sinking or sourcing input

[smsebnaa]

I spending a lot of time for searching on this subject "sinking and sourcing input", but there are not simple answer. I want to add stop/start button to PLC (s7-1200), but i should use sink or source input?
(I think "sink" is good choice!)

 

[Davek0974]

Sink is the most common, it's the one most logical to me at least - you provide a voltage to the input to create an action.

Sink inputs and source outputs seems to be the norm, unless you use relay outputs etc.

 

[smsebnaa]

thank you for quick reply.
1. the capacitance or inductance of each sensor is important?
2. when i attach temperature sensor to plc, i should use sink or source?

 

[Davek0974]

thank you for quick reply.
1. the capacitance or inductance of each sensor is important?
2. when i attach temperature sensor to plc, i should use sink or source?

1, something I've never really worried about, ive used many sensors, probes and inputs and all have worked fine.

2. Depends on the sensors really, obviously the two need to match -a voltage source sensor needs to go to a sinking input.

 

[VaMike]

Like was previously said, Sinking inputs are most common. They tend to be easily visualized. You put 24vdc on one side of switch, close the switch and it sends 24vdc to an input. Voltage High= Input ON

A sourcing input is kind of opposite. You put DC COM on one side of a switch, close the switch and the Input is pull down to DC COMM. Voltage LOW =Input ON.

Many PLC inputs can be configured for Sinking or Sourcing. The trick is that the inputs will have an extra terminal that determines if it is sinking or sourcing. For instance, on micrologix 1400, you will have a COM0, COM1, COM2 and a COM3.
COM0 is used for inputs 0-3
COM1 is used for inputs 4-7
COM2 is used for inputs 8-11
COM3 is used for inputs 12-19

This allows you to use some as Sinking and some as Sourcing.
For example- If I have a mix of NPN and PNP sensors, I will need to use different sets of inputs. If sensor 1 is a NPN proxy switch (ON or OFF), I would connect 24vdc to COM0 and then connect the sensor to Input 0. This would be a sourcing input.
If sensor number 2 is PNP, I can not put it on the same bank of inputs as sensor 1. I must move it to a new bank of inputs. So I can now hook up DC COM to COM1 and connect the PNP sensor to input 4. This is a sinking input.

To say it differently, a sinking input will provide a path to ground for a sensor whose output is High when ON. Current will flow from the sensor, to you sinking input and out through the DC Com that was connected to the COM of that input.
A sourcing input will provide a path from the 24vdc on the COM terminal of the input. When the input is grounded, current will flow out of the input to ground provided by the sensor.

In both cases, the actual sensing of the Input being ON or OFF is simply seeing if a current is flowing though the input terminal. If it is a Sourcing input, the current is actually leaving the plc to the sensor. If the input is a Sinking input, the current is flowing from the sensor to the plc.

All of the above are for discrete inputs. Either ON or OFF. Analog is a little different deal.

 

[andreidragosh]

About sensors

elementary issues: the sensors are called capacitive or inductive regarding of measuring method but their outputs are consisted from pnp/npn transistor stage, for pnp sensors, for example you can supply the sensor with 24V from the PLC source, the output represents logical input for plc.
they are many types of electrical diagrams, depending of sensor output type -pnp, npn, open collector, input or output type of PLC etc;
usually the inputs are linked with the +24V source of the PLC, outputs of PLC are:
-with transistors, in this case plc output sourcing the output element or sink current from external source (pay attention to the current limit);
-with contacts from relays, in this case you use external source (recomended with different types of outputs or for a command that requires free potential contacts);
-with tyristors.

 

[andreidragosh]

About sensors

elementary issues: the sensors are called capacitive or inductive regarding of measuring method but their outputs are consisted from pnp/npn transistor stage, for pnp sensors, for example you can supply the sensor with 24V from the PLC source, the output represents logical input for plc.
they are many types of electrical diagrams, depending of sensor output type -pnp, npn, open collector, input or output type of PLC etc;
usually the inputs are linked with the +24V source of the PLC, outputs of PLC are:
-with transistors, in this case plc output sourcing the output element or sink current from external source (pay attention to the current limit);
-with contacts from relays, in this case you use external source (recomended with different types of outputs or for a command that requires free potential contacts);
-with tyristors.

 

[Aabeck]

For safety reasons I do not use sourcing inputs for switches, pushbuttons, limit switches, etc. Or for safety item proxy's.

If the return wire gets cut & grounded out the PLC will think the switch or PB is now ON & possibly start the operation when not wanted or safe (think HILO damage in stamping plants).

 

[CapinWinky]

No one directly answered the question of why there are two options. The #1 reason anyone would use a sourcing input (NPN) is when the input is provided directly from the output of a small IC chip. These chips may not have enough output power to drive a standard, sinking input. In a full decade as an integrator, I have encountered exactly 1 machine where this was the case. Most people end up with sourcing inputs because they bought a bunch of NPN sensor because they screwed up the part number or found it on ebay. I'm sure there are other valid reasons besides preference, but I have never encountered one.

So, ICs explain NPN inputs, but what about NPN, sinking outputs? I don't know. Maybe there is some benefit to keeping an actuator at a higher potential than ground in some rare circumstances, but I've never encountered it.

 

[just the cowboy]

speed

I was told that NPN input or output is faster. Only by micsoseconds put a sinking output is used for motion control on AD PLCs. Reaction time is less on NPN vs PNP on eyes and prox switchs.
For descrete in or out I don't want ANYTHING that shorts out to go true or pull in a output.

 

[Aabeck]

Another NO I just thought to mention:

Never use sinking outputs where the power is applied directly to the solenoid, motor, valve, etc, that if the output wire gets cut & grounds out the object is then powered up & turns on when unsafe. (Again - HILO damage)

 

[JerryH]

It is really driven for safety reasons depending on if the control voltage is positive or negative grounded.

For a Positive grounded system you should use Sourcing Input Cards and Sinking Output Cards.

For a Negative grounded system you should use Sinking Input Cards and Sourcing Output Cards.

This is done for safety reasons to prevent any unexpected inputs from energizing if an input wired is shorted to ground and prevents any output device from energizing if an output wire is shorted to ground.

I suppose if your DC supply is floating you could go either way.