Boundless Technologies ADDS 3153 ASCII - Manual (Page 20)

Communications 7
Communications
This terminal is equipped with three ports. The two serial ports are intended for
connection to either a host/modem or a serial printer. The remaining port is a
Centronics-compatible printer port. 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 your choice of connecting the terminal to one or two host
computers.
The first host/printer serial port (SES1-EIA) can communicate with your computer or
printer at a baud rate of 110 to 134,400 baud (bits per second). The second
host/printer serial port (SES2-AUX) can communicate at speeds from 110 baud to
38,400 baud. Either port can be used to connect to a host computer/modem or serial
printer.
In single host mode, you have the choice to use the second serial port or to use the
parallel printer port, as your printer interface. In dual host mode, the natural choice is
to use the parallel port, although other options are available (See the “Installation”
chapter). Refer to the following sections for further information on these ports.
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 Scan
Code Key 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).
The terminal’s receive buffer can store 1024 bytes (characters). When “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 a. The terminal will
then continue to process data until its receive buffer is empty. 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 SES1-EIA port has an outgoing
DTR (Data Terminal Ready) signal. If “EIA Rec” is “DT” 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 (serial) host device to stop sending data. On the SES2-
AUX port, the incoming DSR (Data Set Ready) signal serves to signal the host (serial)
device that the terminal is not ready to receive more data, if “Aux Recv” is “DSR” in
Setup.