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I have a device A that sends a signal through an optocoupler, this signal travels through a 7m cable and is used by another device B that has a SN74LVC1G17SingleSchmitt-TriggerBuffer powered by 3.3V, its maximum input voltage is 5.5V. I already have an interface circuit with 24V available on the device B side, so I'm thinking on putting the pull-up resistor on this side, together with a voltage divider and a security zener diode.

The circuit is:
enter image description here

And the result of the simulation:
enter image description here

Will this work in real life? Or do I need to also send the 24V through the cable and place the pull-up resistor on device A side, using a 3 wire cable?

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    \$\begingroup\$ The setup as drawn looks ok. The signal on the A-side cable is either 0 or 12 mA. This is fine. The real worry is the cable from the interface to Device-B. The signal on this cable is either 0 or 5 uA. This will be very susceptible to any noise. \$\endgroup\$ – scorpdaddy May 27 '20 at 13:45
  • \$\begingroup\$ @scorpdaddy Very good point! I'm thinking on using a Molex picoblade 6 pin 50mm cable ref 15134-0600 as device B already has a 6 pin picoblade female connector. Do you think I should get closer or protect this cable with some kind of shielding? Or twist the cable wires? \$\endgroup\$ – Pedro NF May 27 '20 at 14:32
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    \$\begingroup\$ Twisted pair gives good immunity. But the signal level is so low that even that may not work. I usually expect to get 60 dB isolation from other signals, conducted or through the air, and 80 dB if you work hard and twist the wires, shields, etc. So you you would probably be ok if there were no signals higher than 50 mA nearby. Otherwise you should put an extra SN74LVC1G17 on the interface board and a 1k pulldown on Device-B to bump up the signal current by 1000x on the Device-B cable. \$\endgroup\$ – scorpdaddy May 27 '20 at 19:22
  • \$\begingroup\$ @scorpdaddy If I put a 1K resistor on the Device B side then in the simulation the voltage drops to 500mV (it messes up the voltage divider resistor values) and I get a 500uA current flowing through the connection. Is this ok? Or should I make a different circuit with a transistor? \$\endgroup\$ – Pedro NF May 28 '20 at 13:08
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    \$\begingroup\$ Did you put an extra SN74LVC1G17 on the interface board to drive the 1k resistor? \$\endgroup\$ – scorpdaddy May 28 '20 at 14:22
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Followed advice from @scorpdaddy, question closed :-D

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