Apple 310 manual Soh Ctl-A

C H A P T E R 3

Communication Channels

Since any character code can be transmitted as data with this protocol, it can be used for sending PostScript language jobs that contain binary images. Binary protocol is set by default when the LaserWriter Select 310 printer is powered on. It may also be selected by setting softwareiomode to 100.

This protocol requires that a quoting character be inserted at certain places in the data stream and that the character codes for certain characters be altered. Therefore, the host computer must filter binary data before sending it to the printer. When the protocol is enabled, it is used for transmission in both directions. The host should therefore filter any data it receives from the printer. Filtering may be done by an application program or within the operating system of the host. In the case of the LaserWriter Select 310 printer, the Apple LaserWriter driver does this filtering. Table 3-5 lists the control characters for the binary serial protocol.

Table 3-5	Control characters in binary serial protocol
Hex value	ASCII name	ASCII keyboard	Control function
01	SOH	CTL-A	Quote data byte
03	ETX	CTL-C	Abort job and flush to end of file
04	EOT	CTL-D	End-of-job marker
05	ENQ	CTL-E	None
11	DC1	CTL-Q	XON in XON/XOFF flow control
13	DC3	CTL-S	XOFF in XON/XOFF flow control
14	DC4	CTL-T	Request printer status
1C	FS	CTL-\	None

To transmit the control characters as data, the characters must be quoted. 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 51