Husky MP2500 Manual page 149

Transmission
Each character sent by the application is placed in the transmit buffer (if
there is room). Characters are then removed and transmitted when the
remote system is ready for them. Transmission would normally be
controlled by handshaking.
If there is no room in the transmit buffer, the application will be made to wait
for room.
13.5.2.2 XON/XOFF
Reception
If an incoming character causes the receive buffer to have 240 or more
unread characters, the MP2500 transmits an XOFF character (DC3, 13H or
Control-S), which should prevent the host sending further characters. When
there are only 100 characters left unread in the buffer, the MP2500 sends
an XON character (DC1, 11H or Control-Q), which should tell the host to
carry on transmitting.
Once the MP2500 has sent an XOFF, it continues sending them every
three seconds until it is ready for more data. It then sends XONs every
three seconds until it receives more data. (This helps make sure that flow is
controlled correctly, even if the host misses the first XON or XOFF.)
Transmission
While transmitting, the MP2500 continuously examines its input data line for
XON or XOFF characters sent by the host.
If an XOFF or stream of XOFFs are received, the MP2500 stops
transmitting as soon as the character being transmitted is finished. It will
only restart transmission when it receives an XON from the host.
By the very nature of this protocol, indeterminate handshaking situations
may occur if any conditions are altered during transmission.
Note: When the host sends an XOFF, the MP2500 may send up to two
more characters, depending on data link speed. This is due to the
time taken to receive the XOFF and the possibility of just missing
the start of the next character.
The host does not need to send an XON at the start of transmission.
Chapter 13: Introduction to Communications 127