Programmable Logic Controllers LO1: Understand the design and operational characteristics of a PLC system

Learning Outcome 1 LO1: Understand the design and operational characteristics of a PLC system –1.4 describe the internal architecture and operational characteristics of the CPU of a typical programmable logic device

Agenda At the end of this session the students will be able to… 1.Describe the architecture of a PLC. 2.Describe the function of each of the main system elements of the architecture.

The Programmable Logic Controller PLC PROGRAMME INPUTSOUTPUTS

The Functional Components of a PLC The typical PLC has the basic functional components of… –Processor unit (CPU) –Memory –Power supply unit –Input / output interface section –Communications interface –Programming device

Relationship between the Functional Components of a PLC Processor Unit (CPU) Input inter- face Output inter- face Programme & Data Memory Communications interface Power Supply Programming Device

The Central Processor Unit (CPU) The processor unit or central processor unit (CPU) is the unit containing the microprocessor. This unit interprets the input signals and carries out the control actions according to the programme stored in its memory, communicating the decisions as action signals to the outputs.

The Power Supply Unit (PSU) The power supply unit is needed to convert the mains AC voltage (230V r.m.s.) to low DC voltage (5V) necessary for the processor and the circuits in the input and output interface modules.

The Programming Device The programming device is used to enter the required programme into the memory of the processor. The programme is developed in the device and then transferred to the memory unit of the PLC

The Programming Device A programming device could be… –A handheld device –A desktop console –A personal computer

The Memory Unit The memory unit is where the programme containing the control actions to be exercised by the microprocessor is stored and where the data is stored from the input for processing and for the output.

The Input and Output Interfaces The input and output interfaces are where the processor receives information from external devices.

The Communications Interface The communications interface is used to receive and transmit data on communication networks from or to other remote PLCs. It is concerned with such actions as device verification, data acquisition, synchronisation between user applications and connection management

Basic Communications Model Communications Network PLC 1 Supervisory System Plant / Machinery PLC 2

Communications Model: Allen-Bradley

The internal Architecture of a PLC CPU User Program RAM System ROM Data RAM Input / output unit CLOCKCLOCK Data Bus Address Bus Control Bus Opto- Coupler Buffer I/O System Bus DriversDrivers BatteryBattery LatchDriver Interface Inputs Outputs Programme Panel

The CPU The CPU is made up of the following… –An Arithmetic and Logic Unit (ALU) which is responsible for data manipulation and carrying out arithmetic operations of addition and subtraction and logic operations of AND, OR, NOT and EXCLUSIVE OR – HANDOUT 1.

The CPU Memory, termed registers, located within the microprocessor and used to store information involved in programme execution A control unit that is used to control the timing of operations. TXABUAhttps:// TXABUA

The Buses The buses are paths used for communication within the PLC. The information is transmitted in binary form, that is, as a group of bits, with a bit being a binary digit ‘1’ or ‘0’, indicative of on / off states. Bits are arranged into a ‘word’. A word is used for a ‘group of bits’ constituting some piece of information. Typically this might be 8-bits e.g – HANDOUT 2 – ASCII Code Each of the bits in a word is communicated simultaneously along its own parallel wire.

The Data Bus The data bus carries the data used in the processing done by the CPU. A microprocessor termed as 8-bit, has an internal data bus that can handle 8-bit numbers (words). It can thus perform operations between 8- bit numbers and deliver results as 8-bit values

The Address Bus The address bus is used to carry the addresses of memory locations. Each word is located in memory Every memory location is given a unique address so that the CPU ‘knows’ where to look (read) a particular word or to store (write) a particular word. If the address bus consists of 8-lines, then there can be 2 8 = 256 addresses If the address bus consists of 16-lines, then there can be 2 16 = 65,536 addresses

The Control Bus The control bus carries the signals used by the CPU for control, –For example it might be used to inform a memory device whether it is to receive data from an input or output. –It is also used to carry timing signals used to synchronise actions

The System Bus The system bus is used for communications between the input / output ports and the input / output unit

Memory Data and programme instructions need to be stored by the PLC in order for the PLC to process the data. –System (Read Only Memory) ROM: gives permanent storage for the operating system –Random Access memory (RAM) is used for the users programme

Memory –RAM is used for data. This is where information is stored on states of input and output devices and the values of timers and counters and other internal devices. (This is sometimes called the register table or data table). Part of this memory, that is a block of addresses, will be put aside for input and output addresses and the states of those inputs and outputs. Part will be set aside for pre-set data and part for starting counter values, timer values and the like.

Memory –Erasable and Programmable Read-only- memory (EPROM) is used to store programmes permanently –If the memory size is 2048 bits then it can store 256, 8 bit words (bytes). –Similarly 4096 bits can store 512, 8 bytes or 256 x 16-bit words. –1K of memory represents 2 10 bits of storage space i.e bits can be stored.

Memory –Manufacturers supply memory chips with storage locations grouped in 1, 4 and 8 bits. –Thus a 4K x 1 memory has 4 x 1024 x 1 = 1024 bit locations. –A 4K x 8 memory has 4 x 1024 x 8 = 32,768 bit locations.

Memory Remembering that 1 byte = 8 bits, then –4K x 8 means that 4096 bytes can be stored. –For a 16 bit address bus there are 2 16 different addresses. Thus with 8-bit words stored at each address there are 2 16 x 8 storage locations. Thus (2 16 x 8) / 2 10 = 64K x 8 i.e. which might come in the form of 4 x 16K x 8bit memory chips

CPU Operation