Example: Write Modbus RTU multimeter values to KNX/EIB bus group addresses

Task

Read two parameters (3-phase system voltage, 3-phase system current) from ABB DMTME Multimeter and store the data in the KNX group addresses. Make sure to connect LM2 with Modbus device correctly, RS485 A with -, RS485 B with +.

Adding KNX objects

In Objects tab two new KNX objects should be added. In this case:

• 5/5/1 – for writing 3-phase system voltage

• 5/5/2 – for writing 3-phase system current

Modbus program

• In Logic Machine -> Scripting a new Resident script should be added which will periodically run the program and update KNX group addresses with Modbus data readings

• Click on the Editor icon of the respective script to jump to Scripting editor where the following program is added

Source code

-- init modbus on first script execution
if not mb then
     require('luamodbus')
     mb = luamodbus.rtu()
     mb:open('/dev/ttyS2', 9600, 'E', 8, 1, 'H')
     mb:connect()
end
 
-- sets slave ID to read/write data from/to
mb:setslave(20)
 
-- read 3-phase system voltage from 32-bit register
r1, r2 = mb:readregisters(0x1000, 2)
result = bit.lshift(r1, 16) + r2
grp.write('5/5/1',result)
 
-- read 3-phase system current from 32-bit register
r1, r2 = mb:readregisters(0x100E, 2)
result = bit.lshift(r1, 16) + r2
grp.write('5/5/2',result)

 

Note! Modbus data is most often read and written as 16-bit registers. If a 32-bit is required, these values have to be read as a pair of registers, like in this example.

Visualizing Modbus objects

By using grp.write command assign Modbus object to KNX object and then use this new KNX object in the visualization.

More Modbus communication commands

• Open Modbus TCP connection:

Source code

if not mb then
     require('luamodbus')
     mb = luamodbus.tcp()
     mb:open('192.168.1.1', 1502)
     mb:connect()
end

• Reads count registers/coils from the start address and return all values on success and nil, error description on error:

Source code

mb:readcoils(start, count)
mb:readdiscreteinputs(start, count)
mb:readregisters(start, count)
mb:readinputregisters(start, count)

• Write values to registers/coils from the start address. Single write will be used when only one value is supplied, multiple write otherwise. Returns all of values written on success and nil, error description on error:

Source code

mb:writebits(start, v1, [v2, [v3, ...]])
mb:writeregisters(start, v1, [v2, [v3, ...]])

• Read slave internal data. Returns values on success and nil, error description on error:

Source code

mb:reportslaveid()