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25.3 Connecting to AppleTalk
As shown in Figure 25-2, the MPC860 connects to LocalTalk, and, using TXD, RTS, and
RXD, is an interface for the RS-422 transceiver. The RS-422, in turn, is an interface for the
LocalTalk connector. Although it is not shown, a passive RC circuit is recommended
between the transceiver and connector.
MPC860
SCC
6-Bit Sync
Two HDLC
TXD
Sequence
Flags
RTS
Standard HDLC frame handling
Figure 25-2. Connecting the MPC860 to LocalTalk
The 16´ overspeed of a 3.686-MHz clock can be generated from an external frequency
source or from one of the baud rate generators if the resulting output frequency is close to
a multiple of the 3.686 MHz frequency. The MPC860 asserts RTS throughout the duration
of the frame so that RTS can be used to enable the RS-422 transmit driver.
25.4 Programming the SCC in AppleTalk Mode
The AppleTalk controller is implemented by setting certain bits in the HDLC controller.
Otherwise, Chapter 24, ÒSCC HDLC Mode,Ó describes how to program the HDLC
controller. Use GSMR, PSMR, or TODR to program the AppleTalk controller.
25.4.1 Programming the GSMR
Program the GSMR as described below:
1. Set MODE to 0b0010 (AppleTalk).
2. Set DIAG to 0b00 for normal operation, with CD and CTS grounded or conÞgured
for parallel I/O. This causes CD and CTS to be internally asserted to the SCC.
3. Set RDCR and TDCR to (0b10) a 16´ clock.
4. Set the TENC and RENC bits to 0b010 (FM0).
MOTOROLA
Tx Data
TXD
Tx Enable
RTS
Rx Data
RXD
Stored in Receive Buffer
Stored in Transmit Buffer
Destination
Source
Control
Address
Address
Byte
Chapter 25. SCC AppleTalk Mode
Part V. The Communications Processor Module
RS-422
Data
CRC-16
MINI-DIN 8
Connection
Closing
16 Ones
Flag
(Abort)
25-3
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