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

Data Transfer Time 67
A typical data transfer time (based on a typical scan time) is 1 to 2 seconds, and the time-out
limit for a Peer-to-Peer send (including three retries) is 5 seconds. Consequently, the processtolerance
time of the receiving controller must be greater than 5 seconds. Process-tolerance time
is the maximum length of time that can elapse before your control algorithms fail to operate
correctly. If these limitations are not acceptable, further analysis of your process is required.
For an example that shows how to measure the maximum data transfer time and use
SEND/RECEIVE function blocks to transfer-safety critical data, see Example 4: Using
SEND/RECEIVE Function Blocks for Safety-Critical Data on page 69. While testing your
system, you should measure the actual maximum time it takes to transfer data to ensure the
validity of your calculations.
As Table 16 shows, it takes from two to 30 seconds to detect and report time-out and
communication errors. This is why a receiving node that uses the received data to make safetycritical
decisions must include logic to verify that new data is received within the specified time
period. If the data is not received within the specified process-tolerance time, the application
must take appropriate actions depending on requirements.
For an example that shows how to use SEND and RECEIVE function blocks for transferring
safety critical data, see Example 4: Using SEND/RECEIVE Function Blocks for Safety-Critical
Data on page 69.
Refer to the TriStation 1131 Libraries Reference for additional information.
Safety Considerations Guide for Trident v2 Systems