WAGO-I/O-SYSTEM 750
MODBUS – e!RUNTIME 317
750-8207 PFC200 CS 2ETH RS 3G
Manual
Draft version 1.2.1 from 2017-09-13, valid from FW Version 02.06.20(09)
11.3 Estimating the MODBUS Master CPU Load
Due to the real-time characteristics of the Linux
®
kernel used, many data points
can generate many context changes.
For a one-off update (transmitting and receiving of a function code), a CPU time
of approx. 800 µs can be assumed.
The CPU load (cpu_load) in percent can be estimated from the cycle time (t_z)
for a query with the following rule of thumb:
cpu_load = 800 µs / t_z * 100
A cycle time of 100 ms thus results in a CPU load of 0.8%.
A maximum load of approx. 20% can be generated per connection, as this is
limited by the network protocol. To minimize the CPU load:
-
The cycle time must be as high as possible.
-
As many data points as possible must be combined in a query.
-
The minimum query interval can be increased (default value: 0 ms).
