Chapter 1 PLC overview

Programming language

provides rules for combining the instructions so that they produce the desired actions.

Power supply

supplies DC power to other modules that plug into the rack.

Two most common type of PLC programming device

1. hand-held units 2. Laptop computers - most commonly used

Basic Modular controller consists of

1. rack 2. power supply 3. processor module or CPU 4. input/output 5. operating interface for programming and monitoring

1. Central processing unit or CPU 2. Input/output section 3. power supply 4. programming device

Parts of a PLC

Relay Ladder Logic

is the standard programming language used with PLCs, origin is based on electromechanical relay control. Program graphically represents rungs of contacts, coils, and special instruction blocks.

Power supply for larger systems

power to field devices is provided by external AC or DC supplies

numbers

represent the instruction location addresses

Individual symbols

represents instructions

Real-time System

the output of the system controlled by the PLC depends on the input conditions

Power supply for some small micro PLC systems

the power supply may be used to power field devices

Open architecture design allows the system to be connected easily to devices and programs made by other manufacturers while closed is one whose design is proprietary, making it difficult to connect to other systems.

Explain difference between open and proprietary PLC architecture

Fixed I/Os

typical of small PLCs that come in one package with no separate, removable units. The processor and I/O are packaged together and the I/O terminals will have fixed number of connections built in for inputs and outputs. Advantage: Lower cost Disadvantage: lack of flexibility, if any part in this unit fails, the whole unit has to be replaced.

1. Increased Reliability 2. More flexibility 3. Lower Costs 4. communications capability 5. Faster Response time 6. Easier to troubleshoot

6 distinct advantages that PLCs offer over conventional relay-based control systems.

Timing, Counting, Calculating, Comparing, Processing of analog signals

Tasks in addition to relay switching operations that PLCs are capable of performing.

Describe how the I/O modules connect to the processor in a modular-type PLC configuration.

The modules plug into a rack. When a module is slid into the rack, it makes an electrical connection with a series of contacts called the backplane, located at the rear of the rack. The PLC processor is also connected to the backplane and can communicate with all the modules in the rack

1. Fixed I/Os 2. Modular I/Os

Two ways in which Inputs/outputs are incorporated into the PLC

Programmable Logic Controller

an industrial grade computer that is capable of being programmed to perform control functions

Processor or CPU

brain of the PLC, usually consists of a microprocessor for implementing the logic and controlling the communications among the modules. The processor requires memory for storing user program instructions, numerical values and I/O devices status.

architecture

can refer to PLC hardware, software or to a combination of both

Relay logic

capable of performing relay switching tasks

Modular I/Os

divided by compartments into which separate modules can be plugged, increasing options and the unit's flexibility.

Program

is a user-developed series of instructions that directs the PLC to execute actions.