Converting REAL to Byte

[SilverShotBee]

Hi, I'm faced with a rather annoying issue. I'm using CoDeSys 2.3 on an IFM CR0401 PLC which is talking to a CR0451 HMI via a CANfox network. The network is only capable of transmitting BYTES, which is hindering my accuraccy of information. For example, I want to send a number higher than 255, or I want to send a number of 23.56.

In both of the above situations I end up with errors in the transmitted value. When converting a REAL to BYTE, if the REAL value is 256 then the BYTE value is 1... If I send 23.56 as the REAL value then the BYTE value is 24 as it is rounded up.

Is there anyway I can deconstruct the REAL value and reconstruct later to keep my accuracy?

Thanks

 

[SilverShotBee]

I found some information on this thread: http://www.plctalk.net/qanda/showthread.php?t=68817

This works for values over 255, but doesnt help with the preserving accuracy for floating values

 

[rdrast]

All networks transmit bytes. For your floats, you send the 4 consecutive (or eight for a double) that make up the float, and reassemble them at the receiving end.
Depending on the platform, they might arrive in the correct order, or they might high word/low word swapped, or even backwards, but that depends on your platforms.

 

[SilverShotBee]

All networks transmit bytes. For your floats, you send the 4 consecutive (or eight for a double) that make up the float, and reassemble them at the receiving end.
Depending on the platform, they might arrive in the correct order, or they might high word/low word swapped, or even backwards, but that depends on your platforms.

I'm quite new to this and am having difficulty in actually splitting up the floats (or reals?) in to the 4 consecutive bytes, how would I go about this?

 

[L D[AR2P#0.0]]

Something like this (syntax not checked):

VAR
bytARR : ARRAY[0..3] OF BYTE;
ptrValue : POINTER TO REAL;
rValue : REAL;
END_VAR

ptrValue := ADR(bytARR);   
ptrValue^ := rValue;

 

[SilverShotBee]

Thanks LD, that works a treat!

Once I have sent these bytes from the array over the CAN network though, how would I reconstruct the number? Is there a reverse of ADR?

 

[L D[AR2P#0.0]]

To "copy" the array of 4 bytes to a Real var then use

rValue:=ptrValue^;

 

[SilverShotBee]

To "copy" the array of 4 bytes to a Real var then use

rValue:=ptrValue^;

Hmm, I'm not sure i follow here. I am sending the array over the CAN network, when it arrives at the its destination, it is still in an array of bytes, I cant seem to copy this array back in to a real value with the method your using, I get conversion type errors

 

[SilverShotBee]

I sorted it in the end Thanks for your help!

 

[L D[AR2P#0.0]]

Did you have a missing ^

 

[SilverShotBee]

Did you have a missing ^

No, I was getting a bit confused as to how it works in FBD as some of the functions I need to use are not available in ST, I have to use FBD. When in FBD, it looks backwards and weird and took me some time to get my head around