C H A P T E R 3
Communication Channels
After CTL-A is received, the next character received that is not one of the special characters must be the result of XORing one of the special characters with 40 hex. If any other character is received, it is considered to be an input error. Any number of special characters may appear between CTL-A and the XORed character, with the exception of special characters CTRL-D and CTRL-A, which are handled asynchronously. If either of these characters is received between CTL-A and the XORed character, it is considered to be an error condition.
If a special character arrives unquoted and it specifies no control function for the channel, the character is simply discarded. For example, if XON or XOFF is received
and XON/XOFF flow control is not in use, it is discarded.
IMPORTANT
CTL-E and CTL-\ currently specify no control functions and should
not be sent to the LaserWriter Select 310 printer. They are included in the list of quoted characters in case new control functions are added in
the future. ▲
In contrast to simple protocol, with binary protocol there is no mapping between end-of-line conventions. The end-of-line characters (<CR>, <LF>, or <CR><LF>) sent by the host are exactly what is received by the interpreter in the printer. The PostScript scanner handles the different end-of-line conventions in a uniform way, but a program that reads data from the channel directly (via read or readstring) receives whatever characters are sent by the host. Similarly, output is generated by a PostScript program (via print or =) is sent unchanged.
Note
In this connection, the standard end-of-line in the PostScript language (\n in a PostScript language string) is <LF>. ◆
Communicating with an IBM PC
You may connect the LaserWriter Select 310 printer to an IBM PC through either the Centronics 36-pin parallel connector or the RS-232 serial port. The physical characteristics of the connectors are described in Chapter 1, in the sections “Centronics Parallel Connector” and “Serial Port for RS-232C Devices.” This section describes the software support for the interface.
Parallel Interface
To set up the parallel port for communication between the LaserWriter Select 310 and the IBM PC, issue the following command:
MODE LPT1:,,P
This command allows the Centronics busy signal to be active for extended periods of time.
