Why does Modbus have more slave devices, longer polling time, and how to choose the appropriate polling cycle

The reason why the more slave devices there are, the longer the polling time is mainly because in the Modbus protocol, the master communicates with the slave through polling. Each slave has a unique device address in the network. The host performs read and write operations with specific slaves through their addresses.

As the number of slaves increases, the host needs to communicate with more slaves. During a complete polling cycle, the host needs to send more requests and wait for the slave to respond. As the number of slaves increases, the following factors may cause the entire polling process to slow down:

1. More requests: The host needs to send more requests in order to communicate with all slaves. This will result in an overall increase in polling time.
2. Waiting time: The host needs to wait between sending a request and receiving a response from the slave. This waiting time is caused by physical hardware, communication delays, and other factors. An increase in the number of slaves means more waiting time.
3. Communication congestion: When a large number of slaves are connected to the same Modbus network, it may cause communication congestion. This will increase the delay of master-slave communication, thereby slowing down the entire polling process.

To reduce polling time, please consider the following optimization plan:

1. Adjust polling interval: Reasonably adjust the polling cycle based on the real-time data and the requirements of the slave device type.
2. Improving communication speed: By increasing the Modbus communication speed, data transmission speed can be accelerated, thereby reducing polling time.
3. Interface grouping: Classify slaves according to application scenarios and perform group polling on slaves of the same category. This can improve polling efficiency and make network burden more controllable.

There is no fixed standard for the ratio of slave devices to polling cycles, as it depends on many factors such as actual application scenarios, device types, network speed, and real-time data requirements. It is important to find a balance point in actual projects, ensuring that data real-time requirements are met without overcrowding the network or overloading the host (or slave).

For example:

Assuming you have 10 slave devices, each device needs to update data every second. You can set the polling interval to 100 milliseconds to ensure that polling for all devices is completed within 1 second. This will assign the same time window to each slave, which in this example is (1 second ÷ 10 slaves=100 milliseconds).

However, these values are only examples, and the appropriate ratio in actual projects depends on the actual situation. In practical applications, you need to dynamically adjust the polling cycle based on factors such as communication speed, device response time, and application scenarios to meet performance requirements. During the project debugging phase, the optimal polling settings can be determined through experimentation and observation.