RsLogix5 technique

Hi all...I have a question...I am converting a temperature signal into a 4-20madc signal. Going in its 4-20ma = 0 - 100%. I want to invert this signal, changing the temperature signal/0-100% to temperature signal in but 4-20ma = 100 - 0%. I had done this a couple years ago but forgot what I did to accomplish this. Im using an AB Plc5/20 and rslogix5. Anybody able to refresh my memory? Thanks...Jim Schroll

SUB Instruction

Hi Jim,

I believe you can use SUB (Subtract) Instruction to invert the value of the input. Source A will contain a fix value (ex. 100 to represent 100%) and source B contains the changing value from the input. Note the result of subtraction is positive.

Just a thought in mind and hope it helps.

Regards,

Vic Jasareno

I'm Not sure but I believe that can be done in the config of the card. Depending on the card of course. What is the number of the card?

Drewcrew6

Nope, the conversion that is in the analog card can only handle negative to positive rates: Like -100 to 0, or 0 to +100, not 0 to -100. Dont know why, but thats how it is.

And because the MIN/MAX is in BCD the limits are +/- 9999.

So you have to do it in the program if your analog conversion cannot fit into these restrictions.

Edit: Temperature in % ????

I thank you guys for getting back to me on this. Upon reading my original item it might be a little confusing. Im not editing temperature...what Im doing is taking a signal from a Type K thermocouple (0-2400 deg F.) into a moore temperature converter, putting out 4-20 ma. I want to control a small damper by the 4-20 but I want to have it reverse acting. I want to invert that 4-20 output into 20-4. That is what Im trying to do. I am using an analog output card 1771-ofe. Hope this clears things up a bit. Thanks again.

Try using a PID control block. I assume that you are controlling to a set point. In the the PID block set error to SP-PV , as the temperature increases the output will decrease. Turn the integral and derivative off (=0) and set the gain to 1. If you are not using a set point then set the SP to the maximum temperature you will get and the output will be 0 at this point and maximum when the temperature is minimum. Scale your modules to suit the PID block. If the 4-20 mA output does not follow correctly then adjust the gain and/or the scaling until it does.

y = mx + b ... the equation that would not die!

your description is a little ambiguous ... but I think that this is the approach you're looking for ...



if you're working with values of 4 to 20 (as in milliamps) try this:

multiply your original number by -1 and then add 24 ...

that should give you values from 20 to 4 ...




if you're working with values of 0 to 4095 (as in data to the output module) try this:

multiply your original number by -1 and then add 4095 ...

that should give you values from 4095 to 0 ...

Use the PID block! Get to know the PID block. Love the PID block.
The PID block will solve many of lifes problems.....

Thanks Oldgearhead and others. I was able to solve my problem thanks to you experts on here. Very educational bunch of folks!! Thanks again...Jim