Boundless Technologies ADDS 3153 ASCII - Manual (Page 21)

8 Introduction
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.