P L C

PLC Basics
1.1 Function of a PLC
A PLC is a microprocessor-based controller with multiple inputs and outputs.
It uses a programmable memory to store instructions and carry out functions to
control machines and processes.
The PLC performs the logic functions of relays, timers, counters and
sequencers. It has the following advantages:
Low cost
Reliability
Reprogramability

1.2 Inputs and Outputs
The PLC inputs give it information about the machine or process that it is
controlling. These are typically switches and sensors. The switches are connected to
an input module that provides the interface between the switches or sensors and the
PLC.
Input module circuits have opto-isolators to protect the internal PLC
circuitry from damage.

The PLC outputs are connected directly or indirectly (e.g. through a relay) to
actuator controls. Examples include solenoids on directional control valves, motors,
motor contactors, alarms and warning lights.
There are three main types of output module:
1-Relay (volt-free): The signal from the PLC operates a relay within the output
module connecting the control voltage to the output port and hence to the actuator.

2-Transistor: A transistor is used to switch on the output. This is faster than a
relay output but is only suitable for low power direct current applications.

3-Triac: This solid state device is used for switching alternating current
devices. It requires some form of over current protection.
Internal relay
contact
Common port
Output Port
Control Voltage (+)
Solenoid
Intelligent I/O module

EXECUTIVE MEMORY- MAIN MEMORY THAT PART OF PLC BY THIS DONE  THE  EXECUTION OF PROGRAM , COMMUNICATE WITH PERIPHERAL COMPONENTS.

APPLICATION MEMORY- THAT IS STORAGE AREA FOR SAVE USER PROGRAM

sample diagram of PLC 

1.3 PLC Architecture and Wiring Diagrams

1.4 Network Protocols
 Many commonly used devices conform to a 4-20 mA standard whereby signals of 4mA and 20mA form respectively the minimum and maximum values of an analog signal.
With analog devices, a separate cable needs to be run between the end device
and the control system because only a single analog signal can be represented on the
circuit. The 4-20 mA standard is slowly being replaced by network or fieldbus
communications. Fieldbus is a multi-drop digital two-way communication link
between intelligent devices. Fieldbus allows the connection of a number of sensors all
located in the same area to the same cable. Fieldbus comes in many varieties
depending on the manufacturer and application. Examples include ASibus, Profibus,
Devicenet and Modbus.
A more recent trend is the development of Industrial Ethernet which has the
capacity to transport large quantities of data not only for process control but also to
integrate the process with management information systems.

Chapter 2
Ladder Programming

2.1 Conditional Logic
The PLC scans its inputs and, depending on the program, switches on or off
various combinations of outputs. The logic state of the output depends on the input
conditions and so the term conditional logic is used.
A simple example of conditional logic could be stated as follows:
“A machine switches on if either of two start switches are closed and all of
three stop switches are closed.”
The conditions could be realised by a hard wire solution as shown in Fig 2.1.

.
Fig 2.1 Hard-Wire Conditional Logic Example
The two start switches are connected in parallel. Current will flow if one or
the other or both are closed. The start switches are normally open. This means that the
contacts are apart and no current flows when the switches are in their normal (or
unoperated or rest) state.
The three stop switches are connected in series. Current can only flow if the
first and the second and the third are closed. The stop switches are normally closed.
This means that the contacts are connected and current can flow when the switches
are in their normal state.
Start 1 Start 2
Stop 1
Stop 2
Stop 3
Machine
relay
+V
0 V
The relay is a switch with multiple contacts that is operated when its coil is
energised. The contacts are usually capable of carrying a larger current than pushbutton
or limit switches. Large relays for motor starting are called contactors. The
schematic diagram for a typical relay is shown in Fig 2.2.

Fig 2.2 Relay
2.2 Ladder Diagrams
To realise the conditional logic statement from section 2.1 using ladder logic we
connect the switches to a PLC as shown in Fig 2.3.
Fig 2.3 PLC Wiring Diagram

To avoid later confusion regarding the concept of normally open (n/o) and
normally closed (n/c) it is worth looking again at Fig 2.3 and remembering that the plc
scans each input and asks “Is it on or is it off?” The five switches shown are external
devices and the PLC knows nothing about them. As far as the PLC is concerned, at
the moment, inputs X1 and X2 are off and X3, X4 and X5 are on.
I have written the ladder logic using the TriLogi software. (For details of
entering program elements see the Appendix)
Fig 2.4 PLC Ladder Diagram

