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FS-RA-CLX_SlotServer_Instruction_Manual_(T17011) Page 26 of 34
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Appendix A.4. Dealing with ControlLogix RPI Settings
When setting up the SlotServer for ControlLogix, it is necessary to select the Request
Packet Interval (RPI). The RPI is the rate at which data is transferred to and from the
SlotServer IO buffers. The following factors need to be considered when deciding on an
RPI:
1. Minimum RPI setting for the SlotServer is 100ms.
2. The Scan_Interval parameter of the two Logix Map descriptors must be larger than the
RPI to ensure smooth communications and prevent timeouts.
3. The number of message blocks used does not affect the RPI setting, but will affect the
effective update rate for any one message block. The effective update rate for data
to/from the SlotServer’s Data Arrays to the Logix CPU tags is the scan interval of the
block since data is updated to the block every scan interval. Increasing the number of
blocks will decrease the effective update rate per block. This update rate does not
include the time taken to obtain data from the third party network, which is dependant
entirely on the third party protocol involved.
4. The program scan rate should be set to run faster than the RPI. We recommend twice
as fast. If the ladder program scan is slower than the RPI rate, it would be possible to
miss some blocks altogether. It is further recommended that diagnostics be added to
the ladder program to detect missed blocks
The effective update rate can be calculated using the following formula:
Effective update rate (EUR) = Scan interval * (number of msg block pairs
6
)
e.g. (EUR) = 0.2s * 1 = 0.2s
when using only one input and output block
e.g. (EUR) = 0.2s * 25 = 5s
when using the full LonWorks Open Interface configuration.
6
a message block pair consists of an output and input block