A CPU224 that communicates through a PPI cable in STOP but not in RUN has a mode-dependent communication problem, not a basic connection failure. The reported working settings are COM port 1 at 9600 baud. The leading hypothesis is that the PLC communication port is assigned to Freeport or another program-controlled protocol while the CPU is running.
Interpret the STOP-versus-RUN Test
| CPU state | Observed result | Engineering conclusion |
|---|---|---|
| STOP | PPI connection establishes immediately | The cable, selected PC COM port, and 9600-baud setting can establish communication. |
| RUN | No PPI connection | Investigate runtime ownership or reconfiguration of the PLC communication port. |
This comparison does not prove that Freeport is active, but it makes a static PC-side configuration error less likely. Keep the distinction clear: COM port 1 identifies the selected computer interface, while the suspected conflict concerns how the CPU communication port operates in RUN.
Check the CPU Communication Configuration
- Connect with the CPU in STOP using the known working COM port 1 and 9600-baud configuration.
- Open the System Block and review the CPU communication settings, including the configured address, speed, and protocol-related options.
- Inspect the program for writes to system memory bytes that configure Freeport or another protocol. Use the MicroWin help for the applicable system-memory definitions; the evidence does not identify specific byte addresses.
- If PPI programming access is required in RUN, remove or revise the runtime configuration that takes ownership of the PLC port, then download the corrected configuration.
Do not attempt to change communication settings through Module Information. That view reports hardware information, errors, and assigned addresses; configuration changes belong in the System Block or in the program logic that writes communication-related system memory.
Diagnose the EM231 Module
In Step7 MicroWin, open the PLC menu and select Module Information. Review the EM231 entry for reported errors, diagnostic information, and assigned module addresses. This is the relevant view for investigating the red front LED and confirming where the module data appears in the CPU address map.
The available evidence does not identify a specific EM231 diagnostic code or prove why thermocouple signals are absent. Record the displayed module error and address information before changing wiring or configuration; those results determine the next diagnostic branch.
Verify the Repair
After changing the runtime port configuration, place the CPU in RUN and reconnect with the same PPI cable, COM port 1, and 9600-baud setting. A successful RUN-mode connection confirms that runtime port ownership caused the symptom. Separately, reopen Module Information and verify whether the EM231 still reports an error and whether its assigned addresses match those used by the program.
FAQ
Why does a CPU224 connect over PPI in STOP but not RUN?
The leading cause is that Freeport or another program-controlled protocol takes control of the PLC communication port in RUN. Check the System Block and communication-related system-memory writes.
Can I change CPU224 communication settings in Module Information?
No. Module Information displays hardware details, errors, and addresses; review standard communication settings in the System Block and program-controlled protocol settings in the applicable system memory.
How do I check an EM231 red LED in Step7 MicroWin?
Open PLC > Module Information and inspect the EM231 diagnostics, reported errors, and assigned addresses. The red LED alone does not identify the fault, so use the displayed diagnostic result to choose the next check.