Siemens 840SI CNC System HMI Frequent Alarm120203 communication failureInstructions：

The operation panel is connected to NC and PLC through a serial bus.

When MMC DI is started once and NC/PLC boot has not yet ended, or when communication to these components is disrupted, an alarm will appear.

When this alarm occurs, all display values associated with NC/PLC are invalid.

This interference is normal during system startup (such as after reset).

Reaction:Alarm display.

handleOnce the fault situation has ended, the alarm will automatically disappear.

If this alarm keeps occurring, it may be caused by multiple faults. (For example: line disconnection, NC/PLC without upward linear change, incorrect address of bus nodes/data transfer rate configuration)

Wait a minute).

Siemens 840D system HMI alarm 120203The essence of this alarm is the abnormal communication between the operation panel (HMI related MMC human-machine communication unit) and NCK/PLC through the serial bus. Occasional occurrences during the startup phase are normal, but frequent occurrences require troubleshooting and maintenance according to the following steps:

firstExclude temporary alarms caused by normal startup

In scenarios such as system reset and restart, if MMC starts first while NCK/PLC is still booting and initialization is not completed, it may trigger the 120203 alarm. This situation does not require any operation, and the alarm will automatically disappear after the NCK/PLC is started. You can wait for 3-5 minutes to observe whether the alarm self heals, to avoid misdiagnosis.

Check the physical connection of the serial bus

Communication line failure is the high-frequency cause of this alarm. DI opens the control cabinet in one step, finds the serial bus cable connecting the operation panel with NCK and PLC, and checks whether the cable is worn, broken, aged, and whether the joints are loose or oxidized; Next, check if the plug of the bus node is securely plugged in. If there is oxidation, clean the metal contacts of the connector and unplug them again; In addition, the spare cable can be directly replaced for testing to quickly determine whether the cable itself is damaged.

Verify the bus parameter configuration

Mismatching bus address or transmission rate can cause communication handshake failure. Enter the communication parameter configuration interface of the system, verify the address of the bus node, and confirm that there are no conflicts between the addresses of the operation panel, NCK, and PLC (which must comply with the initial configuration or on-site setting specifications of the system); Recheck the data transmission rate parameters to ensure that the rates of the three are consistent, such as common 9600bps, 19200bps, etc. If the rates are not consistent, they need to be modified to match the values. After modification, restart the system to make the parameters effective.

Check the operating status of NCK/PLC

If NCK or PLC does not start normally, it will result in the inability to establish communication with the operation panel. Firstly, observe the status of the indicator lights of the NCK and PLC modules. During normal operation, the indicator lights should display a stable green color. If the indicator lights flash or the red light alarms, it indicates a module malfunction; Subsequently, attempt to perform a reset operation on NCK and PLC. If the reset still fails to start normally, further check whether the power supply of NCK is stable and whether the PLC program is stuck abnormally. If necessary, restore the backup data of NCK/PLC to eliminate startup failures caused by system data confusion.

Check the working status of MMC unit

As the core unit of human-machine communication, MMC's failure will directly trigger communication alarms. Restart the MMC unit and observe whether the alarm is eliminated after the restart; If the restart is ineffective, check whether the power supply voltage of the MMC meets the standard, and connect the MMC through Siemens diagnostic tools to check whether the internal communication driver is normal. If the driver is abnormal, reinstall the driver program. If the MMC module hardware is damaged, repair or replace the module.