Modbus, getting bits in a register

[espen.sorensen@jmhansen.no]

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

[Erwin van der Zwart]

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

[admin]

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')

[espen.sorensen@jmhansen.no]

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?

[admin]

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

[espen.sorensen@jmhansen.no]

Thanks!