Wiring 5 — Pull-up & Pull-down Resistors
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Briefing
An **open-collector** (NPN) output can only pull a line LOW. To produce a valid logic HIGH, you must add an external **pull-up resistor** between the signal line and +24 V. Rule of thumb for 24 VDC logic: - Pull-up resistor: 4.7 kΩ → 10 kΩ (enough current for the input threshold) - Pull-down resistor: 1 kΩ → 10 kΩ (holds signal near 0 V when output is off) Wire the pull-up resistor so the PLC input reads correctly.
Objectives
- Wire the pull-up resistor between +24 V and the signal line (%I0.0)
- Wire the NPN sensor signal to the pull-up junction (%I0.0)
- Wire the NPN sensor 0 V to the common bus
Hints
- The pull-up resistor connects the signal line to +24 V so it defaults HIGH.
- When the NPN sinks the signal line to 0 V, the input sees LOW — that is the active state.
I/O Table
Inputs
PULLUP_RPull-up resistor connection to signal/+V junction
BOOL · %I0.0
NPN_SIGNALNPN sensor signal output → junction with pull-up
BOOL · %I0.1
NPN_GNDNPN sensor 0 V (emitter to common)
BOOL · %I0.2
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