The hardware and software setup described below includes a laptop without an RS232 serial port, an OMRON CPM2AH-60 CDR PLC, and the Cx-Programmer V5.0 programming software.
1. **Unstable Communication with Cx-Programmer V5.0 and PLC:**
The computer is connected to the PLC via a USB-to-RS232 cable. However, communication often becomes unstable. The connection process involves multiple steps: from the computer’s USB port through the USB-to-RS232 cable, then to the PLC’s RS232 port. There are two types of cables used for this purpose. The first one follows the official Omron method, while the second uses a conventional RS232 wiring. The first type provides more stable communication, whereas the second type tends to cause frame loss when using the VC application. It was later discovered that sharing a common ground between the computer and PLC resolves some issues. Therefore, it's recommended to use the first type of cable whenever possible.
2. **Communication Error Messages:**
Occasionally, the system displays a message like "Modem has been selected, continue code?" which is actually a typo and should read "continue?" This error typically appears after a line disconnection. To resolve this, users can go to Device Manager, right-click on the modem, and deactivate it. This step has proven effective in restoring normal communication.
3. **Connection Method and Port Configuration:**
It's important to ensure that the correct COM port is being used. On the author's system, the default COM4 was changed to COM1, but the USB port needed to be inserted in a specific position to match the correct COM port. Additionally, it's advised not to plug or unplug the communication cable while the devices are powered on, as this could potentially damage the ports.
4. **Interference During Operation:**
When the system experiences a sudden power-off (like 20 AC contactors turning off at once), communication errors occur. However, during power-on, no such issues were observed. A potential solution is to add an RC module to each contactor coil, though this hasn't been tested by the author yet.
5. **Testing Direct USB-to-RS232 Connection:**
Attempts to connect the computer directly to the PLC via a USB-to-RS232 cable led to errors like "The selected port is occupied by another application." This issue might be due to improper pin configuration. Unlike the standard connection cable, which only connects 2-2, 3-3, and 5-5, a direct USB-to-RS232 connection may attempt to map all pins, leading to potential conflicts or even damage to the communication port.
Overall, careful attention to cable selection, proper pin configuration, and ensuring a shared ground are crucial for stable communication between the computer and the PLC.
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