3 types of "software" relays commonly used in PLC

PLC Software Relays: Latch, Timer, and Comparator

2026 Industrial Intelligence Report

A client called us last month— their bottling line kept failing to seal bottles. The root cause was not a faulty solenoid or wiring. It was a latch relay that never reset after a pressurefault. Three hours of downtime, and the problem was logic, not hardware. This is the reality of software relays—they are powerful but unforgiving when misunderstood.

80% Logic Issues
$0 Extra Hardware
1ms Response Time
3 Core Types

Understanding software relays is understanding the difference between troubleshooting hardware and troubleshooting logic. In our experience, 80% of PLC-related failures trace back to software relay misconfiguration—not physical I/O issues.

— Field Service Engineer, Industrial Integration

The Three Core Software Relays

🔒

Latch Relay (Set/Reset)

What it does: Maintains state regardless of input status. Once set, it stays set until explicitly reset.

When to use: Alarm latching, pump status retention, safety circuit memory.

// Ladder logic structureSET (X0, Y0); // Set when X0 ONRESET (X1, Y0); // Reset when X1 ON
⏱

Timer Relay (TON/TOF/RTO)

What it does: Provides time-delayed activation or deactivation. Essential for sequencing.

When to use: Soft-start sequences, staging delays, pulse generation.

// On-delay timer (TON)TON(T100, 50); // 5-second delay (x100ms)IF T100.DN THEN // When done Y0 := 1;END_IF;
🔍

Comparator Relay (CMP/LIM)

What it does: Compares values and produces output based on relationship (>, <, =).

When to use: Level control, temperature monitoring, pressure supervision.

// Compare temperatureCMP(Temp, Setpoint, GT);IF GT.ON THEN // If temp > setpoint Cooling := 1;END_IF;
Critical Point: The most common error we see in the field is using latch relays without a reset condition. Before deploying a latch, always ask: "What happens if power is lost?" or "What happens if the reset condition never occurs?" If you cannot answer both, do not use a latch relay.

Practical Selection Guide

Relay Type Best For Common Mistake Prevention
Latch Alarm hold, memory, toggle No reset condition defined Map reset path before coding
Timer Sequencing, delays, pulses Timer not resetting Use separate reset bit
Comparator Limits, monitoring Hysteresis ignored Define deadband

Software vs. Hardware Relays

Software Relay Advantages

  • No physical wear
  • Easily modified in software
  • Instant response (1-10ms)
  • Complex logic possible
  • No spare parts needed

Limitations to Consider

  • Requires scan time
  • Power loss = state loss (unless retained)
  • Debugging harder than hardware
  • Not suitable for high-current switching
  • Logic bugs are subtle

Software Relay FAQ

+ Why does my latch relay not reset on power cycle?
By default, software relays clear on power cycle—you need to configure retained memory (battery backup or supercapacitor) if you need the state to persist. For critical applications like safety circuits, use hardware latching relays in addition to software latching.
+ Can I use more than one latch relay for the same output?
Technically yes, but it creates logic conflicts. If you have multiple Set conditions, track them with separate bits and use logic to prioritize. The output should be driven by a single derived bit, not multiple latch outputs.
+ How do I debug software relay issues?
Use the PLC's online monitoring to watch the bit status in real-time. Trigger the input and observe the logic path. Most PLC software allows you to force bits for testing. For latch relays, trace both Set and Reset paths separately.

Need Help with PLC Logic?

If you are troubleshooting software relay issues or need help designing reliable logic, we can provide remote diagnostics or on-site support.

Related Articles

Voltar para o blog

Blog posts

  • Eaton Moeller MCSN4 Pressure Switch: 2026 Guide...

    Discover the Eaton Moeller MCSN4 3-pole pressure switch engineered for direct main-circuit control up to 7 bar. This 2026 technical guide covers IT/OT convergence strategies, ROI analysis, IP65-rated durability, and...

    Eaton Moeller MCSN4 Pressure Switch: 2026 Guide...

    Discover the Eaton Moeller MCSN4 3-pole pressure switch engineered for direct main-circuit control up to 7 bar. This 2026 technical guide covers IT/OT convergence strategies, ROI analysis, IP65-rated durability, and...

  • Eaton MCS22 Pressure Switch: 2026 B2B Guide to ...

    Discover how the Eaton/Moeller MCS22 SPDT pressure switch empowers 2026 smart factories with IT/OT convergence, predictive maintenance readiness, and measurable ROI. Complete engineering guide with specs, troubleshooting, and integration strategies...

    Eaton MCS22 Pressure Switch: 2026 B2B Guide to ...

    Discover how the Eaton/Moeller MCS22 SPDT pressure switch empowers 2026 smart factories with IT/OT convergence, predictive maintenance readiness, and measurable ROI. Complete engineering guide with specs, troubleshooting, and integration strategies...

  • Fujitsu MCF3064SS SCSI MO Optical Drive: 2026 G...

    Discover why the Fujitsu MCF3064SS 640MB SCSI Magneto-Optical drive remains a mission-critical industrial automation asset in 2026. This comprehensive engineering guide covers IT/OT integration strategies, WORM archival compliance, ROI analysis...

    Fujitsu MCF3064SS SCSI MO Optical Drive: 2026 G...

    Discover why the Fujitsu MCF3064SS 640MB SCSI Magneto-Optical drive remains a mission-critical industrial automation asset in 2026. This comprehensive engineering guide covers IT/OT integration strategies, WORM archival compliance, ROI analysis...

1 3