ModbusRTU.writeRegisters doesn't accept negative int 16 ...

[volkmarnissen]

... You get an out of range exception, when you try it.
RangeError [ERR_OUT_OF_RANGE]: The value of "value" is out of range. It must be >= 0 and <= 65535. Received -10

However, it's easy to convert the value to unsigned int 16:

        if( value < 0 && value >= -32768){
          let b = Buffer.allocUnsafe(2)
          b.writeInt16BE(value)
          return b.readUInt16BE()
        }

This can be a workaround and if implemented also a solution.

[yaacov]

Thank you for the issue, can you make a pull request ?