Introduction
8
Transmit Flow Control
Likewise, the terminal will understand the Xon and Xoff requests from the
host when it is transmitting data (provided the “EIA (or Aux) Xmt” is “Xon-
Xoff” in Setup). This is referred to as “transmit” flow control.
This terminal will stop transmitting data to the host or printer when it
receives an Xoff (DC3) code. If, however, the terminal needs to send a
receive protocol character, it will transmit that character even if it has
received an Xoff code. When the terminal stops transmitting, the data will
be buffered in the transmit buffer (64 characters for the first host/printer
port and 64 characters for the second host/printer port). Once the buffer
is full, additional keyboard data will be lost. When an Xon (DC1) character
is received, the terminal can again send data to the attached serial device.
In addition to software “transmit” flow control (Xon-Xoff), the serial
host/printer ports support “transmit” hardware flow control, whereby DSR
on the SES1-EIA port and DTR on the SES2-AUX port monitor serial
devices to control the flow of data to them (provided EIA and AUX Xmit is
“DSR” and “DTR” in Setup).
For parallel printers, this terminal monitors the BUSY and ERROR signals
which are sent by the printer to determine when data transmission should
be stopped or resumed.
