This means that the character being sent is replaced by a two-character sequence consisting of control character CTL-A, which indicates a quoted data byte, followed by the character itself XORed with 40 hex. For example, if the host wants to send a request for printer status (hex value 14), it sends 01 hex-54. This method of quoting means that whenever one of the eight special characters is received, the control function is intended, regardless of whether or not the preceding character is CTL-A. Asynchronous control characters may therefore be generated and processed at a lower level than the data stream may be generated and consumed. On a host machine, the user program may implement the CTL-A quoting convention, while the operating system is independently performing XON/XOFF processing. Note All byte values other than those of the eight special characters are transmitted by simply sending the value. Communication Protocols CHAPTER 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 (, , or ) 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 reador readstring) receives whatever characters are sent by the host. Similarly, output is generated by a PostScript program (via printor =) is sent unchanged. Note In this connection, the standard end-of-line in the PostScript language (\nin a PostScript language string) is . 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. Communicating with an IBM PC CHAPTER 3 Communication Channels Unlike the serial port, the Centronics parallel input allows no parameters. It always accepts 8-bit data with no parity, and it reserves the same set of characters for communication functions that the serial port reserves, with the exception of XON/XOFF characters, which are passed through the PostScript interpreter, since flow control is maintained by means of the /BUSY signal. The Centronics parallel interface is essentially an input-only channel. This means it basically transfers information from the host to the printer. However, the PostScript interpreter always deals with both the input and output sides of an I/O channel. When operating in Centronics parallel input mode, the printer sets up the 8-pin serial port for the PostScript language print operator. The output from this port may be ignored for simple one-way communication. However, important information may be output via this port. Printer errors are always reported, since the Paper Error signal on the Centronics interface cannot be relied upon to report all printer errors. Table 3-6 summarizes the settings for the serial communications port when it is used as the output port during parallel communication. Table 3-6 Settings for the serial output channel during parallel communication Parameter Settings Data bits 8 Parity None Stop bits 1 Flow control DTR Baud rate 9600 Protocol Binary Serial Interface You may also use the serial port as an alternative means of communicating with the IBM PC, using either DTR or XON/XOFF flow control. DTR flow control is preferred, since this is the default serial flow control for the LaserWriter Select 310 printer. DTR F...
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