Machine Handshake Fault

Walt — Simple Man Takeaway

A handshake fault is two machines talking past each other. Find out who spoke, who heard, and who never answered.

Plain-English Summary

Use this flow when two systems do not agree on ready, request, busy, complete, reset, fault, or transfer timing.

R.E.A.L. firstRead-only firstCapture before changingReverse-trace failed conditionsTrap intermittent ghosts

First-Check Flow

1. Identify both sides of the handshake

Name the sending side and receiving side. Confirm which controller or station owns each signal.

2. List the handshake bits

Record request, ready, busy, complete, fault, reset, clear-to-send, clear-to-receive, and transfer signals.

3. Determine who is waiting

Find which side is ready and which side is blocking the next move.

4. Confirm signal direction and ownership

A bit may be consumed in one PLC but owned by another PLC, robot, drive, or remote I/O device.

5. Reverse-trace the failed handshake bit

Find the OTE/result that drives the false or stale handshake condition.

6. Trap timing-window failures

If both machines appear correct after the event, arm Ghost Busting™ on request/ready/busy/complete timing and context lock the transfer window.

Field Checks

Both sides of the handshake identified
Signal owner and consumer confirmed
Request/ready/busy/complete/fault/reset states recorded
Network/remote I/O/fieldbus health checked
Timing windows and unlatch/reset behavior checked
Failed handshake bit reverse-traced to source
Context window captured if issue is intermittent

Watch Out For

Both machines waiting for the other side first
Wrong bit ownership assumption
Stale remote I/O or network status
Reset/unlatch clears evidence before maintenance arrives
One PLC ready while another machine is faulted or not in auto
Handshake works slowly enough to miss a timing limit

Reverse-Trace or Ghost Busting™?

Use Reverse-Trace Logic Solving™
when the failure is stable and visible live. Open the PLC program, monitor in RUN mode, chase only failed conditions, find the driving OTE, and repeat failure-to-failure.

Use Ghost Busting™
when the failure is intermittent, self-clearing, timing-based, or disappears before maintenance sees it. Select the suspect routine from the HMI and let the comparator trap expected-vs-actual mismatch every scan.

Find the failed result. Identify the blocking condition. Reverse-trace only what is failing. If the truth disappears, trap the ghost.

Related Calculators / SWAT Screens

Industrial Controller™ Remote I/O vs Local I/O Planner Industrial Controller™ Ethernet/IP / Modbus TCP Device Count Readiness

Walt says STOP! - Safety First

Make these checks prior to proceeding.

Do not bypass safety, force outputs, defeat guards, or work live beyond your qualification and site procedure. If motion, stored energy, live electrical exposure, lockout/tagout requirements, or personnel inside guarded areas are involved, stop and follow the required safety process.