Log: I/O Loop Check and Signal Verification Record

This is a ready-to-use loop check log for control system installation and operational qualification. Replace every <<FILL: ...>> placeholder with your own specifics, set your document numbers and dates, and route it through your normal document control. A worked filled specimen follows the blank grid. Verify each cited reference against the current source before you rely on it.

A loop check proves that a physical signal travels correctly from the field device to every place the value is used and recorded. For an input, you inject or apply a known value at the field and confirm the value reads correctly at the controller, at the HMI, and in the historian. For an output, you command a known value and confirm the field element moves as commanded. This is the single test that catches scaling errors, range mismatches, wiring errors, and interface data loss, and it is the backbone of an automation IQ and OQ.

Document control header

1. Purpose

This record documents the verification that each I/O point on <<FILL: SYSTEM NAME>> reads or drives its signal correctly from the field to the controller, the HMI, and the historian, so that recorded process values are accurate and traceable. It provides objective evidence for the loop check portion of installation and operational qualification.

2. Scope

This record covers every I/O point listed in the approved I/O list, revision <<FILL: I/O list revision>>, for <<FILL: SYSTEM NAME>>. It applies to analog inputs, analog outputs, discrete inputs, and discrete outputs. Points designated critical (those that affect product quality, patient safety, or a GMP record) are verified at 100 percent; the sampling basis for any non-critical points is defined in the parent protocol and stated in section 4.

3. Definitions

• Loop check: an end-to-end test that a known field signal produces the correct value at every point it is read, recorded, or acted on.
• Injected value: the known reference applied at or near the field device, for example a calibrated current source at 12 mA, a temperature bath at a set point, or a forced contact state.
• As-found / as-left: the reading before and after any adjustment; a loop check should pass as-found when the system is correctly configured.
• Live zero: the 4 mA bottom of a 4-20 mA range, so that 0 mA signals a fault (broken wire or dead transmitter) rather than a real process zero.

4. Instructions

1. Confirm the field instrument used to inject or read the signal is calibrated and in date; record its ID and due date in the header block of each sheet.
2. Work down the approved I/O list. For each point, record the tag, description, type, and configured range from the I/O list before testing, so a range mismatch is visible.
3. For an analog input, apply at least three points across the range (commonly 0, 50, and 100 percent, for example 4, 12, and 20 mA) and record the value shown at the controller, at the HMI, and in the historian at each point. Confirm each reads the expected engineering value within the acceptance tolerance.
4. For a discrete input, force each state (for example wet/dry, open/closed) and confirm the correct state at the controller, HMI, and historian, and that any associated alarm behaves as specified.
5. For an analog output, command at least three points and confirm the field element position or the measured output matches the command within tolerance.
6. For a discrete output, command each state and confirm the field device actuates (valve strokes, pump starts).
7. Record any deviation, raise it per <<FILL: SOP-ID for deviations>>, and re-test after correction. Do not sign a point as passed until it passes as-left.
8. The tester signs and dates each sheet; the reviewer independently verifies the entries and signs.

5. Acceptance criteria

A loop check point is acceptable when all of the following are true:

• The configured range in the system matches the physical range of the field instrument.
• At each tested point the controller, HMI, and historian values agree with the injected or commanded value within the tolerance stated in the parent protocol (a common default is plus or minus 0.5 percent of span for analog loops, or the loop accuracy defined by risk).
• Discrete points show the correct state at every location and any linked alarm behaves as specified.
• Outputs drive the field element to the commanded state.
• Every deviation is recorded, investigated, corrected, and the point re-tested to a pass.
• The record is complete, signed by the tester, and independently reviewed.

6. The loop check grid (blank)

Test reference instrument: <<FILL: source/meter ID>>, calibration due <<FILL: date>>.

Tester: <<FILL: name, signature, date>> Reviewer: <<FILL: name, signature, date>>

7. References

21 CFR 211.68 (automatic, mechanical, and electronic equipment), 211.63 and 211.67 (equipment suitability and maintenance). EU GMP Annex 11 (computerised systems) and Annex 15 (qualification and validation). GAMP 5 Second Edition, ISPE, risk-based approach to GxP computerized systems (referenced by title; described, not reproduced). The site calibration and metrology program for the reference instruments used.

Confirm the current version and clause numbers of each reference before issue.

8. Revision history

9. Approvals

Filled specimen

The following shows the grid completed for one analog input loop on an example bioreactor skid, so you can see the level of detail expected. The system, tags, and numbers are illustrative; replace them with your own.

Test reference instrument: calibrated current source/meter CAL-CS-04, calibration due 30 November 2026.

Tester: J. Okoro, signed, 14 July 2026 Reviewer: M. Devi, signed, 15 July 2026

In this example the temperature loop passed cleanly across the range, but the pressure loop failed: the injected 20 mA read 99.8 psi instead of the expected 60 psi. The tester found the transmitter had been replaced with a 0-100 psi unit while the controller scaling still read 0-60 psi, so every recorded pressure had been wrong since the swap. A deviation was raised, the scaling was corrected to match the installed transmitter, the historian impact was assessed, and the loop was re-tested to a pass before the point was signed off. That sequence, mismatch found to deviation to corrected scaling to re-test, is exactly what the loop check exists to catch.

Common inspection findings this log prevents

• Recorded process values are wrong because the configured range does not match the installed transmitter, and no loop check caught it.
• The value is confirmed at the HMI but never traced through to the historian, so an interface transformation or data loss goes undetected.
• Loop checks are signed as passed with no recorded injected values, so there is no evidence any signal was actually applied.
• Critical points are sampled rather than verified at 100 percent, with no documented risk basis for the sampling.
• A failed point is corrected but never re-tested to an as-left pass.

How to adapt this log

1. Set your document number, parent protocol, and system ID in the header.
2. Pull the tag list directly from your approved, version-controlled I/O list so the log and the design agree.
3. Set the analog tolerance in section 5 from your loop accuracy requirement and risk assessment, not a generic default.
4. Point the deviation cross-reference to your real deviation procedure.
5. Where a value must also reconcile into an MES electronic batch record, add an EBR column and confirm the value there too.