Communications 9
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).
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.
Host/Printer Port 1
This port, labeled “SES1-EIA,” is designed for connection to the host (computer or
modem) or a serial printer via a 25-pin D-shell (DB25P) female connector. This port
uses an RS-232-C communication interface, is configured as a DTE (Data Terminal
Equipment) device, and can operate from 110 to 134,400 baud. The pins supported
are shown below: