Lowest Temperature

How can I identify the lowest temperature from several RTD's conected to an analog board? What PLC code should I use? Thanks

First things first!

You have to get the signal from the rtd's into the plc before you can do any coding! Do you have an input module for them? The coding and interpretation of the temperature data is rather straight forward after you have it to work with.

hello !
Here is a example of how you could do the comparison

if start
then load first value from DB
load second value from DB
compare
if true then first value smaller if not then second value smaller
put smaller value in memory
load third value
compare with memory value
Loop until number of inputs read then
write the memory value to output

here is one way to do that

It depends on which PLC you use. If you're using a Siemens S7-300/400 you have functions available to find the minimum and maximum of a set of 3 variables. If you have more variables you can determine partial extremes and then find the extremes of these results and so on. Since these are IEC functions, I'm sure there are other PLC's around with the same or even better possibilities.

Kind regards,

The PLC is an A/B PLC5/40. The variables comes from Temperature Transmitters to Analog Boards. Thanks for the inputs so far. But I still need more info.

You need to find the lowest value amongst three values ?
I think you need a basic pogramming course to begin with !

Try this:
(pseudocode)
If (varA < varB) AND (varA < varC) THEN varA is the lowest.
If (varB < varA) AND (varB < varC) THEN varB is the lowest.
If (varC < varA) AND (varC < varB) THEN varC is the lowest.

You can test for some "odd" cases:
If (varA = varB) AND (varA < varC) THEN both var A and varB are the lowest.
If (varA = varC) AND (varA < varB) THEN both var A and varC are the lowest.
If (varB = varC) AND (varB < varA) THEN both var B and varC are the lowest.
If (varA = varB = varC) THEN all of varA, varB and varC are the "lowest".

In AB use the LES and EQU instructions to compare values as shown above.

Sometimes, you just have to give them the answer to get them started....

Store the first RTD.
If the second RTD < Stored value, store the second RTD (overwriting the value there)
If the third RTD < Stored value, store the third RTD.
...continue....

After all the values have been compared, the stored value is the reading from the lowest AI.

First, get all of your values together in a contiguous block. Then apply the SRT instruction to the block. The lowest value will appear in the lowest address of the block.

Muchas gracias amigos!! (Thank you very much my friends!!)

that is easy one
take the input card have it on display and make a short of the wiring you will get the lowest value possible
it should be -250 or close to it.
then open the short and you have maximum. normal for pt100 is 800 celsius
i am using resistor input and i can get all over it as maximum resistance is 400 ohms.
when transmitter is used it is dependent of the converter.