Motorola MPC8260 - Manual (Page 693)

MOTOROLA Chapter 25. SCC AppleTalk Mode 25-1
Chapter 25
SCC AppleTalk Mode
250
250
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
ÔAppleTalk controllerÕ refers to the support that the MPC8260 provides for LocalTalk
protocol. The AppleTalk controller provides required frame synchronization, bit sequence,
preamble, and postamble onto standard HDLC frames. These capabilities, with the use of
the HDLC controller in conjunction with DPLL operation in FM0 mode, provide the proper
connection formats to the LocalTalk bus.
25.1 Operating the LocalTalk Bus
A LocalTalk frame, shown in Figure , is basically a modiÞed HDLC frame.
Figure 25-1. LocalTalk Frame Format
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 speciÞed. 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 ßags 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 0Ð600 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 ßag and a restricted HDLC abort sequence. Then
the transmitterÕs driver is disabled.
> 3 bits 2 or more 1 byte 1 byte 0-600 bytes 2 bytes 12–18 ones 1 byte 1 byte
bytes
HDLC
CRC-16
Flags
Destination
Address
Data
(Optional)
Control
Byte
Sync
Sequence
Source
Address
Closing
Flag
Abort
Sequence