Operating The Localtalk Bus - Freescale Semiconductor MPC850 User Manual

Chapter 24
SCC 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, AppleTalk
protocols have been most closely associated with the LocalTalk physical and link-layer
protocol, an HDLC-based protocol that runs at 230.4 kbps. In this manual, the term
'AppleTalk controller' assumes the support that the MPC850 provides for LocalTalk
protocol. The AppleTalk controller provides 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.

24.1 Operating the LocalTalk Bus

A LocalTalk frame, shown in Figure 24-1, is basically a modified HDLC frame.
Sync HDLC
Sequence Flags
> 3 bits 2 or more
bytes
First, a synchronization sequence of more than three bits is sent. This sequence consists of
at least one logical one bit (FM0 encoded) followed by two bit times or more 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, allowing bit, byte, and frame
delineation or detection. Two bytes of address, destination, and source are sent next,
followed by a byte of control and 0–600 data bytes. Next, two bytes of CRC (the common
16-bit CRC-CCITT polynomial referenced in the HDLC standard protocol) are sent. The
LocalTalk frame is then terminated by a flag and a restricted HDLC abort sequence. Then
the transmitter's driver is disabled.
Destination Source Control
Address Address Byte
1 byte 1 byte 1 byte
Figure 24-1. LocalTalk Frame Format
Chapter 24. SCC AppleTalk Mode
Data
CRC-16
(Optional)
0-600 bytes 2 bytes
Closing Abort
Flag Sequence
1 byte 12-18 ones