Apple LaserWriter Select 310 Printer - Manual (Page 60)

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