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



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=B4s ASCII-Circuit v1.24.140803 Beta

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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 Chris

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I have a device (#1) that has a digital input. In order to put this

input in the "ON" state, you have to connect the input pin the the

ground pin on the device.

I have another device (#2) that has a digital output. It is described

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 to

ground. An "off" condition is an open-circuit.

I want to wire the output on device #2 to the input on device #1. Can

anyone tell me how I can do this and if I need any additional parts

besides wire.

Thank you,


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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.

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John Popelish

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 Dev1.

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 Chris

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Chris & John,

Thanks for your responses, you've been a huge help. Both of you

mentioned that my input might have an internal pull-up resistor. It

does. Here is the description from the manual..."each input has a

4.7K pull-up resistor and is protected for input voltages up to 30

Vdc. Inputs are compatible with CMOS and TTL logic operating from 5V

supplies, with other pump's digital outputs, and with external

switches. An "on" input is less than 1V. An "off"input is more than

3.5 V, or an open circuit. The internal resistance provides the bias

for required external switches. External switches should make a

connection to ground when in the "on" condition.

I think I understand John's response and I can figure out how to wire

this. I assume that there is no problem connecting the system ground

because device #1 and device #2 are actually just two of the same

device (syringe pumps). It looks like I don't have to add a pull-up

resistor myself, so it sounds kind of easy. Of course, I'll run it

by the manufacturer before I connect the wires, but I am just trying

to see if I can learn a little.

Chris, I had a hard time following your schematic. I copied it from

this forum and pasted it into notepad, but the columns didn't seem to

line up right. If you have any suggestions, or if you could possibly

email it to me as a .txt file at (just

remove the NOSPAM), I would really appreciate it. If you could

remove the pull-up resistor if you don't think I need to add it, that

would be great too.

Thanks again to both of you,


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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

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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.

Cheers! Rich

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Rich Grise

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thank you Chris for emailing me the diagram, and to jeff for providing

the link. I can read it now and I'm ready to connect my devices

together. You have all been very helpful

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