Freescale Semiconductor PowerQUICC MPC885 Reference Manual page 719

Powerquicc family

Table of Contents

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 MPC885 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.

Operating the LocalTalk Bus

A LocalTalk frame, shown in

> 3 bits 2 or more

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:

ACK—Enquiry acknowledgment

RTS—Request to send a data frame

CTS—Clear to send a data frame

Freescale Semiconductor

24-1, is basically a modified HDLC frame.

Figure 24-1. LocalTalk Frame Format

MPC885 PowerQUICC Family Reference Manual, Rev. 2