Freescale Semiconductor MPC8260 - Manual (Page 810)

SCC AppleTalk Mode
MPC8260 PowerQUICC II Family Reference Manual, Rev. 2
26-2 Freescale Semiconductor
RTS pin) is sent to request the network, a CTS frame is sent by the destination node, and the data frame is
sent by the requesting node. These three frames comprise one possible type of dialog. After a dialog
begins, other nodes cannot start sending until the dialog is complete. Frames within a dialog are sent with
a maximum interframe gap (IFG) of 200 µs. Although the LocalTalk specification does not state it, there
is also a minimum recommended IFG of 50 µs. Dialogs must be separated by a minimum interdialog gap
(IDG) of 400 µs. In general, these gaps are implemented by the software.
Depending on the protocol, collisions should be encountered only during RTS and ENQ frames. Once
frame transmission begins, it is fully sent, regardless of whether it collides with another frame. ENQ
frames are infrequent and are sent only when a node powers up and enters the network. A higher-level
protocol controls the uniqueness and transmission of ENQ frames.
In addition to the frame fields, LocalTalk requires that the frame be FM0 (differential Manchester space)
encoded, which requires one level transition on every bit boundary. If the value to be encoded is a logical
zero, FM0 requires a second transition in the middle of the bit time. The purpose of FM0 encoding is to
avoid having to transmit clocking information on a separate wire. With FM0, the clocking information is
present whenever valid data is present.
26.2 Features
The following list summarizes the features of the SCC in AppleTalk mode:
• Superset of the HDLC controller features
• FM0 encoding/decoding
• Programmable transmission of sync sequence
• Automatic postamble transmission
• Reception of sync sequence does not cause extra SCCE[DCC] interrupts
• Reception is automatically disabled while sending a frame
• Transmit-on-demand feature expedites frames
• Connects directly to an RS-422 transceiver
26.3 Connecting to AppleTalk
As shown in Figure 26-2 , the PowerQUICC II connects to LocalTalk, and, using TXD, RTS, and RXD, is
an interface for the RS-422 transceiver. The RS-422, in turn, is an interface for the LocalTalk connector.
Although it is not shown, a passive RC circuit is recommended between the transceiver and connector.