Siemens SIMATIC TIWAY 1 User Manual page 190

USING THE BDLC PROTOCOL
B.2.5 Block Checksum Field
This two-byte field is the Block Checksum (BCS) calculated by the transmitter
and checked by the receiver to detect transmission errors. The calculation is a
16-bit one's complement Fletcher Checksum.
The BCS is calculated using all bytes in the transmitted frame except the
starting and ending flags (DLE-STX and DLE-ETX), the BCS bytes, and any
inserted DLEs. The method for calculating and checking the BCS is discussed in
Paragraph B.7.
Figure B–3 illustrates how a TIWAY I Primitive is enclosed in the adapter
command string which is enclosed in the BDLC I-Frame.
Primitive Descriptor Data Unit
Length
Code Field(s) Field(s)
TIWAY I Primitives
Secondary
Command Code Network Data
Address
UNILINK Host Adapter Commands
Adapter Adapter
Control
DLE–STX FSC DLE–ETX
Address Data
BDLC Protocol I–Frame
Figure B–3 Illustrative BDLC I-Frame
B–5 UNILINK HOST ADAPTER
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