PLC RS232 serial port interfacing

When a host computer tries to send a control bite data to PLC via RS232, is an "Open Examine Contact" needed to active PLC serial port communication commands?

Thanks

Every PLC manufacturer has it's own protocols for dealing with the (if there is one) CPU serial port. Many allow switching the port from system mode (default for programming) to user mode (useful for printing, or talking to external devices).

You'd have to try to find either the protocol for your particular PLC, and write a driver for it, or use an OPC driver; either from the PLC manufacturer or a third party.

Good luck!