Parity Settings, DTR Flow Control

C H A P T E R 3

Communication Channels

Parity Settings

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.

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.

Flow Control

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

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.

Serial Communication