Serial cards and flow control with modems.
Title: Serial cards and flow control with modems.
Keywords: Dflex 115, serial, modems
kdb number: kdb-045
Dist: 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.
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THIS IS BACKWARDS.
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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.