Modbus, getting bits in a register

Modbus, getting bits in a register - Printable Version

+- Logic Machine Forum (https://forum.logicmachine.net)
+-- Forum: LogicMachine eco-system (https://forum.logicmachine.net/forumdisplay.php?fid=1)
+--- Forum: Gateway (https://forum.logicmachine.net/forumdisplay.php?fid=10)
+--- Thread: Modbus, getting bits in a register (/showthread.php?tid=522)

Modbus, getting bits in a register - espen.sorensen@jmhansen.no - 21.12.2016

Hi,
How can I read out single bits from a 16-bit register? And write the bit to a group address? The modbus addresses in the readcoils function are not the same as for the readinputs addresses.
I also want a tip how I can read signed 16-bit register? (out side air temperature)

Espen Big Grin

RE: Modbus, getting bits in a register - Erwin van der Zwart - 22.12.2016

Hi,

If you read a 16 bits integer register you can get back value 0 to 65535, but when this register is used as single bit (not coil) you probably get back only value 0 or 1 out of this register.

To use that value for a 1 bit KNX value you can simply transfer it to boolean by using this:

Code:
value = mb:readregisters(123)

if value == 0 or value == 1 then

  value = toboolean(value)

  grp.write('1/1/1', value)

end

To set 16 bits register value to 2 byte signed you can use this code:

Code:
r1 = mb:readregisters(124)

value = lmcore.inttohex(r1, 2)

value = knxdatatype.decode(value, dt.int16)

BR,
Erwin

RE: Modbus, getting bits in a register - admin - 22.12.2016

If your register is not a bitmask but a single bit you can write it directly to a boolean object, any numeric value other than 0 will be treated as true. As for reading int16, you can use this:

Code:
value = mb:readregistervalue(123, 'int16')

RE: Modbus, getting bits in a register - espen.sorensen@jmhansen.no - 22.12.2016

My 16-bit register is a bitmask, e.g.:
Input register 3x0006 consists of:
Bit 0 = Dampers open
Bit 1 = Fire dampers open
Bit 2 = Fire dampers closed
Bit 3 = Status feedback pump
Etc.

I'm interested in getting out the status of a single bit within a register. Not the whole register acting as a bit, 0 or 1.
Is it possible?

RE: Modbus, getting bits in a register - admin - 22.12.2016

You can use this helper function:

Code:
function getbit(value, nr)

  value = bit.rshift(value, nr)

  return bit.band(value, 1)

end

value = mb:readregisters(123)

if value then

  dampers_open = getbit(value, 0)

  fire_dampers_open = getbit(value, 1)

  ...

end

RE: Modbus, getting bits in a register - espen.sorensen@jmhansen.no - 22.12.2016

Thanks!