Communication Processor Module
MOTOROLA
MPC823 USER’S MANUAL
16-259
SCC2
COMMUNICATION
16
PROCESSOR
MODULE
Frames are sent in groups called dialogs, which are handled by the software. For instance,
to transfer a data frame, three frames are actually sent over the network. An RTS frame (not
to be confused with the RS-232 RTS pin) is sent to request the network, then 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. Once a dialog has begun, other nodes
cannot start transmitting until the dialog is complete. Frames within a dialog are transmitted
with a maximum interframe gap (IFG) of 200 microseconds. Although the LocalTalk
specification does not state it, there is also a minimum recommended IFG of 50ms. Dialogs
must be separated by a minimum interdialog gap (IDG) of 400ms. These gaps are
implemented by the software.
Depending on the protocol, collisions should only be encountered during RTS and ENQ
frames. Once frame transmission begins, it is fully transmitted, regardless of whether it
collides with another frame. ENQ frames are infrequent and only sent 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 logic 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.
16.9.18.2 FEATURES. The following list summarizes the features of the SCC2 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 CD interrupts
Reception is automatically disabled while transmitting a frame
Transmit-on-demand feature that expedites frames
Connects directly to an RS-422 transceiver