Masters often encounter the following issue: Why can’t my touch screen communicate with the PLC? Here are two common problems:
1. Weintek touch screen cannot communicate with Siemens PLC, always prompting that the PLC is unresponsive. What settings should I use?
Solution approach:
You need to set the same baud rate, the same data bits, the same stop bits, and the station number must not be the same, meaning the addresses of the PLC and the touch screen cannot be identical. Then check if the communication line is normal…
2. How to determine if there is a communication failure between the touch screen and PLC through programming?
Solution approach:
The usual method is to use heartbeat detection, define a boolean, 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 it does not receive this point as an ON signal within a certain period, say 5 seconds, it is considered a communication interruption.
The PLC sends the memory clock byte to the screen, which uses a script to assign the value of this byte to another address inside the PLC. Then the PLC checks if the returned byte is the same for two scanning cycles. If they are the same, it indicates a communication interruption.
For example, see the diagram, the steps are as follows:
1. Create a DB1 data block in the PLC, with two switch variables: “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 – Events – Value Change, add “Invert”, so that the “HMI Response Switch” variable changes with the “PLC Second Switch”;
4. In the PLC program block, program the “PLC Second Switch” to reverse every 0.5 seconds. Then use the TON delay instruction to output “HMI communication failure” if the “HMI Response Switch” does not act within 1 second, because after the HMI communication exception, the “HMI Response Switch” will no longer change.
In fact, any touch screen that cannot communicate with the PLC usually involves confirming four issues:
1. Are the PLC parameters consistent with 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 the parameters and lines are confirmed to be OK, check if there is an issue with the PLC program or the PLC address.
Method:
First, check the parameters:
1. Use the PLC programming software to connect to the PLC and test the PLC parameters to see if they match the parameters set in the project.
2. Online simulation: Use our configuration software to connect the PLC with its communication cable to the computer and simulate online to see if the project can communicate. You can use a numeric input component or switch to operate it, and check if the previous operations remain after turning off the simulator and then back on, or if it directly prompts NC. (NC indicates that the previous operations were not written down, meaning communication has failed.)
Testing the line:
Use a multimeter to test the wiring according to the wiring diagram.
1. Touch screen parameters; check the parameter settings of the touch screen.
There are several parameters that need special attention:
1. Communication port settings – be sure to confirm whether the PLC is connected to COM1 or COM2 of the touch screen.
2. Device type – this is the most important; if the protocol is not selected correctly, there is no need to discuss the rest.
3. Connection method – confirm whether the connection between the PLC and the touch screen is RS485 or RS232C.
4. Interface parameters and PLC station number – must be consistent with the settings inside the PLC.
2. If the parameter settings are confirmed, proceed to check the line issues.
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 method help document) to check; this is the basic method for troubleshooting communication issues.
Next, I will teach you how to bypass the touch screen issue – online simulation. When communication fails, some customers may suspect it is the touch screen or the interface issue. Online simulation bypasses the touch screen and connects the PLC directly to the computer.
Specific approach:
1. Connect the PLC to the computer via RS232. Some PLCs have an RS232 interface, and some do not; if not, you can connect to the computer via an adapter.
2. Create a simple project. Place two components: a numeric display and a numeric input. Set the address to the address inside the PLC.
3. Ensure the project parameter settings match those in the PLC.
4. Click the online simulation function.
This way, you can clearly see if the PLC can communicate with the PC. If communication is successful, you can rule out issues on the PLC side and parameter settings.
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