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AIX Versions 3.2 and 4 Asynchronous Communications Guide

Transparent Printing

Most terminals have an auxiliary port that can be connected to a serial printer. These terminals support two print modes, Auxiliary and Transparent. If both print modes are OFF, data received by the terminal is simply displayed on the screen. With Auxiliary print mode ON, data received by the terminal is displayed on the screen and is also transmitted to the printer. With Transparent Print Mode ON, the terminal transmits data received directly to the printer, without displaying it on the screen.

Transparent printing allows you to use your terminal in a normal manner, while information is also being sent over the same serial connection from the host to the printer connected to the terminal's auxiliary printer port. This is transparent printing. The transparent printing software determines whether packets of data are bound for the screen or for the printer, and precedes data bound for the printer with the Transparent Print Mode ON command, and follows it with the Transparent Print Mode OFF command.

Data for the terminal screen has the highest priority, and data is sent to the printer only when there is a break in information being sent to the screen. If continuous data is being transmitted to the terminal device, nothing gets sent to the printer.

Whenever an auxiliary printer port is used, flow control to the printer becomes an issue. If the printer falls behind and invokes flow control, output to both the printer and the terminal is stopped. The transparent print feature provides three parameters, accessible through SMIT, to limit printer output and avoid this situation.

The SMIT Transparent Print Maximum Characters per Second parameter limits the maximum printer port character-per-second data rate. This number should be set to the minimum character rate the printer can sustain in typical use.

The SMIT Transparent Print Maximum Character Packet Size parameter limits the number of characters queued to the printer ahead of terminal output. Lower numbers increase system overhead, higher numbers result in keystroke echo delays. Specify a value of 50 for 9600 baud.

The SMIT Transparent Print Printer Buffer Size parameter should be set to a value just below the printer's buffer size. After a period of inactivity, the driver will burst up to this many characters to the printer to fill the print buffer before slowing to the maxcps rate.

The printer on/off strings are also set using SMIT. A cable must be connected between the auxiliary port of the terminal and the printer. The baud rate on the terminal auxiliary port and the printer must be the same, and the printer and the auxiliary port of the terminal must use the same handshaking mode. The auxiliary port must also be enabled. If your terminal is not one of those directly supported, you must know the escape sequence of your terminal.

Refer to your terminal and printer manuals for connection information, escape codes, and to see what handshaking modes are supported (for example, XON/XOFF, busy/ready, RTS/CTS, etc.). Printer devices (xtty1, etc.) must not be in either the /etc/inittab or /etc/ttys files and must not be enabled.

The following sections contain transparent print activation information:


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