Sinking Outputs Vs Sourcing Outputs

I wonder if any one can help me with this one. I have been pondering over which type of outputs is better for some time now. Sinking Or Sourcing Outputs. Can someone please explain the differences between the two as I can never decide which ones to use. I normally end up ordering one type and then the rep telling me that the other is normally what people use. All help is great.

ABTECH

When you're switching most of the loads typically controlled by a PLC, it doesn't make any difference whether the output is source or sink. A solenoid coil works equally well with either type.

Most techs find it more intuitive to check sourcing outputs. That's because when you put your meter probes to the output, a reading of zero volts equals off and a reading of some voltage level equals on. It's the opposite for sinking outputs.

If the output is going to be connected to another transistor device, then you need to pay attention to the details. A sourcing output needs to be connected to a sinking input and vice vesa. This comes into play when you connect a PLC transistor output to a drive's RUN or FWD command for example.

You should also read this:

http://www.mrplc.com/kb/index.php?p...x_v2&id=44&c=38

I would be worried with sinking outputs. That means that a ground (earth) fault on the wire from the output to the device could result in that device actuating.

There are some machine interfaces that require sinking outputs. In my experience these were small definite purpose machines used with electronics assembly equipment. Once in a great while I see them.

For most everything that I do my outputs are sourcing, and my inputs are sinking from the PLC'S point of view. That is mostly for the same reason as the Mr. PLC article above. Search this site this has been discussed many times before.

Sinking Outputs used to be the norm in Japan so Japanese PLC's were often supplied with these by default. I have never liked using them because of one major problem which used to occur with Omron sinking output cards and probably occurs with other manufacturers products as well. If the 24Vdc supply to these output cards was lost the leakage current through the outputs was usually enough to turn the connected devices on. I considered this to be a major safety issue for most installations.

Good source of info:

http://www.patchn.com/npnpnp.htm

Rick Densing said:

I would be worried with sinking outputs. That means that a ground (earth) fault on the wire from the output to the device could result in that device actuating.

Click to expand...

this is indeed one of the biggest issues because NPN is not common
in west so people here often don't know how to use them properly.
trouble is when they someone fit's that group and doesn't think of possible consequences (wiring faults do happen, does't mean that electrician didn't do it right, it means that fault occured).
i would also recommend avoiding it because it can also create lot's of confussion when troubleshooting, making changes or upgrading.

I use NPN outputs all the time and have not had any problems at all. The power supply I use is always a switchmode with a floating output - i never tie the negative from the switchmode to earth.

I did have a problem recently and still do not really know the cause but interestingly the problem affected both NPN and PNP outputs so there was really no advbantage to the type used. I do however always use positive inputs.

See this thread for details
http://www.plctalk.net/qanda/showthread.php?t=25874&highlight=Switchmode

The Japanese tend to love NPN. Many Japanese machines even have NPN type devices as inputs and outputs. In fact many Japanese remote blocks are only available with NPN devices as inputs and NPN transistors as outputs even today. I have heard that in the past PNP transistors were much more expensive than PNP transistors and this may have had something to do with the choice of NPN but do not really know if this is true or not.

Please, can anyone tell are there Siemens PLC modules that are NPN? Are they all PNP modules? I must admit this story about PNP and NPN are pretty much blur to me.

I generally use sinking inputs and sourcing outputs. Seems to always work well for my applications and also it's one less thing to figure out.