Freescale Semiconductor MPC8260 - Manual (Page 809)

MPC8260 PowerQUICC II Family Reference Manual, Rev. 2
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 PowerQUICC II 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 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