A MicroLogix 1400 sends a ViewMarq command through an AWA instruction, but ARL reads the transmitted command followed by a carriage return instead of the expected OK^M^J. The captured response confirms that the command leaves the controller and returns to its receive path; it does not confirm that the ViewMarq accepted or executed the command.
Observed RS-232 Behavior
| Item | Observed value | Engineering conclusion |
|---|---|---|
| Controller | Allen-Bradley MicroLogix 1400 | The PLC uses ASCII instructions for this exchange. |
| Write instruction | AWA | The complete command appears in the receive data. |
| Read instruction | ARL | ARL completes immediately after transmission. |
| Expected reply | OK^M^J |
This reply was not received. |
| Actual reply | Transmitted command plus ^M
|
The receive path contains an echo or returned copy of the outbound data. |
Interpret the Returned String
The immediate character-for-character return is the key diagnostic result. Because the data matches the outbound command, separate the question “Did bytes leave the PLC?” from “Did the ViewMarq parse the command?” The evidence supports the first conclusion but not the second.
The appended ^M identifies a carriage return in the captured data. The expected reply contains both ^M and ^J, so do not treat the returned command plus carriage return as a valid acknowledgment.
Isolate the Echo Source
- Capture the AWA source string and the ARL destination string for the same transaction. Confirm that the returned payload matches the command exactly before the final
^M. - Determine whether the returned characters originate from the ViewMarq or from an RS-232 loopback, echo setting, wiring path, or intermediate interface. The current capture alone cannot distinguish these cases.
- Review whether AWA or AWT is the intended write instruction for this transaction. AWT was specifically raised as an alternative, but the available evidence does not establish which instruction the ViewMarq requires.
- After changing only one communication variable, transmit the command again. Accept the test only when ARL receives the documented acknowledgment rather than the outbound command.
Verification Criteria
Use the following command as the controlled test payload:
<ID 1><CLR><WIN 0 0 287 31><POS 0 0><SL><S S><BL N><CS 0><GRN><T>Hello Duke</T>A successful diagnostic result requires ARL to return OK^M^J. If ARL still returns the command followed by ^M, the echo condition remains and command acceptance is unverified.
FAQ
Why does MicroLogix 1400 ARL return the command I sent?
The exact returned copy indicates that transmitted data is reaching the receive path. Check for device echo, loopback, wiring, or an intermediate interface before assuming the ViewMarq parsed the command.
Does an immediate ARL response prove ViewMarq communication works?
No. It proves that ARL received characters, but the expected acceptance response is OK^M^J; a mirrored command plus ^M does not verify command execution.
Should I use AWA or AWT with a ViewMarq display?
AWA produced the complete echoed string in this case, while AWT was identified as a diagnostic alternative. The evidence does not establish the required instruction, so verify the write method against the applicable configuration documentation and confirm the result by receiving OK^M^J.