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Based on the hardware specifications, there are two basic types of PLC:

1. Compact [the CPU, input module, output module, and sometimes the power supply are integrated into a single module]
2. Modular [the CPU, input module, output module, and power supply are separate; each module is plugged into a common rack/chassis or bus]

The PLC scan cycle is a good way to explain how the PLC works.

The PLC scan cycle is the cycle in which the PLC reads the inputs, runs the PLC programs, performs diagnostic and communication tasks, and updates the output. It’s a repetitive process.

The time taken by the PLC to complete one scan cycle is referred to as the scan time and is measured in milliseconds.

PLC scan cycle may vary slightly depending on the PLC in use.
Basically, the PLC starts by reading the inputs, that is, checking the state of the connected switches and sensors to know which one is ON or OFF or the current value if it’s analog. It then executes the user program based on the state of the inputs. Next, the PLC performs communication tasks, which include communicating necessary information to the control network such as Profibus, Modbus, Ethernet IP, and so on. It also performs diagnostic tasks to ensure the entire PLC system is functioning properly. It then updates the output before it starts a new scan.

Sinking and sourcing in PLC wiring

Sinking and sourcing describe the direction of current flow between devices in a PLC system. These terms apply specifically to DC circuits and are not relevant to AC circuits. In sinking, current flows into a device from the load, while in sourcing, current flows out of a device to the load. Understanding this difference is crucial when selecting input and output modules for PLC system.

PLC input modules interface with devices that send binary signals (ON/OFF). These modules process signals from switches, sensors, or buttons to determine the state of connected field devices. Digital inputs are categorized as:
· Sourcing Inputs: These modules provide a positive voltage to the external circuit.
· Sinking Inputs: These modules provide a ground connection to complete the circuit.

PLC outputs control devices such as motors, heaters, or solenoids by sending binary signals based on program logic. Outputs can also be categorized as sinking or sourcing, depending on their wiring and function:
· Sourcing Outputs: Provide current to external devices.
· Sinking Outputs: Serve as a ground connection for external circuits.

Sinking and sourcing in PLC systems are essential concepts for managing current flow in digital input and output modules. Sinking and sourcing terminology applies only to DC input and output circuits. Input and output points that are sinking or sourcing can conduct current in one direction only. Sinking and sourcing refer to two distinct methods of connecting input and output devices to a PLC’s digital input and output (I/O) modules. They apply only to DC input or output. These methods determine how current flows between the devices and the PLC, influencing the overall circuit design and functionality. The sourcing device is the device that provides the current (positive), while the sinking device is the device that absorbs the current (that is, the device connected to negative or ground). PLC manufacturers usually produce input/output modules that we can either sink or source. There are PLCs where we can only sink their input/output module, and for some, we can sink or source it. It all depends on the manufacturer’s design. So, we need to consult the manual of the input or output module before wiring.
Many PLC manufacturers usually have a common terminal on their PLC’s input or output module which must be connected to positive (e.g. if the module will be sourcing) or connected to negative (e.g. if the module will be sinking). When the common terminal of the input/output module is connected to the positive (+24V) of the power supply, we say the input/output module is sourcing, and when that common terminal is connected to the negative (- or 0V) of the power supply, we say the input/output module is sinking. Key Design Considerations:

Matching: It's crucial to match the sinking/sourcing type of your PLC I/O module with the sinking/sourcing type of your field devices. For example, a sourcing PLC input module needs a sinking sensor (like an NPN sensor).
Polarity: These terms apply primarily to DC circuits because they rely on clear positive and negative polarities. They don't typically apply to AC circuits.
Common Connections: Many PLC I/O cards have groups of inputs or outputs that share a common connection. This common must be wired consistently (either for sinking or sourcing) within that group.
Safety: Proper wiring is essential for system reliability and safety. Incorrect wiring can lead to non-functional circuits, short circuits, or damage to components.
Transistors: Sinking and sourcing typically refer to the type of transistor used for switching within the module or device:

· NPN transistors are generally associated with sinking (they switch the negative side/ground).

· PNP transistors are generally associated with sourcing (they switch the positive side).

Understanding sinking and sourcing is fundamental for anyone working with PLCs, as it dictates how you power and connect your field devices to your control system. Always refer to the manufacturer's documentation for specific wiring diagrams and requirements for your PLC and components.