another device (#2) that has a digital output. It is describe
    as an open-collector MOSFET output rated for up to 170 mA and 40Vdc.
    According to the manual, an "on" condition is a 5-ohm resistance t
    ground. An "off" condition is an open-circuit.

    I want to wire the output on device #2 to the input on device #1. Ca
    anyone tell me how I can do this and if I need any additional part
    besides wire

    Thank you
  2. Chris

    Chris Guest

    Hi, Paul. It's easy. Open-drain outputs can be treated just like
    open-collector outputs. You just need a pullup resistor to the logic
    power supply.

    Assuming you have that power supply available, wire up something like
    this (view in fixed font or Notepad):

    .---------------. .----------------.
    | Device #1 | ___ | Device #2 |
    | Vcc o-|___|--. | |
    | | 1K | | |
    | | | | Open Drain |
    | | | | Output |
    | In o--------o------o |
    | | | |
    | | | |
    | | | |
    | | | |
    | Common/GND o---------------o Common/GND |
    | | | |
    '---------------' '----------------'
    created by Andy´s ASCII-Circuit v1.24.140803 Beta

    Note that the MOSFET will do the exact same thing as connecting the In
    on Device #1 to GND. The pullup resistor may not be necessary, because
    Device #1 may have an internal pullup resistor to keep the In at a
    logic "1" when not active. Or it may have a TTL input which goes to a
    logic "1" when left floating. But an external pullup is usually used,
    and probably won't hurt.

    This is a best guess based on the information you've provided. It
    should work, but you really should contact the manufacturer or read the
    manual on the devices, or provide more information.

    Good luck
  3. That is called an "active low" input.
    That is an "active low" output intended to drive loads connected
    between it and a positive supply.
    Assuming there is no problem connecting the two system grounds
    together, you do that, tie the output to the input and add a pull up
    resistor to that line (say, 4.7 k ohms to a + 5 volt supply) so that
    when the output goes open circuit, there is something to make the
    input voltage positive. The pull up resistor may already be connected
    to the input, internally. Check the input voltage to its common, with
    nothing else connected. If it is near 5 volts, then this is the case.
  4. PaulKraemer

    PaulKraemer Guest

    Chris & John,

    Thanks for your responses, you've been a huge help. Both of yo
    mentioned that my input might have an internal pull-up resistor. I
    does. Here is the description from the manual..."each input has
    4.7K pull-up resistor and is protected for input voltages up to 3
    Vdc. Inputs are compatible with CMOS and TTL logic operating from 5
    supplies, with other pump's digital outputs, and with externa
    switches. An "on" input is less than 1V. An "off"input is more tha
    3.5 V, or an open circuit. The internal resistance provides the bia
    for required external switches. External switches should make
    connection to ground when in the "on" condition.

    I think I understand John's response and I can figure out how to wir
    this. I assume that there is no problem connecting the system groun
    because device #1 and device #2 are actually just two of the sam
    device (syringe pumps). It looks like I don't have to add a pull-u
    resistor myself, so it sounds kind of easy. Of course, I'll run i
    by the manufacturer before I connect the wires, but I am just tryin
    to see if I can learn a little

    Chris, I had a hard time following your schematic. I copied it fro
    this forum and pasted it into notepad, but the columns didn't seem t
    line up right. If you have any suggestions, or if you could possibl
    email it to me as a .txt file at (jus
    remove the NOSPAM), I would really appreciate it. If you coul
    remove the pull-up resistor if you don't think I need to add it, tha
    would be great too

    Thanks again to both of you
  5. Chris

    Chris Guest

    Hi, Paul. It's not much of a schematic -- it basically suggests pretty
    much what John was saying, connecting GNDs or Commons together,
    connecting the innie of Dev1 to the Outie of Dev2, and use an external
    1K pullup resistor from the logic power supply of Dev1 to the innie of

    ASCII circuits with a lot of whitespace sometimes don't copy well in
    some newsgroups. Sorry for that.

    As long as the two devices are close to each other and it's not an
    electrically noisy environment, you shouldn't have any problems with no
    pullup resistor. If you have a problem with false triggering, you
    might want to add an external pullup.

    Good luck
  6. Jamie

    Jamie Guest

    just connect the output of device #2 to the input of Deivce #1.

    the open collector mode is simply pulling what ever you connect
    to it to ground/common
  7. Rich Grise

    Rich Grise Guest

    If they're more than a foot or so apart, you might want to use a
    "twisted pair" - two wires twisted together, to minimize noise
    pickup, but here's your schematic: (I've used '_' to take up space,
    in case you're reading by google or haven't yet figured out how to
    set your newsreader's font to 'Courier')

    Output from device 1 o--------o Input to device 2
    __ Ground of device 1 o--------o Ground of device 2

    You might have to swap "1" for "2" in the above - I don't remember
    which you were connecting to which, but it's the same connection
    in either case, so no worries.

    In Other Words, two wires, and you're good to go.

  8. JeffM

    JeffM Guest

  9. PaulKraemer

    PaulKraemer Guest

    thank you Chris for emailing me the diagram, and to jeff for providin
    the link. I can read it now and I'm ready to connect my device
    together. You have all been very helpfu
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