Convert 32bit unsigned "Real" data type (splitted into two 16 bit signed words) to javascript

Question

i've got a 32 bit unsigned "Real" value splitted into two 16 Bit signed "Word" (0-65535) values. How do I convert them to a javascript number?

Example: Value1: 18584 Value2: 18081 The Real value is: 20644.3

I'm searching for a function like back2Real(18584, 18081) which returns 20644.3. The values come from a modbus application (NodeJS / modbus_stack). The SPS/modbus server sends a "Real" value splitted into two Word-registers.

Regards, root66

Answer 1

You can use the new(ish) typed array functionality to simplify this.

function uint16ToFloat32(low, high) {
  var buffer = new ArrayBuffer(4);
  var intView = new Uint16Array(buffer);
  var floatView = new Float32Array(buffer);

  intView[0] = low;
  intView[1] = high;
  return floatView[0];
}

function float32ToUint16(value) {
  var buffer = new ArrayBuffer(4);
  var intView = new Uint16Array(buffer);
  var floatView = new Float32Array(buffer);

  floatView[0] = value;
  return [intView[0], intView[1]];
}

console.log("Converted ints to", uint16ToFloat32(18584, 18081));
console.log("Converted float to", float32ToUint16(20644.297));

Here's a transcript:

$ node floatsplit.js
Converted ints to 20644.296875
Converted float to [ 18584, 18081 ]
$

Answer 2

Use this function to convert to JavaScript numbers. Since JavaScript uses double precision rather than single precision numbers, some rounding may occur.

function back2Real(low, high){
  var fpnum=low|(high<<16)
  var negative=(fpnum>>31)&1;
  var exponent=(fpnum>>23)&0xFF
  var mantissa=(fpnum&0x7FFFFF)
  if(exponent==255){
   if(mantissa!=0)return Number.NaN;
   return (negative) ? Number.NEGATIVE_INFINITY :
         Number.POSITIVE_INFINITY;
  }
  if(exponent==0)exponent++;
  else mantissa|=0x800000;
  exponent-=127
  var ret=(mantissa*1.0/0x800000)*Math.pow(2,exponent)
  if(negative)ret=-ret;
  return ret;
}

The following function converts JavaScript numbers into 32-bit IEEE floating point numbers, split into the low and high word:

function real2Back(value){
  if(isNaN(value))return [0,0xFFC0]
  if(value==Number.POSITIVE_INFINITY || value>=3.402824e38)
    return [0,0x7F80]
  if(value==Number.NEGATIVE_INFINITY || value<=-3.402824e38)
    return [0,0xFF80]
  var negative=(value<0)
  var p,x,mantissa
  value=Math.abs(value)
  if(value==2.0)return [0,0x4000]
  else if(value>2.0){
   // positive exponent
   for(var i=128;i<255;i++){
     p=Math.pow(2,i+1-127)
     if(value<p){
      x=Math.pow(2,i-127)
      mantissa=Math.round((value*1.0/x)*8388608)
      mantissa&=0x7FFFFF
      value=mantissa|(i<<23)
      if(negative)value|=(1<<31)
      return [value&0xFFFF,(value>>16)&0xFFFF]
     }
   }
   // return infinity
   return negative ? [0,0xFF80] : [0,0x7F80]
  } else {
   for(var i=127;i>0;i--){
   // negative exponent
     p=Math.pow(2,i-127)
     if(value>p){
      x=p
      mantissa=Math.round(value*8388608.0/x)
      mantissa&=0x7FFFFF
      value=mantissa|(i<<23)
      if(negative)value|=(1<<31)
      return [value&0xFFFF,(value>>16)&0xFFFF]
     }
   }
   // subnormal
   x=Math.pow(2,i-126)
   mantissa=Math.round((value*8388608.0/x))
   if(mantissa>0x7FFFFF)mantissa=0x800000
   value=mantissa
   if(negative)value|=(1<<31)
   return [value&0xFFFF,(value>>16)&0xFFFF]   
  }
}

I hope this helps. The code is in the public domain.