Troubleshooting Touch Screen and PLC Communication Issues

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Often, technicians encounter the following problem: Why can’t my touch screen communicate with the PLC?

For example, here are two common issues:

1. Weintek touch screen cannot communicate with Siemens PLC, always prompting that the PLC is unresponsive. How should I set it up?

Solution Approach: Ensure that the baud rate, data bits, and stop bits are the same. The station numbers must not be identical, meaning the PLC and touch screen addresses cannot be the same. Then verify if the communication line is functioning properly…

2. How to programmatically determine if there is a communication failure between the touch screen and PLC?

Solution Approach: The usual method is to use heartbeat detection. Define a boolean variable, and the HMI will set this point at a fixed frequency. After the PLC receives this point as an ON signal, it will reset it. If, within a certain time frame, for example, 5 seconds, the PLC does not receive this point as an ON signal, it will be considered a communication interruption. The PLC sends the memory clock byte to the screen, and the screen uses a script to assign this byte’s value to another address within the PLC, then the PLC checks if the returned byte is the same over two scanning cycles. If they are the same, it indicates a communication interruption.

For example, see the image, and follow these steps:

1. Create a DB1 data block in the PLC containing two digital switches: “PLC Second Switch” and “HMI Response Switch”;

2. Connect these two variables in the HMI variables;

3. In the properties of the HMI “PLC Second Switch” variable—event—value change, add “invert value” so that the “HMI Response Switch” variable changes along with the “PLC Second Switch”;

4. In the PLC program block, program the “PLC Second Switch” to toggle every 0.5 seconds, and use the TON delay instruction to output “HMI communication failure” if there is no action from the “HMI Response Switch” within 1 second, as the “HMI Response Switch” will no longer change after an HMI communication exception.

Good tips, feel free to take them~

In fact, any communication issue between a touch screen and PLC usually boils down to four key questions:

(1) Are the PLC parameters consistent with those in the project?

(2) Is the communication line connected according to the wiring diagram?

(3) Is the COM port set in the project correctly connected to the screen?

(4) If the parameters and wiring are confirmed to be OK, check if there is an issue with the PLC program or its address.

First, check the parameters

Use the PLC programming software to connect to the PLC and test its parameters. Check if the parameters set in the project match those obtained from the test. Several parameters need special attention:

(1) Communication port settings: Confirm whether the PLC is connected to the touch screen’s COM1 or COM2;

(2) Device type: This is crucial; if the protocol is not selected correctly, there is no need to discuss further;

(3) Connection method: Confirm whether the PLC and touch screen are connected via RS485 or RS232C;

(4) Interface parameters and PLC station number: These must match the settings in the PLC.

Online Simulation

Use our configuration software to connect the PLC to the computer with the PLC’s communication cable and perform online simulation to see if the project communicates correctly. You can use a numeric input component or switch to operate it, and check if the previous operations remain after closing the simulator and reopening the online simulation, or if it directly prompts NC. (If NC appears and the previous operation is not recorded, it means there is no communication.) The specific method is as follows:

(1) Connect the PLC to the computer via RS232. Some PLCs have RS232 interfaces, while others do not; if not, you can connect to the computer via an adapter.

(2) Create a simple project. Place two components, one numeric display and one numeric input. Set the address to match the address in the PLC.

(3) The project parameters must match the settings in the PLC.

(4) Click the online simulation function.

This way, you can clearly see whether the PLC can communicate with the PC. If communication is successful, you can rule out issues from the PLC side and parameter settings.

Test the Wiring

Use a multimeter to test the wiring according to the wiring diagram. Confirm that the RS485 and RS232C connections are correct, as the wiring methods may differ for various PLCs. This can be referenced in the manual, and this is a fundamental method for troubleshooting communication issues.

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Troubleshooting Touch Screen and PLC Communication Issues

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