Baumer N 155 - Manual (Page 5)

N 155
www.baumer.com 5
3.3 Check sum
To support an error-free data transmission, a check sum byte CRC is added after the footer token (EOT) to the
string to be transferred. Upon reception of a command the CRC byte received is crosschecked with the self-
calculated CRC byte.
Algorithm:
1) Reset check sum byte.
2) Rotate check sum byte by 1 bit to the left.
3) Link result to first data byte XOR.
4) Rotate result by 1 bit to the left.
5) Link result to the second data byte XOR.
:
x) Rotate result by 1 bit to the left.
y) Link result to the last data byte (always 04H) XOR.
z) Add result as CRC after the footer token (04H) to the sending string.
Note:
1101 0000 RL 1010 0001 Important: Bit 7 is being shifted into bit 0 during rotation
Example:
Transmission string w/o CRC = 01h 20h 43h 04h
CRC-Byte
= 0Ah
0000 0000 RL 0000 0000 XOR 0000 0001 (01h) = 0000 0001
0000 0001 RL 0000 0010 XOR 0010 0000 (20h) = 0010 0010
0010 0010 RL 0100 0100 XOR 0100 0011 (43h) = 0000 0111
0000 0111 RL 0000 1110 XOR 0000 0100 (04h) = 0000 1010 = CRC
Legend: RL = rotate left; XOR = non-equivalence link
3.4 Special features in data transfer
For reasons of compatibility and independent from the number of digits provided in the display, the complete
multicon system features the same data volume (refer to chart „overview of commands). For example, com-
mand “R” (read current value) comprises 6 bytes as data volume to be transferred whilst it is a 5-digit display
only.
3.5 Broadcast commands
Some specific commands are defined as so-called broadcast commands. As soon as such a command is sent
by master to identifier 99, the command will be accepted by each N 155 in the network. Each individual target
display will accomplish the respective function but will not return any confirmation to the master.