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Requirements for Evaluation Unit
10
Requirements for Evaluation Unit
10.1
Hardware Requirements
The design and the structure of the input circuit of the evaluation unit are generally subject to the specifications
of the evaluation unit. When working with a multi-channel structure, the data has to be transmitted to all chan-
nels.
If all errors stated in 10.2 can be detected, the bus drivers (transceiver) and protective circuit may also be real-
ised in a single-channel structure.
10.2
Software Requirements
10.2.1
Requirements for RS-485 Interface
The evaluation unit has to perform at least the following checks on the received data:
The message counter has to match the expected value.
The CRC check sum for segments A to C (
9.5.4.1.2) calculated in the evaluation unit has to match
the CRC check sum received in segment D or its value that has been inverted bitwise (request for transi-
tion into safe state by channel B)
The timing has to be maintained. Two consecutive message packets have to have an interval of 4ms
with a max. tolerance of ±20%. This implicitly includes detection of a time-out when the sensor is not
transmitting anymore.
The number of the received bytes has to be exactly 11. In single-channel operation (
9.2.2), the sen-
sor will not send a verified position. Such a data packet may not be used for safety functions.
Errors in the transmission of symbols (framing errors) have to be detected.
The position has to be valid. The position 0 is the request for transition into safe state by channel A.
If an error is detected in one of the points stated above after suitable filtering
1
, the evaluation unit has to assume
the safe state.
10.2.2
General Requirements
The following measures are not compulsory but recommended:
Repeated check for implausible jumps
Exchange of the received positions between the channels and comparison with the position of the other
channels. If this comparison is not performed, at least the process factors or states derived from the po-
sition information must be compared with each other.
NOTE!
Even when working with two synchronized processors, there is no guar-
antee that the data received from the sensor regarding channel 1 and
channel 2 are always 100 % consistent. Due to slight delay differences,
it is possible that for example channel 1 has already received a new
data packet while channel 2 is still working with the data from the pre-
vious packet. The software of the evaluation unit should take this effect
into account in order to avoid availability problems.
1
The type of filtering is not specified and depends on the design of the evaluation unit. The filtering parameters are basically
determined based on the reaction time for opening the safety circuit.
