PictureTel Concorde 4500 Servicing Manual page 189

Flow Control Between DTE Devices
ASCII codes DC1 and DC3 (decimal 17 and 19 in the ASCII code
table), provide the ßow control functions transmit on (or Xon) and
transmit off (or Xoff) respectively.
The Concorde¥4500 sends a DC1 (Xon) code to signal the device to
start or resume sending data. A DC3 (Xoff) code signals the terminal
device to stop sending data.
D-9