Trigger ACK Arrived, But Was That This Part's Result? Cognex In-Sight–PLC Handshake
MACHCTRL · machine-vision controls | Manufacturer scope: supported Cognex In-Sight firmware 5.1.0+, PROFINET IO modules and the matching GSD. All IDs and timelines below are synthetic teaching examples, not field measurements or validated PLC logic.
The PLC issues a trigger and receives an acknowledgment. Yet the displayed NG may belong to a previous part. Before changing vision thresholds, establish which signal means trigger received, which means inspection finished, and which means this result is ready.
ACK does not mean inspection result
| Signal | What it indicates | What it does not prove |
|---|---|---|
| Trigger Ready | The vision device can receive an acquisition trigger | A new result exists |
| Trigger Ack | The trigger transition was observed | Inspection is complete |
| Exposure Complete | Image exposure has completed | Inspection/result transfer is complete |
| Inspection Completed | Toggles on an inspection completion | A steady high means a new result |
| Results Valid | Inspection data is available | The data belongs to the current part without ID correlation |
Cognex documents that Trigger Ack and Missed Acq may both be asserted when a trigger is issued offline. Do not classify acquisition as successful based on Trigger Ack alone.
Nine-step synthetic example with result buffering enabled
The illustrative Acquisition ID=108 and Inspection ID=108 below are not actual line data. Confirm IO offsets and status mappings against the device model, firmware and GSD.
| Step | Illustrative signal | PLC-side interpretation |
|---|---|---|
| 0 | Online=1; Trigger Enable=1; Trigger Ready=1 | Ready for a new acquisition, not a new result |
| 1 | Trigger ↑; Trigger Ack=1; Acquisition ID=108 | Capture the ID and check Missed Acq |
| 2 | Trigger=0; Trigger Ack=0 | Release the request and confirm acknowledgment clears |
| 3 | Exposure Complete=1 | Exposure ended; inspection may still be running |
| 4 | Inspection Completed toggles; Results Valid=1 | Detect a new result event and prepare to read |
| 5 | Inspection ID=108; Code=NG | Correlate IDs; latch code and payload together |
| 6 | Inspection Results Ack=1; Results Valid→0 | Acknowledge only after copying the result set |
| 7 | Inspection Results Ack=0 | Release ACK to permit the next buffered result |
| 8 | Overrun=0; check ID progression | Check for omissions/duplicates before proceeding |
This is an explanatory state sequence, not a drop-in program for a production reject mechanism or safety interlock.
Five combinations worth logging
| Observation | What to investigate |
|---|---|
| Trigger Ack=1 / Results Valid=0 | No readable inspection result yet |
| Trigger Ack=1 / Missed Acq=1 | Offline or acquisition failure, not success |
| Exposure Complete=1 / Results Valid=0 | Exposure finished but result is not ready |
| Results Valid=1 / Inspection ID mismatch | Hold classification until it can be correlated to the part |
| Results Buffer Overrun=1 | Possible lost results: inspect IDs and the event ledger |
Buffering is not a substitute for acknowledgment
Without result buffering, a newer inspection can overwrite an unread previous result. The applicable Cognex PROFINET reference describes up to eight buffered inspections; once full, further new results may be dropped. Reliable matching requires the PLC to capture the correct result, toggle result ACK as documented, and correlate inspection and acquisition IDs. Inspection Completed itself toggles after each inspection and must not be interpreted as a level-only completion flag.
This applies to the documented In-Sight 5.1.0+ PROFINET IO interface, not all Cognex cameras or EtherNet/IP/SLMP address layouts. Device-specific behavior and PLC scan coordination must be tested separately.
Before production use: verify model/firmware/GSD and addressing, record trigger/ACK/Results Valid/ID/Overrun in an isolated test, and review reject timing and safety dependencies independently. No real camera job or PLC was changed to produce this article.
Primary sources: Cognex In-Sight PROFINET IO reference · Typical acquisition sequence