You guys answered one of my questions and now you'll never be rid of me.

[robpaige]

Lancie, here's what that will look like:

|-----|-MOV-------|-----|

|     |MOVE       |     |

|     |Source  100|     |

|     |Dest   F8:0|     |

|     |___________|     |

|                       |

|-----|-CPT-------|-----|

|     |Dest   F8:1|     |

|     |Expr   (*) |     |

|     |-----------|     |

Etc

Where * is my expression, in this case (5.0|9.0)*(F8:0-32).
The etc indicates that I continued the ladder with his other instructions:
If Temp in Celsius is 50 or greater, activate one output for a red light (used the GEQ instruction)
If Temp is between 20 and 49 inclusive, turn on an amber lamp
(I used the limit function, with F8:1 as my comparator value, 20 as the low limit, and 49 as the high limit)
If Temp is less than 20, a green light is energized. For that I used the LEQ function.

 

[Lancie1]

Very good. Do you have access to RSLogix, or LogixPro? If so, do the actual program. I will post a LogixPro version later, after you have got an answer. What did your conversion from 100 F to C give as a result?

 

[robpaige]

I'm using RSLogix 500. I know the answer should be 37.77777 but I'm not sure how to get RSLogix to show me that. Any suggestions?

 

[Lancie1]

Yes, you can read the temperature directly in the program. When you put your program in RUN mode, the result will appear right below the "Destination" line in the CPT instruction. See my Compute example back in Post #10 of this thread.

EDIT: Here is my LogixPro version. Because this simulator program does not implement the CPT instruction, I have to use Subtract, Multiply, and Divide to get the same function as the Compute does in one instruction. I added a green light!

 

[robpaige]

trying 2 different approaches to see the answer Lancie. I just downloaded the LogixPro demo, and it appears to be lacking the CPT command. Do you know where it's hiding?

 

[Lancie1]

Yes, it doesn't exist in LogixPro, but never fear, there is a way to do it. Just do it using the old-fashioned long method, first Subtract, then Multiply, then Divide.

 

[robpaige]

That's what I'm doing =)

 

[leitmotif]

Rob you said
I'm just stuck on how the "number gets into the PLC in the first place".

Maybe I am interpreting question wrong but here goes.
So you hook a thermocouple to an input on an analog board. That is voltage. Voltage is proportional to temperature. The board converts voltage to a number based on HOW you tell it to convert -- hint y = mx + b and them powers of 2.

What your boss is telling you to do is tell teh PLC to enter it as a number whose place (storage or bit number) you define.

After that you can do math on either inside PLC

Dan Bentler