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CHAPTER 3
Communication Channels
Serial Communication 47
Parity Settings 3
The parity settings for the serial channel may be odd, even, space, or mark. Table 3-2
indicates how the parity setting and number of data bits work together.
Flow Control 3
The LaserWriter Select 310 printer uses one of two conventions for controlling the flow of
characters to and from the host computer. These conventions are DTR and XON/XOFF. If
the host fails to conform to the selected flow control protocol, unexpected occurrences of
ioerror may result.
DTR Flow Control 3
DTR flow control uses the Data Terminal Ready control signal. Normally, the printer
leaves this signal turned on. However, when it needs to stop the flow of characters from
the host, it turns DTR off. The host must then immediately stop sending characters
until the printer turns DTR back on again. Similarly, another signal, Data Set Read (DSR)
may be used by the host to control the flow of data sent to it from the printer. Packet DTR
is an implementation of DTR flow control specifically set and used by the Apple
LaserWriter driver. It uses the DSR signal to control the flow of data sent to the host in
packets of three characters at a time.
Table 3-2 Data and parity choices for the LaserWriter Select 310 printer
Data bits Parity Description
Standard Space 8 bits are sent. The 8th bit transmitted is zero, and
the 8th bit received is ignored.
Standard Odd or even 8 bits are sent. The 8th bit is used for parity.
Standard Mark 8 bits are sent and received. The parity bit is not stripped.
Since the PostScript interpreter expects ASCII data (high
bit zero), a host sending mark parity data will not be
understood, and you should select 7-bit mode in which
the parity bit is stripped.
7 bits Space 7 data bits are sent. A zero is added.
7 bits Odd or even 7 data bits are sent. A parity bit is added to the 7 data
bits in either odd or even parity, depending on the parity
setting.
7 bits Mark 7 data bits are sent. A one is added.
8 bits Space 8 bits are sent.
8 bits Odd or even 8 bits are sent. A parity bit is added to the 8 bits in either
odd or even parity, depending on the parity setting.
8 bits Mark 8 bits are sent.
NOTE Parity is checked on received characters only when the parity setting is even or odd. If a
parity error is detected the PostScript interpreter generates an ioerror.