Mesa 7i96 mixed voltage field input wiring

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05 Oct 2022 18:27 - 05 Oct 2022 18:28 #253554 by PCW
Assuming a 24V field voltage and conventional current flow for simplicity.

Sourcing inputs have the input common tied to +24V (so they "source" current)
They are activated by connection to ground (a sinking or NPN type sensor)

Sinking inputs have the input common tied to ground (negative of the 24V supply so they "sink current)
They are activated by connection to +24V (a sourcing or PNP type sensor)

A sinking sensor must be matched with a sourcing  input and vice-versa.
Last edit: 05 Oct 2022 18:28 by PCW. Reason: clarify

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05 Oct 2022 18:37 #253556 by arvidb
Maybe a diagram will help:


(Diagrams such as these could make a huge improvement to the Mesa manuals IMO.)
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05 Oct 2022 19:06 - 05 Oct 2022 19:18 #253561 by blazini36
Edit: cuz PCW's 100% right. A brain fart had me spun around on sinking and sourcing.

I will say that the comment about the manual section is a bit misleading. The case of a PNP switch doesn't imply that switches have a common positive, just a "less positive" reference. not sure that saying a "negative reference" is correct to say.
Last edit: 05 Oct 2022 19:18 by blazini36.

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05 Oct 2022 19:25 - 05 Oct 2022 19:26 #253564 by PCW
The issue is that ground is common for sinking inputs but the + voltage is the reference
for sourcing inputs.

With sourcing inputs, when you have contacts or NPN sensors,
having a common say 24V reference is fine (since the inputs will
swing the full 24V) but they will not work with 5V outputs (say a MPG).
I guess you could use a 5V common but that is marginal with switches
and NPN sensors with indicators (which have a built in pullup to 24V)

With sinking inputs you have none of these issues with mixed voltages.
Last edit: 05 Oct 2022 19:26 by PCW. Reason: format/clarify

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05 Oct 2022 19:32 - 05 Oct 2022 19:44 #253567 by blazini36
No, once you corrected me in typical PCW fashion I realized you were right on both accounts. Trying to fix my brain to say "sourcing" for positive is gonna be a challenge.....I've been saying it wrong for years lol.

But that's what I mean about the manual page, it pretty much says the same thing thing for NPN and PNP where it is correct for NPN but PNP the sensors just have to be more positive than the input common pin, not that they need to be connected as a common positive between each other.

A negative (ie -15v) input common would be a really tricky thing to explain when the positive input would be the 0v common of other things in the box.
Last edit: 05 Oct 2022 19:44 by blazini36.
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