Monday, April 21, 2014

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

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