Mock a Modbus device — develop your Modbus client on the laptop

A lot of QuickApps speak Modbus TCP — SolarEdge inverters, energy meters, battery systems. Developing one is painful: every change means uploading to the HC3, watching the log, poking the real device. flua flips that around: the same QA runs on the laptop, and a second QA can be the Modbus device. Change the mocked values with a button press and watch your client react — no controller, no inverter.

The mock that ships with flua is a SolarEdge inverter (examples/Modbus Server/): the SunSpec register map straight from SolarEdge's technical note (a copy of the note sits next to the map), built on a small generic Modbus server library (modbus_server.lua — FC3 reads, FC6/FC16 writes, partial-frame buffering). It has buttons — Sun +500W, Grid: Export, Battery SOC 100% — that mutate the registers. One honest caveat: the mock QA itself only runs in flua — it opens local sockets, which the HC3 does not allow. Your client still deploys to the HC3 unchanged.

The client side is ordinary Modbus code. Here is a minimal SolarEdge SunSpec client — powerMeter device, reads AC power (SunSpec register 40083) with its scale factor (40084) every ten seconds:

--%%name:SE Meter
--%%type:com.fibaro.powerMeter
--%%var:host="127.0.0.1"
--%%var:port="1502"
-- --------------- EOH ---------------
-- Minimal SolarEdge SunSpec client: reads AC power (SunSpec register
-- 40083, base-0) with its scale factor (40084) every 10 s.

local socket = net.TCPSocket({timeout = 5000})

local function word(x) return string.char(math.floor(x / 256) % 256, x % 256) end

function QuickApp:readPower()
  -- MBAP header + FC3 (read holding registers), 2 registers from 40083
  local tx = string.char(0, 0, 0, 0, 0, 6, 1, 3) .. word(40083) .. word(2)
  local function schedule() setTimeout(function() self:readPower() end, 10 * 1000) end
  socket:connect(self:getVariable("host"), tonumber(self:getVariable("port")), {
    success = function()
      socket:write(tx, {
        success = function()
          socket:read({
            success = function(response)
              if response and #response >= 13 then
                local power = response:byte(10) * 256 + response:byte(11)
                local sf = response:byte(12) * 256 + response:byte(13)
                if sf >= 0x8000 then sf = sf - 0x10000 end  -- int16
                self:updateProperty("value", power * 10 ^ sf)
                self:updateProperty("unit", "W")
                self:updateProperty("log", os.date("%H:%M:%S"))
              end
              socket:close()
              schedule()
            end,
            error = function() socket:close(); schedule() end,
          })
        end,
        error = function() socket:close(); schedule() end,
      })
    end,
    error = schedule,
  })
end

function QuickApp:onInit() self:readPower() end

Save it and press F5 with the Run & Debug configuration below (it is the standard UI+mobdebug launch with the mock file added as a second argument — both QAs boot in one flua, the mock binds 127.0.0.1:1502, the client connects to it):

{
  "name": "Flua: Debug with Modbus (UI+mobdebug)",
  "type": "luaMobDebug",
  "request": "launch",
  "workingDirectory": "${workspaceFolder}",
  "sourceBasePath": "${workspaceFolder}",
  "listenPort": 8172,
  "stopOnEntry": false,
  "sourceEncoding": "UTF-8",
  "interpreter": ".venv/bin/flua",
  "arguments": [
    "--ui", "8090",
    "-D", "var:host=\"127.0.0.1\"",
    "${relativeFile}",
    "examples/Modbus Server/solaredge_mock.lua"
  ],
  "listenPublicly": true
}

The -D var:host=… is the launch-time override of the QA's own --%%var:host — no code edits to switch between the mock on the laptop and the inverter on the LAN. Open http://127.0.0.1:8090/ and you see both QAs: your client showing 2500 W, and the mock with its buttons. Press Sun +500W — the client's next poll reads 3000 W. The condensed view (the View toggle) shows both QAs as small status cards, and a breakpoint in the read callback stops the client mid-request (post 4).

One address note, because it bites everyone once: SunSpec registers are base-0 wire addresses — AC power is register 40083, sent as address 40083. Some QAs built from the legacy SolarEdge protocol docs number registers 1-based and subtract 40001 instead (40084 → wire 83). The real inverter answers both (verified on device), and so does the mock — its server library serves the map at two bases (base + altBase), so either client convention works.

Already have a Modbus client QA running on your HC3? Bring it over: flua --tool downloadQA <id> (post 7's .env first), then flua unpack name.fqa -d seqa/ — flua regenerates the --%% directives and you get a folder of .lua files to open in VS Code. Point host at 127.0.0.1, press F5, and you are developing against the mock instead of the controller. The QA you test here is the QA you deploy — only the host variable changes.

flua: https://github.com/jangabrielsson/flua · install: pip install fibaro-flua