Rockwell Automation Ethernet Design Considerations Reference Manual - Manual (Page 88)

88 Rockwell Automation Publication ENET-RM002C-EN-P - May 2013
Chapter 5 Predict System Performance
Estimate the Fastest RPI
The fastest RPI for an EtherNet/IP communication module is calculated with
this formula:
RPI
(Fastest)
= (2 x connections)/pps
Example One
A 1756-EN2T module, firmware revision 3.6 or later, supports 25,000 pps. If
there are only 40 connections that are all at the same RPI, the fastest RPI is
the following:
RPI
(Fastest)
= (2 x 40)/25,000 = 3.2 ms
Example Two
Assume there are 23 connections running at an RPI of 2 ms on a 1756-EN2T
module, firmware revision 3.6 or later, that supports 25,000 pps. These
connections are already using some of the communication packets:
pps = (2 x 23)/0.002 = 23,000 pps
The fastest RPI possible for a fourth connection is the following:
RPI
(Fastest)
for 4
th
connection = (2 x 1)/(25,000-23,000) = 1 ms
The same concept can be used for produced tags and consumed tags by replacing
the (2 x connections) with (1 + connections) for produced tags or (2/RPI) for
consumed tags.
IMPORTANT It is not necessary to operate at the fastest RPI.