I would invite you to think about on old DOS based PC if you gave the command:

TYPE BIGFILE.TXT

...then the screen would scroll too fast to read. It was/is possible to use the CTL-S key to start stop the screen output in a sort of flow control mechanism. The CTL-S would stop output and then another key would restart it.

For a XON-only type of Flow Control to function there are three senarios to consider, one of which may describe how your device operates.

1) The device will stop sending upon receipt of the XON control character. Then upon receipt of another XON character it will resume sending. This type of toggle behavior would work for most cases but does have the problem that you could get out of sync with the device. On devices where I have seen this method used the device will reset the "stopped" flag upon receipt of a new command packet (which incidently will most likely be started with a different control character such as <ESC> or <SOH>).

2) The device will stop sending upon recipt of an XON character and will then resume sending upon receipt of any other character or a timeout period.

3) The device, after having data ready to send, will wait and not send ANY information until it receives an XON character. Then it will send a fixed amount of data. When I have seen this mode used the device typically sends one record of data with the record size fitted to the function of the device. After the record is completed the device halts sending until the next XON character is sent.

Hope this info helps you to discover just how your specific device operates.
Michael Karas