Motorola MPC823e Reference Manual page 719

16.9.18 The SCCs in AppleTalk Mode
®
AppleTalk is a set of protocols developed by Apple Computer Inc. to provide a LAN service
between Macintosh computers and printers. Although AppleTalk can be implemented over
a variety of physical and link layers, including Ethernet, the AppleTalk protocols have
traditionally been more closely associated with one particular physical and link layer protocol
called LocalTalk.
LocalTalk refers to an HDLC-based link and physical layer protocol that runs at the rate of
230.4kbps. In this manual, the term AppleTalk controller refers to a support that the
MPC823e provides for the LocalTalk protocol. The AppleTalk controller provides the
required frame synchronization, bit sequence, preamble, and postamble onto standard
HDLC frames. These capabilities, as well as the use of the HDLC controller in conjunction
with DPLL operation in FM0 mode, provide the proper connection formats to the LocalTalk
bus.
16.9.18.1 OPERATING THE LOCALTALK BUS. A LocalTalk frame is basically a modified
HDLC frame.
SYNC HDLC DEST
SEQ FLAGS ADDR
1
> 3 2 OR
BYTE
BITS MORE
BYTES
The frame begins when a synchronization sequence greater than three bits is sent. This
sequence consists of at least one logical 1 bit (FM0-encoded) followed by greater than two
bit times of line idle with no particular maximum time specified. The idle time allows
LocalTalk equipment to sense a carrier by detecting a "missing clock" on the line. The
remainder of the frame is a typical half-duplex HDLC frame. Two or more flags are sent,
which allows for bit, byte, and frame delineation or detection. Then, two bytes of address,
destination, and source are transmitted, followed by a byte of control and 0 to 600 data
bytes. Next, two bytes of CRC (described in Section 16.9.16.8 SCCx HDLC Mode
Register) are sent. The LocalTalk frame is then terminated by a flag and a restricted HDLC
abort sequence. Then the transmitter's driver is disabled.
The control byte within the LocalTalk frame indicates the type of frame. Control byte values
that range from 0x01 to 0x7f are data frames and control byte values that range from 0x80
to 0xff are control frames. There are four types of control frames:
• ENQ—Enquiry
• ACK—Enquiry acknowledgment
• RTS—Request to send a data frame
• CTS—Clear to send a data frame
MOTOROLA
SOURCE CONTROL
ADDR BYTE
1 1
BYTE BYTE
Figure 16-89. LocalTalk Frame Format
MPC823e REFERENCE MANUAL
Communication Processor Module
DATA
CRC-16
(OPTIONAL)
2
0–600 BYTES
BYTES
CLOSING ABORT
SEQUENCE
FLAG
1
12–18
BYTE
ONES
16-265