w4k fails reading modbus rtu float32

w4k fails reading modbus rtu float32 - Printable Version

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w4k fails reading modbus rtu float32 - cgn - 15.04.2026

I've a eastron sdm72d modbus meter and I use the following lua code:

Code:
require('luamodbus')
mb = luamodbus.rtu()

mb:open('/dev/RS485', 9600, 'N', 8, 1, 'H')   -- SDM72 nutzt 8N1
mb:setslave(20)
mb:connect()


local r1, r2 = mb:readinputregisters(70, 2)
local value = mb:readregistervalue(70, "float32")
log(r1, r2, value)


mb:close()

I get the following result:

Code:
* arg: 1
  * number: 16967
* arg: 2
  * number: 57672
* arg: 3
  * number: 0

So in general the code seems to work. Why do I get a 0 for value?
The modbus register can be found here: https://data.xn--stromzhler-v5a.eu/manuals/eastron_sdm72dmv2.pdf

RE: w4k fails reading modbus rtu float32 - admin - 15.04.2026

Use mb:readinputregistervalue(). It's highly recommended to use profiles instead of scripts.

RE: w4k fails reading modbus rtu float32 - cgn - 16.04.2026

Using a profile was my first try, but it didn't work. So I tried so solve it with this script.

Code:
{
  "manufacturer": "Eastron",
  "description": "3-phasiger Energiezähler SDM72D-M",
  "mapping": [
    {
      "name": "Frequency",
      "address": 70,
      "type": "inputregister",
      "bus_datatype": "float32",
      "unit": "Hz"
    }
  ]
}

With this profile I get the value 16967.000. Which is wrong, it should be around 50.

readinputregistervalue() - seems to be the problem.

Code:
require('luamodbus')
mb = luamodbus.rtu()

mb:open('/dev/RS485', 9600, 'N', 8, 1, 'H')
mb:setslave(20)
mb:connect()

local r1 = mb:readregistervalue(70,"float16")
local r2 = mb:readregistervalue(71,"float16")

local value = mb:readregistervalue(70, "float32")
log(r1, r2, value)

mb:close()

log:

Code:
* arg: 1
  * number: 0
* arg: 2
  * number: 0
* arg: 3
  * number: 0

RE: w4k fails reading modbus rtu float32 - admin - 16.04.2026

Your profile is missing "datatype": "float32"

RE: w4k fails reading modbus rtu float32 - cgn - 16.04.2026

Thank you verry much. Now I can use the profile.