serial port read

Has anyone worked with the modbus slave program for rslogix 5000 before. I put it in and it worked ok. I kept on getting crc errors and faulting out the processor. I put in a patch to fix the fault and I thought I did some coding to make it work, but I kept on getting timeouts. Taking a closer look at the SlaveRecMonitor routine I noticed that before they read the serial port they check how many chars. are on the serial port, do some checking and then read the serial port chars again. why would they do this. I put an afi comment on the rung and it works perfect now. Is it possible that I could be getting bad data?
The program is in the project folder in your rslogix 5000 folder(its only in v13 and up) if you want to look at it. Anybody has any clues let me know.

Thanks
Tim