IEEE decimal units

The Historian HMI uses the Institute of Electrical and Electronics Engineers (IEEE) 754 standard to transform 32-bit binary values to floating-point decimal numbers.

IEEE 754 32-bit numbers are stored in 16-bit Programming Logic Controllers (PLCs) as two 16-bit words. The floating point registers in the PLC are usually sequential in their numbering scheme for the low and high hexadecimal words. The current generation of 32-bit personal computers use a single 32-bit register. The register’s bit numbering scheme follows the same format as two sequential 16-bit registers.

To use floating point numbers in an Historian application, the Historian must be able to transform the values stored in the two 16-bit PLC registers. Bit conversions must be made because Historian always regards the raw PLC register values as individual integers. It is not possible to perform a Boolean AND operation of the two register integers and transform them into a real number. Historian cannot perform a type conversion for dual integer registers.