将寄存器值转换为 float 32 big endian

Question

I have two signed register values 17147 and -27394. They are in order of High-Low bits. I want to get the value float 32 big endian value using these value. I have used following code to convert to value.

My code

int intValue = 17147;
intValue <<= 16;
intValue += -27394;
byte[] bytes = BitConverter.GetBytes(intValue);
double readingValue = BitConverter.ToSingle(bytes, 0);

According to above code reading shows as 125.291 . But modbus tools & other online convertions says that value should be 125.791 .

Please note that some times reading is okay for some values. Actual value and converted value difference is not the same.

Answer 1

You're combining the two registers together incorrectly.

• 17147 is hex 42 FB (or 00 00 42 FB as a 4-byte value)
• -27394 is hex 94 FE as a 2-byte value, or FF FF 94 FE as a 4-byte value
• Therefore intValue should have the hex value 42 FB 94 FE , but your way of combining 42 FB and 94 FE gives you 42 FA 94 FE .

You're being caught out by the fact that -27394 is presented as a negative number, but that's just the way that your software has chosen to interpret those 2 bytes. You're then treating it as a signed 4-byte value rather than an unsigned 2-byte value when you come to combine it with the 17147.

When we combine 17147 and -27394, we need to make sure that we just look at the lower 2 bytes of each. That will undo any damage that was done by interpreting the values as signed 4-byte numbers.

int upper = 17147;
int lower = -27394;
int intValue = ((upper & 0xFFFF) << 16) | (lower & 0xFFFF);

Now:

byte[] bytes = BitConverter.GetBytes(intValue);
double readingValue = BitConverter.ToSingle(bytes, 0);
Console.WriteLine(readingValue);

Gives 125.79100036621094, as expected.