fabiantao
2020-04-29T22:44:27Z
I am relatively new to Allen Bradley programing. I would like to know how an analog input signal ( 4-20 mA) can be change to and output signal (20 - 4 mA).

Is it possible to do it in RSLogix 500 ? if yes, could someone please help.

Thank you...
Guest
2020-04-29T22:45:02Z
Some of this depends on which SLC you are using and what version of the firmware you have. In other words some of the new SLC's have more functionality than the older ones. In addition a SLC 5/05 has a lot more functionality than the SLC 5/01. With the latest processors and firmware, you can simply use the SCP or scale with parameters instruction. We have videos on that instruction so you might want to check those out. With the SCP instruction, you can simply scale the input to something usable in the PLC and then scale it as necessary on the output also. I believe you should always scale your inputs to something that makes sense in the PLC. So if the 4-20ma corresponds to 10 - 150 C then it should be scaled that way and the scaled values used in the PLC logic. None of us think in mA or bits or other PLC representations of numbers so we should always convert inputs to numbers that make sense. I know thats not the primary focus of your question, but I had to get that in. Lets talk a little about the SCP. It has six parameters. Input register; Input min and input max; scaled min and scaled max; and the output register. The input min and max should correspond to how the input card is setup. It may be 4000-20000, 0-16383 or something else depending on the card and how it is configured. The output is whatever value the 4 and 20 mA represent (10 and 150 in my example above). On the output side you would simply reverse that process. Put your scaled min and max into the input side of another SCP and the output min and max would correspond to the numbers your output card expects to see for 4 and 20 mA. If you want to flip the input signal you would need to have 150 as your min and 10 as your max in our example. If you are not using the input values in your program and are simply echoing them reversed you could just use one SCP. Your input min/max would be as required by the input to correspond to 4 and 20 mA. On your output you would reverse the output min/max. So the value you expect for your output 20 ma would go into the SCP scaled minimum value and the value you would expect for your 4 mA would go into your SCP scaled maximum value. The output register would be inserted into the destination register. If you dont have the SCP function, you would have to do all the scaling manualy or use the dreaded SCL block.
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