User`s guide

9
Communications
This terminal is equipped with three communications ports. The two serial ports are intended for connection to
either a host (or modem), an alternate input device such as a scanner, or a serial printer. (The EIA port has both
an RJ45 and a DB25 interface; either, but not both, can be used.) The last port is a Centronics-compatible
printer port which could be used with a parallel printer or other output device (such as a cash drawer) connected
to a parallel interface. The port you use as the printer port depends on whether you have a serial or a parallel
interface to the printer. Another factor affecting your port selection is whether the terminal is being used in a
single or dual host environment.
Either “EIA” serial port (DB25 or RJ45) can communicate with your computer or printer at a baud rate of 110 to
115,200 baud (bits pe
r s
econd). The second host/printer serial “Aux” port can communicate at speeds from 110
baud to 38,400 baud. Any of these ports can be used to connect to a host computer/modem or serial printer.
Receive Flow Control
Because devices can receive data faster than they can process it, data flow control (selectable in the PORTS
menu of Setup) should be used to prevent data loss. Software flow control relies on the Xon and Xoff characters
(“g” and “e” characters in PC-Term mode) to indicate when the terminal is able or unable to store further data.
The Xon signal transmits the DC1 character (11h) and the Xoff signal transmits DC3 (13h).
This terminal has a receive buffer capable of holding 256 bytes (or characters). When the buffer becomes half full
(128 bytes), a
nd “EIA (or Aux) Rcv” in the Setup menu for PORTS is “Xon-Xoff,” the terminal issues an Xoff
character, indicating to the host that it should stop transmitting data. The terminal will then continue to process
data until its receive buffer holds only 32 bytes. It will then issue an Xon character to the host, indicating that it
can resume sending data to the terminal.
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 EIA ports have an outgoing DTR (Data Terminal Ready) signal. If the DTR signal on the
terminal is low and “EIA Recv” is “DTR” in Setup, then the serial device will stop sending to the terminal. On the
AUX port, the incoming DSR (Data Set Ready) signal serves to signal the device that the terminal is not ready to
receive more data, if “Aux Recv” is “DSR” in Setup.
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 EIA port and DTR on the 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
det
ermine when data transmission should be stopped or resumed.
Host/Printer Port 1 (DB25)
This port, labeled “EIA,” located to the left of the video port, 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
115,200 baud. The pins supported are shown below.