Usually in my experience the serial port is used to catch the data from the scanner. I guess that you could command the ASCII port to enable the scan, but this is not the way that I would do it. I would just use the ASCII port for reading and writing, and use a P/E to discretely enable the scan for a pre-determined time.
Lots of examples. Look in the AB knowlegebase.
There is a Control-logix example (Not an SLC but similar) on the RS5000CD as well.