PLC Hardware: I/O Section

• PLC Hardware Components are discussed.

The IO Section (Input/Output)

• The I/O section acts as the connection point for all field devices to the PLC and provides an interface between these devices and the CPU.
• Input/output arrangements are integrated into fixed PLCs.
• Fixed I/O is common in small PLCs, where the processor and I/O are packaged together in one non-removable unit, offering low cost but limited flexibility.
• Modular types offer more flexibility by using external I/O modules that can be plugged into the PLC.
• Rack-based I/O sections consist of individual I/O modules.
• Input interface modules convert signals from machines or processes into signals readable by the controller.
• Output interface modules convert controller signals into external signals to control machines or processes.
• The hardware assembly containing I/O modules, processor modules, and power supplies is the chassis.
• Chassis are available in different sizes, typically with 4, 8, 12, or 16 slots.

IO Section Addressing

• The PLC's memory system stores the status of inputs, outputs, and other data objects.
• Addressing is used to track this information.
• An address indicates the location of specific information in PLC memory using a label or number.
• Direct addressing syntax starts with a % symbol, followed by a memory location prefix that indicates the memory zone. Prefixes include:
• M for internal variables
• N for network variables
• K for constants
• I for input variables
• S for system variables
• Q for output variables

IO Modules

• I/O modules can be classified into three types:
• Discrete I/O modules use a bit addressing format.
• Analog I/O modules use a word addressing format.
• Specific function modules include high-speed counters, thumbwheel inputs, TTL, encoder-counters, stepper motors, and BCD outputs.
• An I/O module comprises a printed circuit board and a terminal assembly.
• The printed circuit board includes electronic circuitry.
• The terminal assembly facilitates field wiring connections.
• I/O modules are commonly available as 8, 16, 32, or 64 point cards, where "points" indicate the number of available inputs or outputs.
• Most PLC modules feature plug-in wiring terminal blocks.
• During module replacement, only the module strip is removed and replaced without disturbing the wiring.
• Combination I/O modules can accommodate both input and output connections in one physical module.

Discrete IO Modules

• Discrete I/O modules interface with on/off field input devices.
• These I/O are bit oriented, dealing with individual inputs and outputs.
• Each discrete I/O module is powered by a field-supplied voltage source or the IO module itself.
• I/O modules are compatible with various AC and DC voltage ratings.

Power and Voltage

• Modules receive voltage and current from the backplane of the rack enclosure.
• Backplane power is supplied by the PLC module power supply and powers the electronics on the I/O module circuit board.

The Central Processing Unit (CPU)

• The CPU resides within fixed PLCs, while modular PLCs generally use a plug-in module.
• The terms CPU, controller, and processor refer to the same module that executes the primary functions.
• A processor module consists of a CPU section and a memory section.
• The CPU carries out the program instructions.
• Memory stores active programs and retrievable data.
• The CPU contains a similar microprocessor to those found in a personal computer.
• PLC microprocessors are designed for industrial control rather than general-purpose computing.

Memory Design

• Memory is the storage location for information, programs, and data in a PLC.
• The amount of memory needed depends on the complexity of the program.
• A memory location refers to a specific address within the CPU's memory where a binary word is stored.
• Each binary piece of data is a bit , and eight bits make up a byte.
• The program and numerical data are stored in memory as ones and zeros.
• 16 bits are generally considered a word
• Sections of memory are dedicated to storing the status of inputs and outputs.

Memory Types

• PLCs use programmable non-volatile memory, ensuring that control programs are retained even if power is lost.
• ROM stores the operating system and system software for the PLC, preventing alteration of programs and data.
• RAM provides temporary storage for data that needs quick changes, such as ladder logic programs.
• Electrically Erasable Programmable Read-Only Memory (EEPROM) is a non-volatile memory allowing programming flexibility equivalent to RAM but is slower.
• The EEPROM can be electrically overwritten with new data, removing the need for ultraviolet light erasure.
• The EEPROM doesn't lose memory when the power is off, so does not require a continuous power supply or battery backup.

Programming Terminal Devices

• A programming terminal device is essential for entering, modifying, and troubleshooting the PLC program.
• A hand-held proprietary programming terminal connects to a PLC's programming port via a cable.
• Hand-held programmers are compact, inexpensive, and have limited display capabilities.
• PLC programming is commonly done on a personal computer using the manufacturer’s software and sometimes special cables.