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11-80 SA-1100
Developer’s Manual
Peripheral Control Module
11.9.1.2 Frame Format
SDLC uses a flag (reserved bit pattern) to denote the beginning of a frame of information and to
synchronize frame transmission. The flag contains eight bits that start and end with a zero, and
contains six sequential ones in the middle (01111110). This sequence of six ones is unique because
all data between the start and stop flags is prohibited from having more than five consecutive ones.
Data that violates this rule is altered before transmission by automatically inserting a zero after five
consecutive ones are detected in the transmitted bit stream. This technique is commonly referred to
as “bit stuffing” and is transparent to the user. The information field within an SDLC frame is
placed between two flags and consists of an 8-bit address, an optional 8-bit control field, a data
field containing any multiple of 8 bits, and a 16-bit cyclic redundancy check (CRC-CCITT). The
user can also program the SDLC to insert an optional second start flag. Note that each byte within
the address, control, and data fields is transmitted and received LSB first, ending with the byte’s
MSB. However, the CRC is transmitted and received MSB first. Figure 11-23 shows the SDLC
frame format.
Figure 11-23. SDLC Frame Format
11.9.1.3 Address Field
The 8-bit address field is used by a transmitter to target a select group of receivers when multiple
stations are connected to the same set of serial lines. The address allows up to 255 stations to be
uniquely addressed (00000000 to 11111110). The global address (11111111) is used to broadcast
messages to all stations. Serial port 1 contains an 8-bit register that is used to program a unique
address for broadcast recognition. It also contains a control bit to enable or disable the address
match function. Note that the address of received frames is stored in the receive FIFO along with
normal data; it is transmitted and received starting with its LSB and ending with its MSB.
11.9.1.4 Control Field
The SDLC control field is typically 8 bits, but can be any length. Serial port 1 does not provide any
hardware decode support for the control byte; it treats all bytes between the address and the CRC as data.
Note that the control field is transmitted and received starting with its LSB and ending with its MSB.
Figure 11-22. FM0/NRZ Bit Encoding Example (0100 1011)
NRZ
Data
FM0
Data
Bit
Value
11010010
LSB
MSB
8 Bits
(optional)
8 Bits 8 Bits
8 Bits
(optional)
Any Multiple
of 8 Bits
16 Bits 8 Bits
Start Flag
0111 1110
Start Flag
0111 1110
Address
Control Data CRC-CCITT
Stop Flag
0111 1110