Safety Considerations Guide for Trident v2 Systems - TUV ...

Examples of Peer-to-Peer Applications 69
Example 4: Using SEND/RECEIVE Function Blocks for Safety-Critical Data
This example shows how to use SEND/RECEIVE function blocks for transferring a limited
amount of safety-critical data between the two applications as fast as possible. It also shows how
to measure the actual maximum time for transferring data from the sending node to the
receiving node.
Sending Node #1 Parameters
• Scan time (SS) = 125 milliseconds.
• Number of aliased variables in bytes = 2000.
• Time to transfer aliased data over the communication bus in milliseconds (TS) =
(2000/100,000) * 1000 = 20 milliseconds.
• The sending controller has one SEND function block in the application, meeting the
requirement to have five or fewer SEND function blocks. The sendflag parameter is in
the SEND function block so that the sending controller initiates another SEND as soon
as the last SEND is acknowledged by the receiving controller.
Receiving Node #3 Parameters
• Scan time (SR) = 100 milliseconds.
• Number of aliased variables in bytes = 15,000.
• Time to transfer aliased data over the communication bus in milliseconds (TR) =
(15,000/100,000) * 1000 = 150 milliseconds.
• Process-tolerance time = 4 seconds.
• Estimated data transfer time = 2 * 125 + 2 * 150 = 550 milliseconds.
If the sending controller does not receive acknowledgment from the receiving controller in one
second, it automatically retries the last SEND message. Because of network collisions,
communication bus loading, etc., the sending controller occasionally has to retry once to get the
message to the receiving node. This is why the general rule for data transfer time is one to two
seconds, even though the estimated time is 550 milliseconds.
The receiving node has a network to measure the actual time so you can validate the assumed
two-second maximum transfer time. Since the process-tolerance time of the receiving node is
four seconds, the maximum time-out limit is set to two seconds (half the process-tolerance
time). The receiving node should receive at least one data transfer within the maximum timeout
limit. Using this criteria meets the basic requirement for using peer-to-peer communication
to transfer safety-critical data.
Safety Considerations Guide for Trident v2 Systems