Analog Inputs en AB Micrologix 1500

Hi People,

I´m using for 1st time a Micrologix-1500, I have a 1769-IF4 ANalog input Module attached to it. In Analog input 1 I have a Temperature sensor (4-20 mA). I see this value in I:1.0 converted to digital, but I don´t know how to handle it. i.e I want to scale this digital converted value to a process value of 0-100 (ºC) does any one know how to do this.

I tried to copy I:1.0 to an integer value, but this isn´t working.

I know my question is simple.. sorry.

Thanks.

Ner

In the manual that came with the analog module, it will tell you what the numeric value is at 4 mA and what it is at 20 mA. Use the SCP (scale with parameters) instruction to scale the data from 0 to 100. Use the rslogix help screens to understand the SCP instruction. If that doesn't help, go to the AB website and download the SLC 500 instruction set reference and look up the SCP instruction there. This reference is an invaluable tool when programming SLC's and explains things much better than the logix help screens.

good luck

Thanks John,

I tried this but it seems that it´s not taking the I:1.0 as input to any of the instructions. I see that I:1.0 change but I cant see in the output words being N,L, etc.

Regards,

Ner

I don't use the 1500 yet but we have some 1200's. Make sure you have the module configured correctly (I assume you do cause you can see the data in the input word) Make sure you're in the run mode. Make sure the SCP rung is unconditional.

That run mode thing sounds simple but I've done some pretty silly things before. The 1200 offers no "on line" programming. You have to program off line, then download. I have actually tried to look at on line type things while offline--before I downloaded!! Make sure you're in the run mode.

Is the rung in ladder file 2?? If in some other file, make sure there is a JSR to that file in ladder file 2.

You want to use the SCP (SCale with Parameters) function to convert the raw data into engineering units. This function is described in your manual, but there are a couple of cautions. First off, on the 1500 you need to do the 20% offset for a 4-20 mA signal. Second, the nominal input data range is 6241 at 4 mA and 31206 at 20 mA, using the default set-up. However, I have found these modules to be less precise than I wanted, so you may want to use a 4-20 mA simulator, and find out the actual data values for 4 mA and 20 mA.

The function looks like this:


This example will take the raw data in channel 2 of the first I/O card and convert it to engineering units of 0 to 100. The result of the conversion will be stored in register N10:2

If you want resolution to a tenth of a degree, use 1000 instead of 100 to get an implied decimal place. Just remember everywhere you use register N10:2 that 254 is really 25.4 degrees C!

You can re-configure the I/O to read 4000 to 20000 (Actual mA reading with two implied decimal places) or 0 to 10000 (0% to 100% range with two implied decimal places) by changing the configuration in software using RSLogix, but if you're converting to engineering units anyway it isn't worth the effort.

Thanks, John... I was using JSR and I forgot to tell in ladder 2 to go to this new ladder...

Regards,

Ner

glad to see I'm not the only one that does things like that!

OG

Thanks Tom, that´s a good one too.

Regards,

Ner

Micrologix 1500 - Scaling an Analog Input

I also had problems with a 4 - 20 ma input. I ended up using the PID to scale my input. Then I took the the PV (Process Variable) and divided it by 10 to get my decimal point. My 1500 INTERGER register doesn't handle floating point decimals, so I put the answer into a FLOAT register.

Just a different solution.

HI Tom ,
I see lots of comments are your reply , I need your support and guide , I have very new to analog instruction using rslogix 5oo , My application is level transmitter i have to control one pump start and stop using level transmitter , Say iam using 1762-IF4 , 4 channel analog input module , signal : 4 - 20mA , with Rslogix500 instruction can you help me how to create this programme , my height of the tank is 10 mtrs i need to convert this is signal to 4-20mA . Please waiting for your reply.

SamSam,

You’re using RSLogix 500, what is your PLC Processor?

On most PLC's you should be able to scale your 4 - 20mA input that would equate to your tank volume or level, which ever you are using. I usually use the PID function to do the scaling. It's quick and easy and takes minimum lines of Code. Then it would be just a matter of doing a compare function. If Volume/Level is Equal to or Less Than ### then Turn on Output/Bit O:1/7 (random example) for pump. Then another compare function If Volume/Level is Equal to or Greater Than ### then Turn off Output/Bit O:1/7 (random example). for pumpOf course you would have the safety bits in the same line. Safety bits being for E-Stops, MAX Level Sensor, etc…

Sorry about NOT including the logic. I don’t have RSLogix on this computer. I'll try and get my laptop up and get the logic I use and send it.

Bill