Student again. For a lab rig we have a gateway with 4 digital inputs that we want to use for a door contact, a float switch and two NPN proximity sensors, with the states going to Hermetry. The datasheet table says for the inputs:
Input type: sinking, IEC 61131-2 type 1/3
On voltage: > 11 V
Off voltage: < 5 V
Input current: 3 mA @ 24 V ("current load")Why would an input “load” anything, isn’t it just measuring voltage? Does the 3 mA matter for my wiring? Also the prox sensors are NPN and I think the input is sinking, which seems like it would not work?
Before the theory: check the gateway model actually supports what you plan with those inputs. Pulse counting on digital inputs depends on the model, for example.
We only need on/off states, no counting. Good to know though.
Fair questions, they confused me as well when I started.
The input deliberately draws a few mA when it’s on. An input that only “measured voltage” with a very high impedance would see 15 V picked up on a long cable next to a motor lead and call it ON. By needing 3 mA, a weak induced voltage can’t hold it up. That’s what the type 1/3 classes in IEC 61131-2 define: voltage and current ranges for ON and OFF.
It also matters for 2-wire sensors. A 2-wire prox still leaks a little current when it’s off. If that leakage is above what the input accepts as OFF, the input never turns off. Type 3 inputs are designed to tolerate that leakage, type 1 inputs may not be.
Your wiring: “sinking” input means current flows into the input and out to 0 V, so it needs a device that sources +24 V into it. That’s a PNP sensor or a dry contact between +24 V and the input. An NPN sensor pulls the input towards 0 V, so on a sinking input it does nothing. Options: buy PNP versions of the prox sensors (easiest), or use an interposing relay.
The door contact and float switch are dry contacts: wire them from +24 V to the input. 3 mA through a reed contact is fine.
On the Hermetry side send the states as 0/1 numbers so they can be charted and alarmed on.
That explains it, thanks Ciaran. We swapped the NPN proxes for PNP ones (the lab had some), the door and float switch go from +24 V to the inputs, all four show 0/1 in Hermetry now.
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