Section 6.3: PLC Fundamentals & I/O Systems

PLC Fundamentals & I/O Systems in Mail Processing

In postal automation, the physical movement of mail, switching of gates, and reading of sensors are controlled by a Programmable Logic Controller (PLC). A PLC is a ruggedized industrial computer designed to operate reliably in harsh environments characterized by dust, electrical noise, and vibration. Technicians must understand the internal architecture of a PLC, discrete and analog Input/Output (I/O) configurations, electrical wiring concepts of sourcing and sinking, and the cyclic nature of the PLC execution loop.

PLC Architecture and Hardware Modules

A standard PLC system consists of several modular components housed in a chassis or rack with a backplane that provides electrical power and communication buses:

1. Power Supply: Converts high-voltage incoming AC line power (such as 120VAC or 240VAC) into clean, low-voltage DC power (typically 24VDC and 5VDC) required by the internal circuitry of the CPU and I/O modules.
2. Central Processing Unit (CPU): The processor that stores and executes the control program. It reads the states of input devices, processes the programming logic (ladder logic), and updates the states of output devices. It also contains memory partitions for program storage (non-volatile) and data storage (volatile).
3. Input Modules: Interface circuits that receive signals from field sensors and convert them into low-voltage digital signals for the CPU.
4. Output Modules: Interface circuits that receive control signals from the CPU and convert them into the appropriate electrical power to drive field actuators.
5. Programming Device: A personal computer running manufacturer-specific software connected to the PLC via serial, USB, or Ethernet, used to write, upload, download, and debug program code.

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Discrete and Analog Input/Output Systems

Field devices connected to a PLC are classified as either discrete or analog, which determines the type of I/O module required.

Discrete (Digital) I/O

Discrete signals are binary, representing only two states: ON or OFF (1 or 0).

• Discrete Inputs: Send a simple switch state to the PLC. Examples include proximity sensors (detecting the presence of a mail tray), photoelectric eyes (detecting a letter blocking a light beam), limit switches, and pushbuttons.
• Discrete Outputs: Controlled by the PLC to turn field devices on or off. Examples include solenoids (pneumatic valves redirecting a mail piece), motor starters, and pilot lights.

Analog I/O

Analog signals represent continuously variable physical quantities. They must be converted into digital values by the PLC using an Analog-to-Digital Converter (ADC) for inputs, or back to analog signals using a Digital-to-Analog Converter (DAC) for outputs.

• Analog Inputs: Convert physical values into variable electrical signals. Common standard signals are current loops (typically 4-20 mA, which is highly resistant to noise over long distances) or voltage loops (typically 0-10 VDC). Examples include temperature RTDs, pressure transmitters, and speed potentiometers.
• Analog Outputs: Allow the PLC to control devices at variable rates, such as modulating a control valve or setting the speed command of a Variable Frequency Drive (VFD) controlling a conveyor belt.

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Sourcing vs. Sinking in DC I/O Circuits

A critical concept in wiring DC discrete I/O modules is the distinction between sourcing and sinking circuits. These terms define the direction of conventional current flow between the PLC module and the field device. In a DC circuit, conventional current flows from the positive terminal (+24VDC) to the negative terminal (0VDC/Common).

• Sinking: A device or module is sinking if it provides a path to the common ground (0VDC). Current flows into a sinking device.
• Sourcing: A device or module is sourcing if it provides the positive electrical source (+24VDC). Current flows out of a sourcing device.

For an electrical circuit to be complete, a sourcing device must always be wired to a sinking device, and vice versa.

For example, if an input module is configured as a sinking input module, its internal circuitry is connected to the common ground. Therefore, the field sensor connected to it must be a sourcing sensor (often a PNP transistor type) that outputs +24VDC when active. If the module is a sourcing input module, its internal circuitry is tied to +24VDC, meaning it requires a sinking sensor (often an NPN transistor type) that connects the input terminal to ground when active.

[Sourcing Input Module (+24V)]  ----(Current Flow)---->  [Sinking NPN Sensor (0V Ground)]
[Sinking Input Module (0V Ground)] <---(Current Flow)---- [Sourcing PNP Sensor (+24V)]

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The PLC Scan Cycle

Unlike a standard PC which runs multiple applications concurrently in an unpredictable order, a PLC executes a single program in a continuous, deterministic loop known as the PLC scan cycle. This cycle consists of four distinct phases:

1. Input Scan (Read Phase): The PLC CPU reads the electrical status of all physical input modules. It copies these states (ON or OFF) into a dedicated memory area called the Input Image Table (or Input Register). Once copied, the states are locked for the duration of the cycle.
2. Program Execution (Logic Phase): The CPU executes the user program (such as ladder logic) rung-by-rung, starting from top to bottom and left to right. It uses the input states stored in the Input Image Table to solve the logical conditions and writes the calculated results into the Output Image Table.
3. Output Scan (Write Phase): The CPU transfers the calculated states from the Output Image Table to the physical output modules, energizing or de-energizing the connected solenoids, contactors, or indicators.
4. Housekeeping and Communications (Overhead Phase): The CPU performs internal self-diagnostics (checking watchdog timers, battery status, memory integrity) and processes communications with programming laptops, human-machine interfaces (HMIs), or networked controllers.

The time required to complete one full cycle is called the scan time, typically measured in milliseconds (ms). If an input event (such as a fast-moving letter blocking a photoeye) occurs and resolves in a time shorter than the PLC scan time, the PLC will fail to register the event. To prevent this, technicians install high-speed counter modules or program hardware interrupt routines that immediately pause the normal scan cycle to handle the event.