3 types of "software" relays commonly used in PLC

PLC Software Relays: 3 Types

2026 Industrial Intelligence Report

Software relays replace hardware—saving cost and space. Here's the three types—from field experience.

Three Types

1. Latch Relay (OTR)

Function: Holds state

Use: Toggle on/off

Example: Motor start

2. Timer Relay (TON)

Function: Time delay

Use: On-delay/off-delay

Example: Delayed start

3. Counter Relay (CTU)

Function: Count events

Use: Batch counting

Example: Parts produced

Implementation

Latch (OTR)
Set (OTU) to set, Reset (OTU) to clear. Remains set until reset.
Timer (TON)
Timebase (1ms, 10ms, 100ms, 1s). Preset = delay time. ACC = elapsed.
Counter (CTU)
CU = count up input, R = reset. PV = preset value. DN = done when ACC >= PV.
Compare
Use EQU, LES, GRT to compare counter values and trigger actions.

90% of relay logic can be done in software—only safety relays need hardware. Save cost and space.

— Senior Controls Engineer

Common Mistakes

Relay Type Mistake Result>
Latch No reset Stays on forever
Timer Wrong timebase Wrong delay
Counter No reset Never resets
Counter PV = 0 Immediate done
Pro-Tip: Always have a reset path—latch without reset is a common mistake. Plan reset logic first.

Timer timebases: 1ms (fast), 10ms (normal), 100ms (slow), 1s (very slow). Choose based on needed precision.

FAQ

+Latch vs OTL?
LATCH is the instruction—OTL (latch) sets, OTR (unlatch) resets. Both require hardware replacement for safety.
+TON vs TOF?
TON = timer on-delay (delays turn-on). TOF = timer off-delay (delays turn-off).
+When use hardware?
Safety circuits always require hardware relays. Emergency stop, safety door, motor overload.

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