There are three popular types of output (Digital Output) cards.
Relay cards are very easy to interface but suffer from the mechanical limitation of low-speed operation and limited lifespan.
DC Sinking and Sourcing output card can switch output quickly and dont suffer from mechanical wear but exhibit some interface limitations.
Sourcing Output Card
Sourcing output cards are called P-N-P because the switching element was traditionally a P-N-P transistor.
The PLC card connects V+ to the load.In sourcing output module during switch closure V+ is connected to the load and current flows as shown.
Advantages of Sourcing Output Card are -
1.Easier interface since only one power supply is required.
Disadvantage of PLC sourcing output card.
1.Voltage to the load is limited to the card supply voltage.
Sinking Output Card
Sinking output cards are called NPN because the switching element was traditionally a NPN transistor.
These PLC cards connect ground to the load.
In sinking output module during the switch closure the load is connected to ground to the switch.The switch is hooked to ground and load is connected to the V+.
Advantages of PLC sinking output card.
1.Ability to switch voltages higher than card supply voltage.
Disadvantages of PLC sinking output module
1.Two power supplies are required for proper interfacing.