Why Can’t Your Touch Screen Communicate with PLC? Here Are the Reasons!

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Users often encounter this problem: Why can’t my touch screen communicate with the PLC? For example, here are two issues:

1. The Weintek touch screen cannot communicate with the Siemens PLC, always prompting that the PLC is unresponsive. What settings should be made?

Solution approach:

Ensure that the baud rate, data bits, and stop bits are the same, and the station numbers must not be the same, meaning the addresses of the PLC and touch screen cannot be identical. Then, verify whether the communication line is functioning correctly…

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

Commonly, a heartbeat detection method is used. Define a bool, and the HMI sets this point at a fixed frequency. After the PLC receives the ON signal for this point, it resets it. If the ON signal is not received within a certain period, say 5 seconds, it is deemed that communication has been interrupted.

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. The PLC then checks if the returned byte is the same for two scanning cycles. If they are the same, it indicates a communication interruption.

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

1. Create a DB1 data block in the PLC, and set 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 variable “PLC Second Switch” — Events — Value Change, add “Invert”, so that the “HMI Response Switch” variable changes with the “PLC Second Switch”;

4. Program in the PLC program block to reverse the “PLC Second Switch” every 0.5 seconds, and use the TON delay instruction to output a communication failure if the “HMI Response Switch” does not act within 1 second, because after a communication exception, the “HMI Response Switch” will no longer change.

In fact, any touch screen on the market not communicating with the PLC usually requires checking four issues:

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 on the screen?

4. If parameters and lines are confirmed to be okay, check if it is a PLC program or address issue.

Method:

First, check the parameters:

1. Use the PLC programming software to connect to the PLC and test to see what the PLC parameters are, and whether they match the parameters set in the project.

2. Online simulation: Use our configuration software to connect the PLC itself with the communication cable to the computer, and simulate online to check if the project can communicate. You can use a numeric input component or a switch to operate it, and check if the previous operations remain after turning off the simulator and then re-enabling online simulation. If it directly prompts NC (NC means that the previous operation was not recorded, indicating no communication).

Test the line:

Use a multimeter to test the connections according to the wiring diagram.

01Touch Screen Parameters

Check the touch screen’s parameter settings; there are several parameters that need special attention:

1. The communication port settings — be sure to confirm whether the PLC connects to COM1 or COM2 on the touch screen.

2. Device type — this is the most important; if the protocol is not selected correctly, then nothing else matters.

3. Connection method — confirm whether the connection between the PLC and touch screen is RS485 or RS232C.

4. Interface parameters and PLC station number — must match the settings in the PLC.

02Confirm Parameter Settings and Check Wiring

Confirm whether the wiring for RS485 and RS232C is correct; the connection methods between the touch screen and various PLCs differ. This can be referenced in the Weikong (PLC and touch screen communication wiring help document) as a basic method for troubleshooting communication issues.

Next, I will teach you how to bypass touch screen issues — online simulation. When communication fails, some customers may guess that it might be a problem with the touch screen or the interface. Online simulation bypasses the touch screen and connects directly to the PLC with the computer.

Specific steps:

1. Connect the PLC to the computer via RS232. Some PLCs have RS232 interfaces, while others do not; if not, a converter can be used to connect to the computer.

2. Create a simple project with two components: a numeric display and a numeric input. Set the address to the address in the PLC.

3. Ensure that the project parameter settings match those in the PLC.

4. Click on the online simulation function.

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

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