controllogix data type REAL's Struecture

[wilsonzhu]

Hi all,

As you know, when reading from modbus floating point data (32 bits) by prosoft MCM card, we use real data type in controllogix to COP two int modobus register into it. Now I want to test in the controllogi how it's converting, I need to know the structure of real type, for example, how 12.34 was represent in two int registers which I COP to REAL data type?
Thanks for you help.

 

[zankorel]

Hi all,

As you know, when reading from modbus floating point data (32 bits) by prosoft MCM card, we use real data type in controllogix to COP two int modobus register into it. Now I want to test in the controllogi how it's converting, I need to know the structure of real type, for example, how 12.34 was represent in two int registers which I COP to REAL data type?
Thanks for you help.

AFAIK, the mantissa and exponent of type REAL numbers are not explicitly addressable in Control Logix. You will need to construct them manually.
That assuming I understand your question properly.

 

[kamenges]

This topic of floating point representations has been covered very extensively on the forum. If you do a search you should find a tone of stuff.

Take a look at this website:
http://babbage.cs.qc.edu/IEEE-754/Decimal.html

This link was posted in a thread last week by Alaric. When you look at the bit patterns remember that the exponent is for a power of 2, not a power of 10 and that the decimal point in the mantissa is inferred.

There is no magic in the COP thing to get a float out of two integers. The reason they arte integers in the first place is a restriction in the data transfer path somewhere. In one of the links you are only allowed to pass integers. So on one side the processor copies the bit pattern for a float into two integers. At that point the bit pattern in each integer has not real meaning by itself. When the intergers get to the PLC the COP command copies the contents of the two integers byte by byte into the REAL tag and you get your floating point value back.

Keith