Best way to decode Modbus - Printable Version

Best way to decode Modbus - Printable Version

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+--- Thread: Best way to decode Modbus (/showthread.php?tid=3450)

Best way to decode Modbus - gtsamis - 29.06.2021

Hi,

I have a DSE7320 power generator controller and i need to read some alerts.
I use Modbus mapper to store the alert registers to virtual objects and then decode them with script.

Code:
dword ='0x'.. event.datahex
myobject = grp.find(event.dstraw)

function getbit(value, nr)
  value = bit.rshift(value, nr)
  return bit.band(value, 1)
end

for i = 0, 15, 4 do 
  obj=math.floor((i+1)/4)
  msg=''
  val=''
  for x = 0, 3, 1 do
    ix=i+x
      val = getbit(dword,ix)..val
  end
 
  bin = string.reverse(val)
    local sum = 0

    for i = 1, string.len(bin) do
      num = string.sub(bin, i,i) == "1" and 1 or 0
        sum = sum + num * math.pow(2, i-1)
    end
  
  msg=msg .. ' Bin:'..val..' Dec:'.. sum
    obj=obj..msg
  alert(myobject.name..'-'..obj)
end

Can someone please verify if this is the most eficient way or sugest something better?

Thank you in advance
George

RE: Best way to decode Modbus - admin - 30.06.2021

The most correct way to do this is via bit shifts (bit.rshift and bit.band) but since you need to split the value into 4 bit values you can simply use the datahex string:

Code:
value = event.datahex

values = {}
for i = 1, 4 do
  char = value:sub(i, i)
  values[ i ] = tonumber(char, 16)
end

log(values)