MPC8280 datasheet(813/1386 Pages) FREESCALE

813 / 1386 page

MPC8280 PowerQUICC II Family Reference Manual, Rev. 1

Freescale Semiconductor

26-1

Chapter 26

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’ refers to the support that the MPC8280 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.

26.1

Operating the LocalTalk Bus

A LocalTalk frame, shown in Figure 26-1, is basically a modified HDLC frame.

Figure 26-1. 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 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.

The control byte within the LocalTalk frame indicates the type of frame. Control byte values from

0x01–0x7F are data frames; control byte values from 0x80–0xFF are control frames. Four control frames

are defined:

•

ENQ—Enquiry

•

ACK—Enquiry acknowledgment

•

RTS—Request to send a data frame

•

CTS—Clear to send a data frame

Frames are sent in groups known as dialogs, which are handled by the software. For instance, to transfer

a data frame, three frames are sent over the network. An RTS frame (not to be confused with the RS-232

> 3 bits

2 or more

1 byte

0-600 bytes

2 bytes

12–18 ones

1 byte

bytes

HDLC

CRC-16

Flags

Destination

Address

Data

(Optional)

Control

Byte

Sync

Sequence

Source

Address

Closing

Flag

Abort

Sequence

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