Title: Serial Cards and Flow Control with Modems
Keywords: Dflex 115, serial, modems
KDB: KDB-045
Revision: 0.00
Author: Support
Distribution: External
CTI recently had a reported problem where a Dflex-2 would not run properly at 115K baud with USR 28.8K modems.
The customer had XON/XOFF Flow Control turned ON and RTS/CTS Flow Control turned OFF.
******************
THIS IS BACKWARDS
******************
The proper settings are RTS/CTS Flow Control turned ON; XON/XOFF Flow Control turned OFF.
When transferring binary data (like .ZIP and .EXE files) over a modem link, you can NEVER use XON/XOFF Flow Control because these two characters can and will appear in the data stream.
When XON and XOFF appear in the data stream, the flow control mechanisms kick into play and remove the characters from the stream, causing CRC and Timeout errors; plus they do nothing to “pace” the flow of data.
On lower speed links with high speed CPUs, you can quite often get away without turning on any flow control. With the newer higher speed modems, flow control will become increasingly more important so that problems like the one experienced today will be seen more often by people in Tech Support.
End of KDB-045