INFICON PEG100-D Medium to Ultra-High Vacuum Gauge - Manual (Page 9)

5 Format of real values
According to the IEEE-754 standard real values are stored in floating point format. The floating point values are trans-
mitted according to the following format:
Byte 2 3 4 5
–––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––
Content
SEEE EEEE EMMM MMMM MMMM MMMM MMMM MMMM
S means: Sign Bit, which means 1 = negative, 0 = positive
E means: Two-complement exponents with offset 127
M means: 23 bit mantissa. The most significant bit is always 1 and is, therefore, not stored.
Example:
The value -12.5
Byte number of Byte 3: Byte 2: Byte 1: Byte 0:
the floating C1 hex 48 hex 00 hex 00 hex
point value
–––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––
Content
SEEE EEEE EMMM MMMM MMMM MMMM MMMM MMMM
Content in this
example
1100 0001 0100 1000 0000 0000 0000 0000
binary binary binary binary
Sign bit:
The bit S in this example is 1. That means the sign bit of the whole value (or of the mantissa) is „minus“.
Exponent:
The EEEE EEEE have the value: 1000 0010 binary. This value converted in decimal it is: 130 decimal. This value has
the offset 127. So the exponent is: 130 - 127 = 3
Mantissa:
Because the mantissa is normalized the most significant bit has the value 1, the next bit has the value 0.5, the next bit
has the value 0.25.
Bit number Value of the bit, if the bit is set to 1
bit 24 (MSB) 1
bit 23 0.5
bit 22 0.25
bit 21 0.125
bit 20 0.0625
bit 19 0.03125
bit 18 0.015625
bit 17 0.0078125
and so on
The MMM MMMM MMMM MMMM MMMM MMMM (23bit) have the value 100 1000 0000 0000 0000 0000. The
most significant bit (MSB) is always 1 (and not stored). You have to implement this most significant bit.
So the value of the mantisse is: 1100 1000 0000 0000 0000 0000 (binary).
Bit number Value
Bit 24 is set to 1
→
1
Bit 23 is set to 1
→
+ 0.5
Bit 20 is set to 1
→
+ 0.0625
So the mantissa has the value 1.5625
Whole Value:
The whole value is: -1.5625·2
3 = -12.5
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tina12e1 (0106) PEG100-D.ga