Download 1. Introduction - Beijer Electronics

4. Configuration
Communication Performance
Communication Rate of a MODBUS Server Device
The MODBUS devices configurable in the Nexto CPU run in the background, with a priority below
the user application and cyclically. Thus, their performance varies depending on the remaining time,
taking into account the difference between the interval and time that the application takes to run. For
example, a MODBUS device in an application that runs every 100 ms, with a running time of 50 ms,
will have a lower performance than an application running every 50 ms to 200 ms of interval. It
happens because in the latter case, the CPU will have a longer time between each MainTask cycle to
perform the tasks with lower priority.
It also has to be taken into account the number of cycles that the device, slave or server takes to
respond to a request. To process and transmit a response, a MODBUS RTU Slave will takes cycles
(cycle time of the MODBUS task), whereas a MODBUS Ethernet Server task takes only one cycle.
But this is the minimum time between receipt of a request and the reply. If the request is sent
immediately after the execution of a task MODBUS cycle time may be equal to 2 or 3 times the cycle
time for the MODBUS slave and from 1 to 2 times the cycle time for the MODBUS server.
In this case: Maximum Response Time = 3 * (cycle time) + (time of execution of the tasks) + time
interframe chars) + (send delay).
For example, for a MODBUS Ethernet Server task with a cycle of 50 ms, an application that runs for
60 ms every 100 ms, the server is able to run only one cycle between each cycle of the application.
On the other hand, using the same application, running for 60 ms, but with an interval of 500 ms, the
MODBUS performs better, because while the application is not running, it will be running every 50
ms and only each cycle of MainTask it will take longer to run. For these cases, the worst performance
will be the sum of the Execution Time of the user application with the cycle time of the MODBUS
task.
For the master and client devices the operating principle is exactly the same, but taking into account
the polling time of the MODBUS relation and not the cycle time of the MODBUS task. For these
cases, the worst performance of a relationship will be performed after the polling time, along with the
user application Execution Time.
It is important to stress that the running MODBUS devices number also changes its performance. In a
user application with Execution Time of 60 ms and interval of 100 ms, there are 40 ms left for the
CPU to perform all tasks of lower priority. Therefore, a CPU with only one MODBUS Ethernet
Server will have a higher performance than a CPU that uses four of these devices.
CPU’s Local Interfaces
For a device MODBUS Ethernet Server, we can assert that the device is capable to answer an x
number of requisitions per second. Or, in other words, the Server is able to transfer n bytes per
second, depending on the size of each requisition. As smaller is the cycle time of the MODBUS
Server task, higher is the impact of the number of connections in his answer rate. However, for cycle
times smaller than 20 ms this impact is not linear and the Table 4-60 must be viewed for information.
The Table 4-60 exemplifies the number of requisitions that a MODBUS Server inserted in a local
Ethernet interface is capable to answer, according to the cycle time configured for the MODBUS task
and the number of active connections:
Number of Active Connections
Answered requisitions per second
with the MODBUS task cycle at 5 ms
Answered requisitions per second
with the MODBUS task cycle at 20
ms
1 Connection
160
47
2 Connections
300
95
3 Connections
395
142
4 Connections
470
190
5 Connections
515
237
6 Connections
570
285
111