MC68000 MOTOROLA [Motorola, Inc], MC68000 Datasheet - Page 208

• Manufacturer Part Number: MC68000
• Manufacturer: MOTOROLA [Motorola, Inc]

Communications Processor (CP)
The purpose of the control characters table is to enable automatic recognition (by the BI-
SYNC controller) of the end of the current block. See 4.5.13.14 Programming the BISYNC
Controllers for more information. Since the BISYNC controller imposes no restrictions on the
format of the BISYNC blocks, user software must respond to the received characters and
inform the BISYNC controller of mode changes and certain protocol events (e.g., resetting
the BCS). However, correct use of the control characters table allows the remainder of the
block to be received without interrupting the user software.
Up to eight control characters may be defined. These characters inform the BISYNC con-
troller that the end of the current block has been reached and whether a BCS is expected
following this character. For example, the end of text (ETX) character implies both an end
of block (ETB) and a BCS should be received. An enquiry (ENQ) character designates end
of block without a subsequent BCS. All the control characters are written into the data buffer.
The BISYNC controller uses a table of 16-bit entries to support control character recognition.
Each entry consists of the control character, an end-of-table bit, a BCS expected bit, and a
hunt mode bit. The control characters table is shown in Figure 4-31. To disable the entire
control characters table, write $8000 to the first table entry.
CHARACTER8–CHARACTER1—Control Character Value
E—End of Table
4-88
OFFSET + 0
OFFSET + 4
OFFSET +2
OFFSET + E
These fields define control characters.
0 = This entry is valid. The lower eight bits will be checked against the incoming char-
1 = The entry is not valid. No valid entries exist beyond this entry.
acter.
15
E
E
E
E
When using 7-bit characters with parity, the parity bit should be
included in the control character value.
14
B
B
B
•
•
•
B
Figure 4-31. BISYNC Control Characters Table
13
H
H
H
H
12
11
MC68302 USER’S MANUAL
10
NOTE
9
8
7
CHARACTER1
CHARACTER2
CHARACTER3
CHARACTER8
MOTOROLA
0