Runnable simulator field guide
PLC pull-up and pull-down wiring scenario: test checklist
Direct answer
The learner can predict current and voltage for open and closed conditions and distinguish a deliberate bias from an unsafe or incompatible field interface.
Written for pLC and electronics learners examining how resistive bias establishes a defined input state when a switch or transistor output is open.
Scenario setup
Supply, reference, input impedance, pull-up or pull-down resistor, switch or transistor state, leakage, threshold, default state, power loss and fault response.
I/O and state map
Source through bias resistor and switching element to input voltage, module threshold, software state and observable result.
Normal run
Open and closed switching conditions produce the intended stable default and active state within declared input thresholds.
Boundary run
Open wire, short to supply, short to return, leakage, wrong resistor, floating input, shared common, power loss and restart.
Injected fault
A source, reference, resistor, switch, transistor, wiring, threshold or software-interpretation mismatch.
Target check
The circuit checked against the exact field device and module electrical specifications.
What does a pull-up resistor do?
It biases a signal toward the positive supply when no stronger path drives the node, creating a defined default state.
Why should a PLC input not be left floating?
A floating node can cross the input threshold unpredictably because of leakage, coupling and noise, so the interface needs a documented electrical state.