Freescale Semiconductor PowerQUICC MPC885 Reference Manual page 739

Powerquicc family

Table of Contents

SCC BISYNC Mode

The byte-oriented BISYNC protocol was developed by IBM for use in networking products. There are

three classes of BISYNC frames—transparent, nontransparent with header, and nontransparent without

header, shown in

discussed in

Chapter 28, "SCC Transparent Mode."

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 fields.

The bulk of a frame is divided into fields 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

defined 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-stuffing. 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

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 transmission, an underrun must not occur between the DLE and

its following character. This failure mode cannot occur with the MPC885.

An SCC can be configured as a BISYNC controller to handle basic BISYNC protocol in normal and

transparent modes. The controller can work with the time-slot assigner (TSA) or nonmultiplexed serial

interface (NMSI). The SCC supports modem lines by connecting to port C pins or general-purpose I/O

pins. The controller has separate transmit and receive sections whose operations are asynchronous with the

core and either synchronous or asynchronous with other SCCs.

Freescale Semiconductor

26-1. The transparent frame type in BISYNC is not related to transparent mode,

Nontransparent with Header

Nontransparent without Header

Figure 26-1. Classes of BISYNC Frames

MPC885 PowerQUICC Family Reference Manual, Rev. 2

Transparent BISYNC mode allows full binary data to