TR-Electronic CDH 75 M - SSI Format; Safety-Specific Functions; Sign of Life Counter; CRC Checksum

Printed in the Federal Republic of Germany  TR-Electronic GmbH 2009, All Rights Reserved
12/13/2018 TR - ECE - BA - DGB - 0070 - 17 Page 99 of 108
6.3 SSI format
Bit 0-11
Bit 12-24
Bit 25
Bit 26
Bit 27-31
Bit 32-39
MSB LSB
MT, 12 bit ST, 13 bit Err1, 1 bit Err2, 1 bit SoL, 5 bit CRC, 8 bit
Multi-Turn
Resolution
Single-Turn
Resolution
Error bit
Master system
Error bit
Safety system
Sign of life
counter
Checksum
2
24
-2
13
2
12
-2
0
2
0
2
0
2
4
-2
0
2
7
-2
0
Master system and safety system use the same SSI data format.
6.3.1 Safety-specific functions
6.3.1.1 Sign of life counter
The 5-bit sign of life counter is incremented in dependence of the scanning
procedures and is inserted into the SSI telegram. The control of this incrementing
event by the control guarantees that the newly transferred position value comes from
a current scanning procedure.
The sign of life counter is increased if the control has read the current position values.
It is prerequisite that internally new position values were generated and provided.
Thus a
n incremented sign of life counter, parallel to the last telegram, guarantees that
the new transmitted data originate from a new position scanning.
=> If the SSI requests take place faster than the internal scanning period
of 500 µs
for the Master channel and
Safety channel, position data and the sign of life counter
are unchanged.
6.3.1.2 CRC checksum
In the SSI telegram, about all user data, an 8-bit CRC checksum is calculated and is
inserted into the SSI telegram.
Polynomial: x
8
+ x
5
+ x
4
+ 1 (Maxim/Dallas)
Start value: 0xFF
Secured data: MT- and ST-data, Error bits, Sign of life counter
Min. Hamming distance = 4: three faulty data bits are detected in any case,
one faulty data bit can be corrected
An incorrect CRC checksum is not a reference to a measuring system error, but to a
communication problem. A cause for it can be an interference signal.
Communication
problems at SSI interfaces can also occur by too long cable lengths o
r to high SSI
sampling rates. Usually an individual
destroyed telegram, e.g. by an interference
signal, is not safety critical. Reason: The time up to entering a critical state is normally
definitely longer, as the time between two position transmissions. In this case
the
appropriate
reaction to an individual incorrect position transmission is application
dependent.