Siemens SIMATIC TIWAY 1 - Manual (Page 190)

USING THE BDLC PROTOCOL
UNILINK HOST ADAPTER B–5
USER MANUAL
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.
Length
Primitive
Code
Descriptor
Field(s)
Data Unit
Field(s)
Command Code
Secondary
Address
Network Data
TIWAY I Primitives
DLE–STX Control FSC
UNILINK Host Adapter Commands
Adapter
Address
DLE–ETX
BDLC Protocol I–Frame
Adapter
Data
Figure B–3 Illustrative BDLC I-Frame