Siemens SIMATIC MV440 Operating Instructions Manual page 316

Process interfacing via an automation system (PLC, PC)
10.6 Control via TCP/IP and RS-232
Checksum
The user data contains a checksum byte whose use is recommended when using the RS-
232 interface.
● If the checksum is not used and will not be checked by the reader, the checksum byte
must have the value 0.
● To calculate the checksum, all bytes as of the total length (inclusive) up to the byte of the
checksum (exclusive) are XORed.
Bad frames
If an XMATCH frame with an incorrect structure is received, the error is logged in the
diagnostics data records as a temporary error (transient error) and the frame is discarded.
● Bad structure: For example String length cl > 3 001 characters or checksum incorrect.
Even with a correct frame structure, the XMATCH command can be invalid. The error is also
logged in the diagnostics data records as a temporary error (Page 345).
Examples of XMATCH user data
Application: 1D/2D code reading
● Current program number is "5".
● new match string for trigger number 42
● No OCR
● No multicode
Total length
Trigger ID
Flags
Code No Cnt
Code No
Code Idx Cnt
Code Idx
String length
String
ROI ID Cnt
XOR checksum
314
19
42
0
1
5
1
1
8
"SN012345"
0
0
SIMATIC MV420 / SIMATIC MV440
Operating Instructions, 04/2013, A5E02371045-06