SSP North AB
30/08/2018
Manual V2f
86
13.10.5 Global and CAN Memories
As mentioned in earlier chapters, Global Memories and CAN Memories can be used to send safety
information between nodes in a system via Radio or CAN. The Global Memories are sent via both
radio and CAN, while the CAN memories are only sent via CAN. A maximum of 16 Global Memories
can be defined for each node, and an additional 16 CAN Memories can be defined for each node. This
means a maximum of 16 signals can be sent from each node via radio, and a maximum of 32 signals
can be sent from each node via CAN. All Global and CAN memories can be used by any node in the
system.
13.10.5.1 Choosing Memory Numbers
There is no difference between the memories in the radio communication. Either all memories from a
node are received, or no memories are received, and all memories are received at the same time. This
means that no memory number is better than another, and there are theoretically no rules for
assigning memory numbers. There are however some good guides to follow, to make debugging
easier.
It is recommended that similar functions are assigned to the same memory number in every node. For
example, it is good practice to put all E-Stops on GM1, and all Reset buttons on GM6 for all nodes that
have these functions. This makes debugging easier with the display, as the status of all E-Stops is
displayed in the same position for all nodes.
It is also recommended that the memory numbers follow the I/O:s. If a Push Button is assigned to
terminal 6, it makes sense that it is sent on GM Number 6 as well.
These are only guidelines and cannot always be followed for all applications. The user is free to make
their own rules for how memories are assigned, but it should be as consistent as possible, and make
sense for the application.
13.10.5.2 Timeout and StartUp test
For every Global or CAN memory, apart from Memory Number, two more properties exist; Timeout and
StartUp test. These properties are important for the safety communication and determine how the
receiving node uses the Memory signal. There is also a third property for Global memories called Do
Not Repeat, which is discussed further down in this chapter.
Timeout
The response time of a complete function that is sent over radio (Input
→
Logic
→
Output) is given by
T
input
+ T
logic
+ T
timeout
+T
logic
+T
output
. The T
timeout
time is given by the timeout specified on the memory that
is being used to send the signal. Generally, the longer the communication timeout, the more reliable
and fault tolerant radio communication between nodes. Setting shorter timeout guarantees shorter
maximum response times for safety signals sent over radio, but also results in less robust
communication over radio. The question that should determine the timeout values is “What is the
maximum theoretical allowed response time for the safety device I am using?”. For an E Stop, the
maximum response time can be set to higher values (between 500ms to 1000ms is acceptable). For a
Light Curtain/ESPE device, the response time requirement is much stricter, so a timeout of 200ms or
less can be required. This means that the E Stop would be more resistant to radio disturbances than
the ESPE but would also have a longer theoretical maximum reaction time than the ESPE. Reaction
time must always be weighed against communication robustness when choosing radio timeouts.
Two different values for timeout can be set for a project; Long and Short. These are default set to Long
= 500ms, and Short = 200ms. For each global or CAN memory one of these values can be specified.
This is so that safety devices that require shorter timeouts (such as light curtains) can use the short
timeout value, and other devices that allow longer reaction times can use the long timeout value.
Imagine that a node (A) receives a radio packet from another node (B). Two countdowns start
internally in node (A); one for the long timeout and one for the short timeout. If the long timeout is set
to 500ms and the short timeout is set to 200ms, the long timer is set to 500ms and the short timer is
set to 200ms, and they start counting down immediately. When the timer for short timeout reaches
zero all memories from node (B) with short timeout are internally set to zero at the receiver node (A).
The same happens when the long timeout reaches zero for the memories with long timeout. Every
time a packet is received from node (B) both timers are restarted at their initial values.
Repeated information does not change this behaviour, as the timing information about all data is
repeated as well. If a third node (C) sends data to node (B), and node (B) repeats it to node (A), node
