Dropping some decimal points on a REAL number

I'm utilizing a REAL number on a Controllogix platform, and want to drop some of the decimal points is there a easy way to accomplish this. example number displays 30.170505 I only care about 30.170. I just want to deal with or carry three decimal places.

Greetings and welcome to the forum.

Are you asking about this for displaying the number or for a computation?

Most HMIs allow you to configure how many decimal points you want to display. If it is for display then what HMI device are you using?

In RSLogix5000 the software will display as many decimals as is relevant for the number. Your PLC uses 32 bit IEEE-754 floating point format to represent real numbers. This format has a 23 bit mantissa plus one implied bit (24 bits) so the actual float as stored in the PLC memory will have as many decimal places as can be accommodated in the mantissa. Put in plain English, that means the number of decimal places in the number varies according to how large the number is. There is some good information on the IEEE-754 format on wikipedia if you want to dig deeper. We've also discussed it a lot on the forum so you can search for IEEE-754 to find it.

If you are performing a computation then don't worry about how many decimal places there are. No matter what you do you will still have 24 bits in the mantissa. That is just how computers handle real numbers.

One trick that is sometimes used is to multiply a real by 10, 100, or 1000 and make the destination a DINT. Then you set up your display for an implied decimal point and display the DINT value. This will also take care of rounding for you, for example if RealTag = 30.170505 then MUL RealTag 1000 DintTag will put 30171 in DintTag.

This is a pretty generalized answer. We can probably give you a little better answer with more information on what exactly you are trying to accomplish.

I'm attemptin a computation as I want to compare a change in the number to see if it stabalizes. I move the number every second to a snapshot number and want to campare the current vs the snapshot, I want to calculate a rate of change so that I can test this rate of change to be within a certain limit. I only care about comparing it to a .000 of an inch I don't care after that

You can use the ALM block in a function block routine to check and alarm on rate of change.

Probably the best way to do this is by using the TRN (truncate) instruction inside of a CPT block.

Your instruction would be something like:
CPT Destination_Tag (TRN (Source_Tag * 1000) / 1000)

Flicker52 said:

I'm attemptin a computation as I want to compare a change in the number to see if it stabalizes. I move the number every second to a snapshot number and want to campare the current vs the snapshot, I want to calculate a rate of change so that I can test this rate of change to be within a certain limit. I only care about comparing it to a .000 of an inch I don't care after that

Click to expand...

If you follow Troy's advice, and MUL by 1000 with Destination: DINT, then DIV that DINT by 1000 with Destination: Real, that will effectively truncate the value past the third decimal point.

Why not calculate the rate of change?

Flicker52 said:

I'm attemptin a computation as I want to compare a change in the number to see if it stabalizes. I move the number every second to a snapshot number and want to campare the current vs the snapshot, I want to calculate a rate of change so that I can test this rate of change to be within a certain limit. I only care about comparing it to a .000 of an inch I don't care after that

Click to expand...

Then compare rate of change to an upper and lower limit? It sounds like the rate of change is a velocity in your case. What are you really trying to do? My guess is you want some indication that something that moves has stopped.
What is your definition of stopped?

+1 for Peter solution - It has advantages

If not use the following

All tags real
Tag1 = Current Value
Tag2 = Last Value

Tag3= Tag1 - Tag2

IF ABS(Tag3) < 0.001 THEN Stopped

---CMP ----------------------------()

ABS((Tag1-Tag2)< 0.001          Stopped



MOV 

Tag1

Tag2