By Dovydas Kersys · Chapter 14 of 58 in Machine Commissioning.
Before you begin: Follow the machine documentation, risk assessment, site procedures and lockout/tagout requirements. Commissioning can expose personnel to electrical energy, stored pressure and unexpected motion. Never defeat a safety function merely to make a test easier. Examples and diagrams are generic and must be checked against the actual machine.
A digital input loop check proves the complete path from field condition to PLC tag and operator indication. The best test changes the real field device rather than forcing the PLC input.
A loop check follows the signal from the field all the way to the operator interface.
Why this matters during commissioning
Input mistakes often survive visual wiring inspection because the signal may arrive at a valid but wrong channel. Identity is as important as voltage.
Commissioning procedure
- Place the machine in a safe state where the input can be actuated.
- Operate the real switch, sensor or contact.
- Confirm the correct PLC input LED/channel changes.
- Confirm the intended software tag changes with correct polarity.
- Confirm HMI indication or diagnostic text is correct.
- Test both states and record any debounce or timing requirement.
Common failure patterns
- Normally closed device interpreted as normally open.
- Two sensor plugs swapped.
- Input LED changes but wrong PLC tag is mapped.
- HMI shows inverted status.
Evidence to record
- Device tag
- PLC address / tag
- Expected inactive state
- Expected active state
- Observed result and correction
FIELD RULE: Test the real device whenever possible. A software force proves logic, not field wiring.
Appendix B. Digital I/O Loop-Check Sheet
Print or duplicate this page as needed. Record actual measurements and references rather than relying only on check marks.
| Tag | Type | PLC address | Expected state | Observed | Result |
|---|---|---|---|---|---|
Continue the series
- Previous chapter: Software Backups and Version Control: Commissioning Guide
- Next chapter: Digital Output Loop Checks: Commissioning Guide
- Browse all Machine Commissioning chapters
Adapted from Machine Commissioning for Maintenance and Automation Technicians, by Dovydas Kersys. © 2026 Dovydas Kersys. Featured cover image is an AI-generated editorial illustration.