I get an error id OL9-1631 when I try to connect a pc to a Modicon PLC via serial interface. I was told to cycle power to clear a stuck bit however the error id is still indicated when trying to connect.

What's the error OL9-1631? What prevents me from connecting the PLC and PC via the Modbus serial comm link?

 

Sounds like you are trying to use Modbus RTU through WIndows NT4, which never worked very well from Concept. If so, give up now and use Ethernet or Modbus Plus or try Modbus ASCII. Good luck.