sinking or sourcing

What is better out of - sinking and sourcing inputs and outputs and why. One site says it's better to sink the inputs and source the outputs. I am using a Direct Logics 06 PLC 24 VDC discrete ins and outs and 4-20 mA ins and outs.
Thanks, James

re

depends what type of sensor you have

vikas1947 said:

depends what type of sensor you have

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Mate - Answer the question

The choice of NPN or PNP (sinking or sourcing) is generaly site driven. What type of parts are required by the company you are working.
NPN Or PNP is optional
BUT
Can not or should not be both.

Inputs and Outputs Same choice.
Analogues - Investigate What you are using.

in Practice
Japanese machinery - usually NPN
Others more comonly PNP

James

Thanks. I don't have any solid state relays that the PLC will connect to. I think I will source the outputs and sink the inputs.
James

That will do - it is your choice

Sinking and sourcing - different manufactures describe these different ways.
I normally use positive input which some describe as sourcing and others as sinking. The input switches on high.
Outputs, do not really care. Usually use npn (switch to 0V) sometimes pnp (switches high).
Sometimes it depends what is in stock.
The only thing I will not compromise is I use a 24VDC/24VDC or 240VAC/24VDC switch mode power supply in front of the PLC, nearly always use 24VDC PLCs, and float the output - never ground the output negative to earth - introduces lots of noise sometimes - have been caught out badly once before.
The other reason to adopt this approach is if you are using 0V switching inputs and the output of the switch mode is grounded, if you short an input to earth it will turn on in the PLC. Got caught with tha one years ago too.

BobB when you say that you leave the power supply floating do you mean the power supply for the I/O?
A floating power supply can have catastrophic consequences using Siemens
24Vdc output modules. This situation can occur if some reason the +24
is brought to Earth or ground potential which will bias all the transistors to turn on regardless of logic, which means everything will start up without control. On a floating supply no protective device activate in this situation.
I have seen this happen 2 times, once on the old S5 115, and also on S7 300.
In Europe the negative logic method is not used, the Positive logic method being preferred.
I would always Ground the 24-, so if there is a ground fault a circuit breaker will isolate the fault.

I do not ground the negative output from the switch mode power supply to either the processor or the I/O. The whole PLC 24VDC supply floats to the CPU and I/O . As there is NO ground reference at all a short to ground does nothing. It cannot turn anything on or off or cause a short to ground due to the fact that there is NO ground reference.
I have never had a problem with this method ever and that is using a 240VAC/24VDC or 24VDC/24VDC switch mode power supply. I do a lot of generator work and the systems run on batteries and battery chargers as well - sometimes on the generator starting batteries (I hate that - noisy as heck when the generator starts). The switch mode reduces the 24VDC supply from the batteries from, usually, 27.5VDC to 24VDC and boosts the supply from around 19VDC to 24VDC if the batteries are low and extends the operation range of the battery supply to the PLC. Additionally it supplies isolation to the PLC - grounding the negative output of the switch mode removes isolation and introduces noise from switching components and battery chargers - particulalrly el cheapos.
A circuit breaker will still work on a short circuit between + and -.

Do you not ground the PLC Chassis?

Yes but this does not really matter as the DC switch mode power supply is still floating and there is still no reference to earth on the power supply. The PLCs I use do not have a ground reference on the negative to earth either for the I/O or the CPU power supply. The earth is generally used only to satisfy an inspector who believes the earth should be connected no matter what. It really does nothing.
I do use the earth reference for analogue input cards and this is grounded through the PLC earth but this is only for filtering out noise and the analogue card is isolated internally from the PLC circuits anyway so once again no bearing on shorts to ground.

I was just making the point that using a Siemens plc with isolated transistor outputs that if a fault occurs which brings the 24+ to ground potential it causes all the outputs to turn on.

I hardly ever use Siemens PLCs at all. Perhaps they have some inbuilt ground reference in them that causes this - I really do not know. I will keep this firmly in mind the next time I have to use one. Will have to have a close look at the internal wiring to see what has to be done to prevent the occurance.
I do not have a problem with Omron at all thankfully.