Knowledge Base

The purpose is to examine the performance of the Modbus protocol in master mode during data acquisition.

In this architecture there are two PCs, one acting as master and the other as slave. The slave machine is also a PcVue project with Modbus Slave protocol simulating 100 PLCs.

The sample project used for both master and slave stations was developed by the Technical Support team.

PC capacities

The power of the Modbus Slave station must be sufficient so that it is never at 100% of CPU capacity during the tests. The capacity above is more than enough.

Simulated features of the project 

Results of the tests

For 400 frames scanned in 1 second:

For 400 frames scanned in 200ms:

For 400 frames scanned in 50ms:

Conclusions

To ensure that the Ethernet network functions well, the following are advisable in the above example:

• Not to exceed 30% of the network’s bandwidth.
• On a 10Mbit network, the period for processing the frames should be around 2 seconds.
• On a 100Mbit network, a period of 200ms can be sustained easily which makes it possible to read all of the frames in 250ms.

With a 50ms period on the 100Mbit network, the CPU is overloaded. It would be worth repeating the test on a more powerful machine.