Siemens SIMATIC TIWAY 1 - Manual (Page 188)

USING THE BDLC PROTOCOL
UNILINK HOST ADAPTER B–3
USER MANUAL
B.2 BDLC PROTOCOL FRAME STRUCTURE
Information is transferred in single frames without any blocking. Each frame
includes the destination address, the BDLC control field, the information, and
the block checksum as shown in Figure B–2. Rather than using ASCII codes to
indicate where one field begins and ends, BDLC uses positional significance,
meaning that the position of the bits in the frame have a particular meaning. All
of the fields are on byte boundaries and are represented hexadecimal rather
than as ASCII characters, so that hexadecimal ”1E” is transmitted as 1E. The
maximum length of the frame is 300 bytes, which includes all bytes except the
inserted DLEs.
DLE–STX Address Control Information BCS DLE–ETX
Figure B–2 BDLC Protocol Frame
B.2.1 Flag Fields — DLE-STX, DLE-ETX
These two-byte flag fields are the BDLC protocol message delimiters. DLE-STX
is the pair of hexadecimal values 10 02 and signals the beginning of a message.
DLE-ETX is the pair of hexadecimal values 10 03 and signals the end of a
message. Any other occurrence of the DLE character is considered to be part of
the information.
NOTE
An extra DLE must be inserted at the point where the
DLE character occurs in the data stream by the
transmitting station and removed in the data stream
by the receiving station. This differs from the ANSI
X3.66–1979 standard which uses zero-bit insertion.