When Three Brains Think at Once: Synchronization Jitter in the Triconex 3101 - Excellent PLC

By Robert Klein – System Integration Lead

If you’ve ever commissioned a Tricon system, you know the comfort of triple modular redundancy.

Three processors.
Three independent opinions.
One decision.

With the Triconex 3101 main processor module, that promise holds — but under extreme load, the way those three “brains” stay in sync becomes visible in subtle ways.

Project Background

This happened during the final phase of a refinery upgrade:

New safety logic added to an existing Tricon system
Scan cycle tightened to meet faster interlock response
Large number of analog inputs sampled at once
Redundancy channels fully loaded

Everything passed FAT.
SAT revealed something different.

What We Actually Saw on Site

Control outputs remained correct
No safety trips occurred
System logs showed minor timing jitter between channels
In rare cases, one channel lagged a few milliseconds behind

From an operator’s perspective, nothing was “wrong.”
From a systems perspective, timing drift had entered the picture.

Why This Happens

The 3101 MPM performs:

Real-time logic execution
Continuous cross-channel comparison
Input sampling and voting
Output commit only after agreement

Under high computational load:

Task scheduling becomes denser
Channel execution phases can slip slightly
Synchronization logic realigns channels before committing outputs

This realignment introduces tiny timing shifts — not failures, but temporal friction.

How We Proved It

We instrumented scan cycles across all three channels:

Under nominal load, deltas were negligible.
Under peak load, deltas widened — still within tolerance, but noticeable.

Engineering Lessons

Redundancy synchronization has a cost under heavy load
Timing jitter does not equal functional failure
Scan cycle tuning matters more than most teams expect
FAT rarely reflects worst-case site load

What We Changed

Slightly relaxed scan cycle timing
Redistributed logic execution
Reduced burst sampling of non-critical inputs
Added performance monitoring during peak operations

The jitter disappeared without touching hardware.

Closing Thought

The Triconex 3101 did exactly what it was designed to do:
maintain safety first, timing second.

When three processors argue, the system pauses just long enough to make sure they agree — and that pause is part of the design, not a defect.

— Robert Klein

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