What Is a PLC Communication Error?

By Laurie Reeves

A programmable logic controller (PLC) can be found in a business or industrial setting. It typically is used to control electromechanical equipment in a complex setting such as a manufacturing assembly line or for simple applications, such as turning on and off the lights a business uses at night or during the day. A PLC communication error happens when the PLC doesn't respond to communication requests or responds negatively to such requests. A PLC communication error can occur between the programmable computer and the devices it controls or when the technician attempts to program the device from afar.

Error Codes

Programmable logic controllers are built by a variety of manufacturers and are designed for specific uses. Each company has a series of error codes that refer to its specific PLC. For instance, a timeout error can cause a device to not communicate properly, or a checksum error can be generated when the communication package is the incorrect size. Another communication error can occur when the PLC does not reply to the last message request or does not have enough time to respond to the request.

Communications

PLCs can communicate via serial ports to other equipment or by hooking up a monitoring device on the RS-232 modem port available on the PLC. A modem connection that isn't seated properly in the port can cause a communication error to occur when trying to access the device using a cellular telephone or other communication device. When connected to a remote monitoring device, a technician can program the PLC from afar, send and receive data and view PLC logs. Interruptions in the signal will also cause communication errors.

Defective Parts

Defective communication boards within a PLC can also be a cause of communication errors. A "receive" or "transmit" error can occur during communications, resulting in a lack of all the data being received or sent by the PLC. Other communication errors can occur because of faulty communication cabling, or if water gets inside the communication cable connection points. Any of these types of error typically require a visual inspection of the device; none of them can be fixed by accessing the device from afar.

Software and Firmware Updates

As with any computer equipment, to prevent communication errors and other problems, the software on the device must be compatible and up-to-date with all the other devices with which it interfaces. Setting the device so that it automatically searches for firmware or software updates or scheduling it for updates can prevent communication errors from occurring. Power outages can also affect the PLC and may require a reboot to reset the device for communication and avoid these errors.