Boundless 4000/160, User Manual, Page: 29 / 107

10 Introduction
If the “EIA (or Aux) Rcv” is “No Protocol” in Setup, the terminal
will continue to accept characters into its receive buffer until it
is full. Additional characters will be lost. Xon-Xoff protocol must
also be set on the host computer or printer for proper
handshaking.
In addition to software “receive” flow control (Xon-Xoff), the serial
host/printer ports support “receive” hardware flow control. The
SES1-EIA port has an outgoing DTR (Data Terminal Ready) signal.
If “EIA Rec” is “DTR” in Setup, and the terminal’s receive buffer
fills to the level mentioned above, the terminal will set the DTR
signal low to inform the host (serial) device to stop sending data.
On the SES2-AUX port, if “Aux Recv” is “DSR” in Setup, the
outgoing DSR (Data Set Ready) signal serves to signal the host
(serial) device that the terminal is not ready to receive more data.
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. 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. 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 are “DSR” and “DTR”, respectively, in
Setup).