Logic Programming – Function Blocks - Mitsubishi Electric WS0-CPU0 Operating Manual

9
Logic programming – Function blocks
ATTENTION
Logic programming – Function blocks
9.1 General description
The function logic of the MELSEC-WS safety controller is programmed by using
function blocks. These function blocks are certified for use in safety-relevant functions
if all safety standards are observed during implementation. The following sections
provide information on important aspects of using function blocks in the MELSEC-WS
safety controller.
9.2 Safety notes for the logic programming
Observe the relevant safety standards and regulations!
All safety related parts of the installation (cabling, connected sensors and actuators,
configuration settings) must be according to the relevant safety standards (e.g. IEC
62061 or EN/ISO 13849-1) and regulations. Only safety-relevant signals may be used
in safety-relevant logic. Ensure that the application fulfills all applicable standards and
regulations!
You are responsible for checking that the right signal sources are used for these
function blocks and that the entire implementation of the safety logic fulfills the
applicable standards and regulations. Always check the mode of operation of the
MELSEC-WS module and of the logic program in order to ensure that these behave in
accordance with your risk avoidance strategy.
Take additional safety measures if the safe value may lead to a dangerous
condition!
The safe value of process data and outputs is Low, which is applied if an error is
detected. If the safe value (signal = Low) may lead to a dangerous condition in the
application, additional measures must be taken, e.g. evaluation of the status of the
process data and switching off the related output signals if the status evaluation
detects an error. This needs especially to be considered for inputs with edge
detections.
Take unexpected rising or falling edges into account!
Special attention is required for applications where an unexpected rising or falling
edge at an input with edge detection may lead to a dangerous situation. An error on
an input can generate such edges (e.g. network or EFI communication interruption,
cable interruption at digital input, short circuit at digital input connected to test output).
The safe value is applied until the error reset condition is fulfilled. Due to this the
related signal can behave as follows:
 It changes temporarily to High, instead of remaining Low (rising edge and falling
edge, i.e. transition from Low to High to Low),
or
 it changes temporarily to Low, instead of remaining High as under faultless
condition (falling edge and rising edge, i.e. transition from High to Low to High),
or
 it remains Low, instead of changing to High as under faultless condition.
Chapter 9
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