PLC PROGRAMMING

PLC Programming LanguagesThe standard IEC 61131 (International Electro technical Commission) was established to standardize the multiple languages associated with PLC programming by defining the following five standard languages:• Ladder Diagram (LD) —a graphical depiction of a process with rungs of logic, similar to the relay ladder logic schemes that were replaced by PLCs.• Function Block Diagram (FBD) —a graphical depiction of process flow using simple and complex interconnecting blocks.• Sequential Function Chart (SFC) —a graphical depiction of interconnecting steps, actions, and transitions.• Instruction List (IL) —a low-level, text-based language that uses mnemonic instructions.• Structured Text (ST) —a high-level, text-based language such as BASIC, C, or PASCAL specifically developed for industrial control applications.

Ladder Diagram and Programming: Load: The load (LD) instruction is a normally open contact

A Load (contact) symbol

Load Bar: The Load Bar instruction is a normally closed contact.

A Load Bar (normally closed contact) symbol

Out :The Out instruction is sometimes also called an Output Energize instruction. The output instruction is like a relay coil

An OUT (coil) symbol

matrix limitation diagram for a typical PLC. A maximum of seven parallel lines and 10 series contacts per rung is possible.

Addressing format

Logic elements

Programming a PLC :

In order to create or change a program, the following items are needed:

PLC Programming Device Programming Software Connector Cable

You can use a personal computer as a programming device

Instructions

• Timers• Counters• Master control • Jump control• Data handling• Shift registers

TIMERS

• In general, there are three different PLC timer types: – On-delay timer (TON) – Off-delay timer (TOF) – Retentive timer on (RTO)

ON-DELAY TIMER (TON)

An on-delay timer is used when you want to program a time delay before an instruction becomes true.

Contd..

OFF-DELAY TIMER (TOF) and RETENTIVE TIMER ON (RTO)

The off-delay timer (TOF) operation will keep the output energized for a time period after the rung containing the timer has gone false.

A retentive timer (RTO) accumulates time whenever the device receives power, and it maintains the current time should power be removed from the device.

OFF-DELAY TIMER (TOF)

RETENTIVE TIMER ON (RTO)

COUNTERS

UP-COUNTER

Contd..

DOWN-COUNTER

• The down-counter instruction will count down or decrement by 1 each time the counted event occurs.

• Each time the down-count event occurs, the accumulated value is decremented.

MASTER CONTROL

JUMP CONTROL

DATA HANDLING INSTRUCTIONS

• Moving data• Comparison of magnitude of data( greater

than, equal to, less than)• Arithmetic operation (Addition & subtraction)• Conversion between binary coded decimal,

binary, octal.

MOVE INSTRUCTION

DATA COMPARISON

EQUAL TO NOT EQUAL TO

GREATER THANGREATER THAN OR EQUAL

DATA COMPARISON

LESS THAN LESS THAN OR EQUAL

LIMIT TEST

ARITHMETIC OPERATION

ADDITION SUBTRACTION

MULTIPLICATION DIVISION

CONVERSION

TO BINARY CODED DECIMAL

The convert to BCD (TOD) instruction is used toconvert 16-bit integers into binary-coded decimal (BCD) values.

FROM BINARY CODED DECIMAL

The convert from BCD (FRD) instruction is used to convert binary-coded decimal (BCD) values to integer values.

BIT SHIFT REGISTERS• A bit shift register is a register that allows the shifting

of bits through a single register or group of registers.• The bit shift register shifts bits serially (from bit to bit)

through an array in an orderly fashion.• A shift register can be used to simulate the

movement , or track the flow, of parts and information• Common applications for shift registers include the

following:– Tracking of parts through an assembly line– Controlling of machine or process operations– Inventory control– System diagnostics

Data in the shift register could represent the following:• Part types, quality, and size• The presence or absence of parts• The order in which events occur• Identification numbers or locations• A fault condition that caused a shutdown

ANALOG I/O MODULES

• Discrete devices are inputs and outputs that have only two states: on and off.

• In comparison, analog devices represent physical quantities that can have an infinite number of values.

• Typical analog inputs and outputs vary from 0 to 20 milliamps, 4 to 20 milliamps, or 0 to 10 volts.

Contd..

• The analog input interface module contains the circuitry necessary to accept an analog voltage or current signal from the level transmitter field device.

• This input is converted from an analog to a digital value for use by the processor.

• The circuitry of the analog output module accepts the digital value from the processor and converts it back to an analog signal that drives the field tank level meter.

• Common physical quantities measured by a PLC analog module include temperature, speed, level, flow , weight, pressure, and position.

Contd..

• For example, a sensor may measure temperature over a range of 0 to 500°C, and output a corresponding voltage signal that varies between 0 and 50 mV.

Selection of PLC

• Input/output capacity Future?• Types of Input / Output required• Size of memory• Speed and power of CPU

Areas of Application

Manufacturing/Machining Food/beverage Metals Power Mining Petrochemical/Chemical

Examples of PLC Programming Softwares:

1.Allen-Bradley – Rockwell Software RSLogix 500

2. Modicon - Modsoft3. Omron - Syswin4. GE-Fanuc Series 6 – LogicMaster65. Square D- PowerLogic6. Texas Instruments – Simatic7. Telemecanique – Modicon TSX Micro