mpc823rg Freescale Semiconductor, Inc, mpc823rg Datasheet - Page 715

• Manufacturer Part Number: mpc823rg
• Description: Mpc823 Powerquicc Integrated Communications Processor For Portable Systems
• Manufacturer: Freescale Semiconductor, Inc

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 RTSx 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 must 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 serial
communication controller 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 CDx interrupts
• Reception is automatically disabled while transmitting a frame
• Transmit-on-demand feature that expedites frames
• Connects directly to an RS-422 transceiver
Freescale Semiconductor, Inc.
For More Information On This Product,
MPC823 REFERENCE MANUAL
Go to: www.freescale.com
Communication Processor Module
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