Motorola MPC8260 - Manual (Page 633)

MOTOROLA Chapter 22. SCC BISYNC Mode 22-1
Chapter 22
SCC BISYNC Mode
220
220
The byte-oriented BISYNC protocol was developed by IBM for use in networking
products. There are three classes of BISYNC framesÑtransparent, nontransparent with
header, and nontransparent without header, shown in Figure 22-1. The transparent frame
type in BISYNC is not related to transparent mode, discussed in Chapter 23, ÒSCC
Transparent Mode.Ó Transparent BISYNC mode allows full binary data to be sent with any
possible character pattern. Each class of frame starts with a standard two-octet
synchronization pattern and ends with a block check code (BCC). The end-of-text character
(ETX) is used to separate the text and BCC Þelds.
Figure 22-1. Classes of BISYNC Frames
The bulk of a frame is divided into Þelds whose meaning depends on the frame type. The
BCC is a 16-bit CRC format if 8-bit characters are used; it is a combination longitudinal
(sum check) and vertical (parity) redundancy check if 7-bit characters are used. In
transparent operation, a special character (DLE) is deÞned that tells the receiver that the
next character is text, allowing BISYNC control characters to be valid text data in a frame.
A DLE sent as data must be preceded by a DLE character. This is sometimes called byte-
stufÞng. The physical layer of the BISYNC communications link must synchronize the
receiver and transmitter, usually by sending at least one pair of synchronization characters
before each frame.
BISYNC protocol is unusual in that a transmit underrun need not be an error. If an underrun
occurs, a synchronization pattern is sent until data is again ready. In nontransparent
operation, the receiver discards additional synchronization characters (SYNCs) as they are
received. In transparent mode, DLE-SYNC pairs are discarded. Normally, for proper
Nontransparent with Header
SYN1 SYN2 SOH Header STX Text ETX BCC
Nontransparent without Header
SYN1 SYN2 STX Text ETX BCC
Transparent
SYN1 SYN2 DLE STX Transparent
Text
DLE ETX BCC