Apple 310 manual SCC Operator Encoding, XON/XOFF Flow Control

C H A P T E R 3

Communication Channels

XON/XOFF Flow Control

XON/XOFF flow control uses two special characters, XON and XOFF, that may be sent either to the printer or to the host to control the flow of characters. When the LaserWriter Select 310 printer sends an XOFF character to the host, the host must immediately stop sending characters to the printer. When the printer sends XON to the host, the host may start sending characters again.

SCC Operator Encoding

The serial communications parameters are set by setsccbatch, a 1-byte options argument that holds four SCC encoded parameters: stop bits, data bits, flow control, and parity. The argument is an integer parameter with values in the range 0–255. The bits are assigned to different fields, as shown in Table 3-3, with the most significant bit representing the field stop bits, and so on. Table 3-3 lists the proper field value for each setting. You may determine the value of the options integer by simply selecting one decimal value for each field and adding those decimal integers together.

Table 3-3	SCC compatibility operators options byte values
Bit
position	Field	Setting	Decimal value	Field value
Bit 7	Stop bits	1 stop bit	0	0
		2 stop bits	128	1
Bits 6	Number of	Standard	0	0
and 5	data bits	7 bits	32	1
		8 bits	64	2
		Undefined	96	3
Bits 4–2	Flow control	XON/XOFF	0	0
		DTR	4	1
		Undefined	8, 12, 16, 29,	2, 3, 4, 5, 6, 7
			24, 28
Bits 1	Parity	Space	0	0
and 0		Odd	1	1
		Even	2	2
		Mark	3	3

Figure 3-1 shows an example of SCC operator encoding for an 8-pin serial port, using standard data bits with even parity, one stop bit, and XON/XOFF flow control.

In PostScript Level 1, the data bits and parity bit interact in a nonorthogonal way, to produce a table of possible choices for data and parity, as shown in Table 3-2. The choices include many commonly required methods of sending data. The Standard data bits setting provides compatibility with earlier versions of the PostScript Level 1.

48 SCC Operator Encoding