Safety Instructions

SYMEO LPR
®
LPR-1D
Product Documentation
Protocol Description Binary XP (1D messages)
Copyright © Symeo GmbH 2010
Page 68 of 87
The TYPE-field is following the DATA-field. The DATA field contains the real data of the
packet of the type TYPE. The CRC-field contains a check sum. The check sum is applied to
all previous data fields except the START data field.
All multi byte integers (e.g. CRC field) are encoded in Network-Byte-Order (Big Endian). All
signed integers are encoded in two's complement representation.
7.1.3 Byte Stuffing
The two symbols 0x7E and 0x7F are unique for START and STOP-fields. If those symbols
occurs within any other field (TYPE, DATA or CRC), they must be replaced by the following
order:
original symbol
replaced by
0x7D
0x7D 0x5D
0x7E
0x7D 0x5E
0x7F
0x7D 0x5F
This byte stuffing scheme ensures that the receiver of the protocol can identify definitely the
START-field within a flow of data, even if the symbol of the start field occurs within the
DATA-field.
Example: If the symbol 0x7d is read, it must be cancelled. The following symbol must be
XOR combined with 0x20 to recreate the original symbol.
Remark: Byte stuffing is deactivated for the fixed frame protocol (compare chapter 7.3).
7.1.4 CRC
The CRC-16-IBM with polynomial x
16
+x
15
+x
2
+1 is used for the CRC. The CRC is calculated
over all data fields (TYPE and DATA), but not for the START and END field.
The CRC-calculation is only applied to the original symbols. The appropriate calculation for
coding must applied before byte stuffing. If receiving the data from the LPR
®
-B system the
byte stuffing must be reserved to get the original symbol. Then the CRC is updated with the
original symbol.