Modbus Server Split Out of modbus_controller

May 14, 2026

J. Nick Koston

ESPHome Maintainer

Modbus server mode has been split out of modbus_controller into a new dedicated modbus_server component. Configurations that used modbus_controller as a Modbus server (the role: server setting lives on the modbus: bus, not on modbus_controller) must move their server_registers: and server_courtesy_response: blocks under a new top-level modbus_server: entry.

This is a breaking change for YAML configurations using server mode in ESPHome 2026.5.0 and later.

Background

PR #15509: Split modbus_server from modbus_controller

Server-mode register handling was wedged into modbus_controller alongside its client-mode polling. The two roles share a transport (the modbus: bus) but almost nothing else: server mode dispatches inbound requests to user lambdas, while client mode polls outbound register reads on a schedule. Keeping both in the same component made the controller heavier than it needed to be, complicated review of server-side fixes, and prevented the server logic from sharing helpers with future modbus-based components.

modbus_server is a thin Component that implements modbus::ModbusDevice and owns the register table plus the optional courtesy response, with no client-mode machinery. The migration in this PR is intentionally close to a copy/paste — functional improvements to server mode (out-of-range handling, multi-register transactions, parser fixes, etc.) come in follow-up PRs.

Flash savings (approximate)

Configuration	Before	After
Pure server	modbus_controller ~4.5 KB	modbus_server ~1.8 KB
Pure client	modbus_controller ~6.4 KB	modbus_controller ~3.9 KB

Devices that previously combined client and server in one modbus_controller will now have two separate components, but each is much smaller.

What’s Changing

YAML keys moved

Old (under modbus_controller)	New (under modbus_server)
server_registers:	registers:
server_courtesy_response:	courtesy_response:

modbus_controller is now client-only

role: server on the bus is still supported — that selects the transport-layer role. What changed is that the register table is no longer accepted on modbus_controller. Put it on modbus_server instead.

Who This Affects

YAML configurations that used modbus_controller as a Modbus server — typically battery management bridges (BMS protocol bridges, yamBMS forks), EVSE bridges, energy/inverter proxies, and any “act as a register slave” project. Pure-client modbus_controller users (the more common case — reading from third-party Modbus devices) are not affected.

Migration Guide

# Before
modbus:
  role: server

modbus_controller:
  - address: 1
    server_registers:
      - address: 0x0001
        value_type: U_WORD
        read_lambda: "return id(my_sensor).state;"
    server_courtesy_response:
      enabled: true
      register_value: 0

# After
modbus:
  role: server

modbus_server:
  - address: 1
    registers:
      - address: 0x0001
        value_type: U_WORD
        read_lambda: "return id(my_sensor).state;"
    courtesy_response:
      enabled: true
      register_value: 0

A modbus: bus has a single role and your device occupies one side of it: modbus_controller requires role: client, modbus_server requires role: server, and the bus schema enforces this — you cannot mix the two on the same modbus: instance. Modbus only allows one client per bus, so the “both on one device” pattern was never valid; if a previous config had server_registers: under a client-mode modbus_controller, those registers were silently inactive. Run a second modbus: bus on a different UART if you need a device that’s both a server on one wire and a client on another.

Timeline

• 2026.5.0: Server-mode keys removed from modbus_controller; new modbus_server component active.

This is a clean break — there is no deprecation period or migration shim. modbus_controller configs that still contain server_registers: or server_courtesy_response: will fail validation on 2026.5.0. (For contrast, the earlier modbus_controller → modbus::helpers namespace refactor in 2026.4.0 kept deprecation shims through 2026.10.0 — that pattern doesn’t apply here because there’s no in-place rename to forward, and the affected configs are caught at config-validation time with a precise error message rather than at runtime.)

Finding Code That Needs Updates

# YAML — find server-mode keys still under modbus_controller.
# -E (extended regex) keeps the alternation portable across GNU and BSD/macOS grep.
grep -rEn 'server_registers:|server_courtesy_response:' your_configs/

Questions?

If you have questions about migrating your configuration, please ask in:

• ESPHome Discord - #devs channel
• ESPHome GitHub Discussions

Related Documentation

• PR #15509 — Split modbus_server from modbus_controller
• PR #15291 and PR #14172 — Earlier helper-function refactor that enabled this split (blog post)
• ESPHome Modbus Server documentation (new page)
• ESPHome Modbus Controller documentation

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