Hey there! Let me break down what that unit parameter is all about, since the source comment can feel a bit cryptic at first.

What does unit mean? #

The unit parameter maps directly to the Unit Identifier field in the Modbus TCP protocol. To put it simply:

• Modbus is built on a master-slave architecture. Even though Modbus TCP uses TCP/IP connections (which are peer-to-peer by nature), the protocol retains this field to identify specific "slave" devices.
• If you're communicating through a Modbus TCP-to-RTU/ASCII gateway, this unit value corresponds to the address of the serial slave device connected to the gateway.
• For standalone Modbus TCP devices, this value is a unique identifier configured on the device itself (think of it like a device ID for the Modbus layer).

The source comment's "slave unit this request is targeting" just means: this is the specific device you want to send your read/write command to.

Why did your code break when upgrading to pymodbus 1.4.0? #

Older versions of pymodbus (like 1.2) likely used a default value for unit (often 0 or 1) without requiring you to specify it explicitly. The 1.4.0 update made this parameter mandatory, forcing you to explicitly define which device you're targeting.

What values can you use for unit? #

The valid range for unit is 0 to 247, with specific rules:

• 0: This is the broadcast address. You can use it to send write commands to all slave devices on the network, but you can't use it for reads (since multiple devices would respond at once, causing conflicts).
• 1 to 247: These are valid, assignable slave addresses. The exact value you need depends on the configuration of your target device—check its manual or setup interface (like a web UI, dip switches, or configuration software) to find the correct Unit Identifier.
• 248 to 255: These are reserved for special use cases and shouldn't be used for regular device communication.

In your case, using unit=1 works because your target device is configured with a Unit Identifier of 1. If you were communicating with a different device, you'd need to replace 1 with that device's specific configured value.

内容的提问来源于stack exchange，提问作者Joshua Clayton