Difference between "Source" and "Sink" inputs

Difference between "Source" and "Sink" inputs

There is a variety of terminologies for characterizing digital inputs and outputs;

Sinking Inputs  / Outputs - Most Common for SIEMENS Automation Systems

Sourcing Inputs  / Outputs - Used by some other manufacturers

Furthermore, you can characterize or order the digital inputs and outputs as follows:

• the logical status
• the electrical signal
• the wiring and switching

The two tables below show the relationship between terminology and assignment:
Digital inputs (24V)
 

Terminology	Logical status	Electrical signal	Switching
P-lesend Sinking input Switch is located between DC24V and the module	1 (true)	24V	Fig. 01
	0 (false)	0V (or open)
M-lesend Sourcing input Switch is located between the module and ground	1 (true)	0V	Fig. 02
	0 (false)	24V (or open)

Table 01
Digital outputs (24V) 

Terminology	Logical status	Electrical signal	Switching
P-schaltend Sourcing output PNP Transistor Load located between the module and ground	1 (true)	24V	Fig. 03
	0 (false)	0V (or open)
M-schaltend Sinking output NPN Transistor Load located between DC24V and the module	1 (true)	0V	Fig. 04
	0 (false)	24V (or open)

Table 02 

In case of SIEMENS Automation Systems, all Inputs / Outputs are "Sinking" types unless Specifically mentioned. 

Sourcing type Input / Output modules are however also available for SIMATIC S7 -300, S7-400, S7-1200, S7-1500 PLCs and  ET200S , ET200M Remote I/Os