2.4 Outputs and latches
Output states (on or off) can be used in programs as conditions for other actions.


2.5 Internal relays
These have the same properties as outputs but they only exist in software.

Note: Most PLCs include a function called a Set and Reset or a flip-flop which
latches and delatches an output or an internal relay. Throughout this book I use the
latch as described in section 2.4, because of the visual resemblance of the ladder rung
to the equivalent hard-wire circuit, in which a relay coil is latched on by a normallyopen
contact connected in parallel with the start button.
2.6 Timers
The delay-on timer introduces a delay between the start of one event and the
start of another.
For example, when a start push button is pressed, the pneumatic cylinder shown
in Fig 2.16 extends, remains extended for 5 seconds and then returns. Draw the PLC
wiring diagram and the appropriate ladder logic.

2.7 Counters
A counter allows a number of occurrences of input signals to be counted. The
counter is set to a preset number value and when this value of input pulses has been
received, it will operate its contacts. A second input or software coil is provided to
reset the current value of the counter to zero.



Chapter 3

Sequential Programming
3.1 Introduction
Most machine operations are sequential in nature so it is necessary for the
PLC to switch outputs depending not only on the input combinations but also on the
current stage in the sequence. An output operating at the wrong time could cause
damage or injury so the correct programming technique is critical.
Summary of this differences between SCADA and HMI 
• SCADA defined as supervisory monitoring and control in the plant, and HMI is a division of SCADA. 
• HMI is stand for Human Machine Interface, it links with all the processes and then display or show this data to the operator. 
• The HMI function as well as required is critical for the good operating SCADA.
SCADA is stand for Supervisory Control and Data Acquisition

HMI-human machine interface

There are some different of Modbus and Profibus which are sometimes called "Fieldbus". It is a common name used to tell any network industry that links the field devices. As a generic explanation it was fashionable at one point, but looks to have fallen out of use recently. 


It is also occasionally applied as the simple form of a particular protocol called Foundation Fieldbus. It is only another protocol such as Profibus or Modbus, but it has attributes which are planned specifically for industrial processes. 


It was also used as the short form for a failed effort of IEC at standardizing the networks industries. That standard is much forgotten now, it was said that the proprietary of protocols could define itself as standard. 


There are 2 different communications protocols. Profibus has convinced protocol characteristics that allow certain versions of it run in multi-master mode using RS-485, while Modbus can be operate at only single master. Though, Modbus can run on Ethernet, include the multiple masters. While Profibus can't operate on Ethernet, but for Profinet it can operate on Ethernet, but Profinet is not same with Profibus. 


Profibus is a real sensor or actor of the fieldbus. It can work at different speed of transmission from 1.2 kbps - 12Mbps and is highly optimized for the speed. A fieldbus is a term of technical for serial buses which is used for communication industry, such as USB (Universal Serial Bus), but it is integrated industrial devices and not devices of multimedia. 


The fieldbus define as data communication for industry with serial bus systems is the main of the I/O system and for every distributed control. The systems of serial bus designed for real time communication data are defined as fieldbuses. Fieldbuses are networks industries to determine real time communication data between masters of bus, distributed sensors and actors. They are constructed for low transmission volumes and low latency. The fieldbuses packet sizes are in common in the range of 1 until 200 to 300 bytes. The speed of transmission can be selected in a wide range from 12 Mbps down to 1200 bps. This transmission speed flexibility is a great advantage against, for example, Ethernet based networks. The useable length of segment depends on the speed of transmission, media of transmission, and the individual fieldbus topology. 


Normally, high speed of transmission reduces the segment length of bus oriented fieldbuses to a 50 to 100meters; low speed of transmission lets a segment length in the range of some kilometers. Fieldbuses with a ring topology such as EtherCAT or INTERBUS are operating frequently with a physical expansion in the kilometers range due to the signal refreshing is done by the I/O modules of fieldbus. The transmission Medias of fiber optic allow in common extended segment lengths and increased immunity of noise. 


In other hand, Profibus was OK in its day, but its specialized characteristics tied it directly to RS-485, which is going to obsolete even though there are still lots of used in industries. The utilization of Profibus is still increasing, specialized sensor/actor fieldbus such Profibus have their market in the automation industry now and future. 


Modbus is still going grow because its simplicity allows it develops to adjust to the Ethernet without major change. Modbus currently one of the main Ethernet protocols applied in automation. The older versions of serial like RS-232 and RS-485 are still around as well. 


There is a group of fieldbus systems standardizes by the IEC 61158. Each of the group is open specified, standardizes, and are not proprietary protocols. The process to get the standard of IEC 61158 is not easy. There is no such simple look like.