Modbus Controller (as Server): Support for Default Values in Unused Modbus Registers

[carlessole]

Component name

Modbus Controller

Link to component documentation on our website

https://esphome.io/components/modbus_controller.html

Describe the enhancement

Sometimes, in a Modbus Server Device, its Modbus Map includes empty or unused registers reserved for future use.

For example, a device may expose data at the following addresses:

• Registers 0–6: Device info and configuration
• Registers 10–13: Temperature data
• Registers 20–28: Humidity data

Since a Read Holding/Input Registers request can span up to 125 registers in a single transaction (see Modbus spec - Function 0x03), some Modbus clients may attempt to read large blocks of contiguous addresses to reduce the requests amount.

On this scenario, if a client sends a request like:

Function: 0x03 Read Holding Registers
Start Address: 0x00
Quantity: 0x30

ESPHome responds with Exception Code 0x02 (Illegal Data Address), since some of the requested registers are not defined.
The behavior is correct, but can limit interoperability with clients that expect to read blocks of data that may include gaps.

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Proposed Solution

Introduce an optional configuration in the Modbus controller to define a default value for undefined registers up to a given address.

max_reg_address_with_default_value: 0..65535 (default 0)
reg_default_value: value (default 0)

modbus_controller:
  max_reg_address_with_default_value: 100
  reg_default_value: 0

With this configuration, any Read Holding/Input Registers request for addresses <= max_reg_address_with_default_value will be accepted, and undefined registers within that range will return the specified default value (e.g., 0).

Use cases

Since a Read Holding/Input Registers request can span up to 125 registers in a single transaction (see Modbus spec - Function 0x03), some Modbus clients may attempt to read large blocks of contiguous addresses to reduce the requests amount.

On this scenario, if a client sends a request like:

Function: 0x03 Read Holding Registers
Start Address: 0x00
Quantity: 0x30

ESPHome responds with Exception Code 0x02 (Illegal Data Address), since some of the requested registers are not defined.
The behavior is correct, but can limit interoperability with clients that expect to read blocks of data that may include gaps.

Anything else?

No response

[github-actions[bot]]

🏷️ I've automatically added the component: modbus_controller label to help categorize this feature request.