Analog Input

[davidt 0]

:-p Hellow I need to know whether can i connect one transmitter of 4-20ma into two diffrent analog current inputs in the PLC, and how can i do it ? thanks David

[Guest JorgeR]

I assume I can be done, but I can't see why you would want to. Basic OHM's law says that a current will be split between two loads in parallel. So, assuming that the loads on each input are exactly the same, your 4-20ma would be evenly split in half to each input. However, nothing is perfect, so one input will probably be slightly higher than the other. Anyway, is there a specific problem you are trying to solve? If you are trying to use the current value in more than one place in your program, you should be able to do this with just one input. If the current value is too high for your input card, there are better ways to reduce this value. Let us know what you want to do.

[panic mode 246]

Most analog inputs use 250 Ohm resistor to convert 4-20mA into 1-5V signal which is than sampled and and passed to PLC (multiplexed with other channels). If the current loop is floating (like two wire transducer) and two of the analog inputs are not single ended, referenced to ground, you can connect both analog inputs in series and run same current through both of them. If this is not possible, you can still connect it to one of the inputs configured for current and then connect the second input in parallel but configure it for voltage (leave it in high impedance state). Of course, you can always use transducers but the first two scenarios are cheaper alternatives.

[Sleepy Wombat 27]

If you are using two PLC's then i would do as Panic Mode suggested...yep should work...AS the reason to put into the same PLC i don't understand sorry, kepp in mind that you can always scale the same value differently anyways. Is it a redundancy issue perhaps ????

[KinK 0]

I have done this before and what sleepy and Panic mode said is correct. the reason is that I had 2 different plcs (omron and a slc500) and it was easyer to do it that way then hooking up some sort of communication for just one input.