Pull Request Description:

This pull request introduces two key efficiency enhancements to the Modbus plugin, designed to reduce unnecessary data traffic and allow for more flexible polling strategies.These changes enable users to prioritize high-frequency data from critical devices while minimizing network and cloud resource usage for static or infrequently changing data points.

Motivation

In many industrial environments, it's necessary to monitor certain devices (like critical sensors) at a high frequency (e.g., every second), while other devices only require infrequent updates. Similarly, some registers, such as status flags, only need to be reported when their value changes. This implementation addresses that need by providing more granular control over data polling and publishing.

Features Implemented

1. Per-Device Polling Interval

This feature allows a custom polling interval to be set for each individual Modbus device, overriding the global default.

• How it Works: The plugin checks for an optional poll_interval key within a device's configuration in the devices.toml file. If this key is present, its value is used for that specific device. If it is absent, the plugin falls back to the global pollinterval defined in modbus.toml, ensuring full backward compatibility.
• Configuration: To set a custom rate, add poll_interval = <seconds> to a [[device]] table in devices.toml.

2. On-Change Publishing

This feature prevents the plugin from publishing data for a register if its value has not changed since the last poll.

• How it Works: For any register configured with on_change = true, the plugin caches the last published value. A new MQTT message is only published if the newly polled value is different from the cached one, or on the very first poll for that register. If the on_change key is false or absent, the default behavior of publishing after every poll is maintained.
• Configuration: To enable this, add on_change = true to a [[device.registers]] table in devices.toml.

Example Configurations

Test modbus.toml:

[modbus]
ip_address = "127.0.0.1"
port = 502
pollinterval = 5
loglevel = "INFO"

[thinedge]
mqtthost= "127.0.0.1"
mqttport = 1883

Test devices.toml with new updates:

[[device]]
name = "mock_battery"
address = 1
ip = "127.0.0.1"
port = 502
protocol = "TCP"
poll_interval = 1  # new update
[[device.registers]]
number = 0
startbit = 0
nobits = 16
signed = false
multiplier = 1
input = false
measurementmapping.templatestring = '{"power": %%}'

[[device.registers]]
number = 100
startbit = 0
nobits = 16
signed = false
input = false
measurementmapping.templatestring = '{"error_flag": %%}'
on_change = true # new update