PLC inputs are giving out 24V

Hey. I just joined this forum, mainly to hopefully get some help with a problem i've encountered. I just bought myself a Mitsubishi FX1S 10-MR PLC. When i had connected it to the supply (230V), and was going to run it, i noticed that all of the INPUTS (X0,X1,X2 etc) gives out 24V. This is without me having wired anything to them yet. To clearify, i can measure 24V from between every input and com/0V on the PLC. Can anyone please help me solve this problem? Is it a sense of wiring that must be adjusted, or programming? Thank you!

By the way, this is my plc:
http://www.asianproducts.com/company/index.php?op=autoplay&cno=A13503565243763349&item_id=P13521085146230278

The PLC needs to detect whether the input is open or closed. This is often accomplished by using a sensing voltage at minimal current levels. Some IO is configurable by jumper or software to eliminate this. If you are using dry contacts for your inputs, there won't be a problem.

Why does it need a sensing voltage if i'm going to supply the inputs with 24v through a switch anyway?

Check out the manual - section 5.2.1

http://www.meau.com/Files/JY992D83901-M.PDF?ID=010000000000000001000000210202000

Most likely your inputs are sinking inputs (if you have the Japanese version it is). A sinking input will have 24v to com at the terminal, and when the input device "closes" the voltage 'sinks' to ground. Check out page 5.2 of the manual for your PLC for the wiring diagram. Also check out this: http://www.plcs.net/chapters/dcin25.htm

Mark S

oh ok. Didn't know the japaneese version was different. But as i understand this will not be a problem, and i can connect switches from the 24V PLC supply, to the PLC inputs as usual, right?

sontun said:

oh ok. Didn't know the japaneese version was different. But as i understand this will not be a problem, and i can connect switches from the 24V PLC supply, to the PLC inputs as usual, right?

Click to expand...

Probably not. This is where the manual comes in handy - I recommend you study the wiring diagrams in it.

You only want one voltage source in any circuit. If your field device has powered contacts you need to either eliminate one power source or use a relay between the two.


Remember, until a voltage is present and drives current through a device nothing happens. This applies to a light bulb or a PLC input. When a circuit is open your PLC inputs may have voltage on them, but without continuity there is no current flow and the PLC won't sense an "on" condition.

When you close a switch with a dry contact continuity is created, a small current flows trhough the PLC input channel, and the PLC logic senses the "on" status. As FluxCap says, with a dry contact you shold be fine. Polarity and current direction don't matter to a simple mechanical contact.

On the other hand, if you have a solid state switch llike a proximity or photo switch polarity and current direction matter greatly. There is only one connection sequence that will work properly. Get the manual out, draw your proposed circuit, and follow the current path through the entire circuit from + to - .

I agree with Moggie and Mark, It is likely set up for sinking inputs. Is there a jumper between S/S and 24v?

No, there is no S/S on this version of the PLC. It's weird, because i've used the same model before, which then had the S/S connection. Im not all familiar with the dry contacts..i'm mostly using simple 1-poled switches, and some photoelectric switches with 3 wires (+, - and output)

But from what i can understand from the term "dry-contacts" online, a regular 1 poled switch is a dry contact?

A dry contact is simply one in which the switched voltage is supplied from an external source. The contact isn't different. In your case the term dry contact is used because you are providing the voltage from your PLC and the contacts just make or break.

From what you have said, and the model number you gave (from your source link also) I believe you do have Japanese spec model. If your model number is FX1S 10MR, with no further number than it is a Jap spec model with ac power supply. Here is a link to the manual I found:

http://www.taiwan-control.com.tw/products/plc/fx1s.pdf