Programming PLCs

External programming units can be used to download programs into the program memory of the CPU.
The external field programmers provide several software features that facilitate program entry in
graphical form. The programmers also provide comprehensive aids for debugging and execution
monitoring support logic and sequence control systems. Printer can be connected to the programmers
for the purpose of documenting the program. In some cases, special programming packages that run on
Personal Computer can also be used as programming units.

There are two ways of entering the program:

1. Direct program entry to the program memory (RAM) plugged into the central controller. For this
purpose, the programmer is connected to the processor or to the programmer interface modules.

2. Programming the EPROM sub modules in the programmer without connecting it to the PC (off-line).
The memory sub modules are then plugged into the central controller. 0A programming device can be a
handheld device, a desktop console, or a computer. Only when the program has been designed on the
programming device and is ready is it transferred to the memory unit of the PLC.

• A handheld programming device normally contains enough memory to allow the unit to retain programs
while being carried from one place to another.

Desktop consoles are likely to have a visual display unit with a full keyboard and screen display. Personal
computers are widely configured as program development workstations. Some PLCs only require the
computer to have appropriate software; others require special communication cards to interface with the
PLC. A major advantage of using a computer is that the program can be stored on the hard disk or a CD
and copies can be easily made

The important features for programming PLC’s are

1. Programming: You can choose between two different languages: Ladder and FBD/ Grafcet.

2. Simulation: You can test the result of your programming in real time.

3. Downloading: You can transfer your programs directly to the modules or remotely via local wired or
wireless (Bluetooth) modem solutions.

4. Supervision: You can view the status of your application, locally or remotely, by the communication
solutions.

1. Ladder Programming (LD):

The most popular language used to program a PLC is ladder Programming. Ladder Programming (LD) is a
graphic Language. It can be used to transcribe relay diagrams,

In the ladder programming method, the PLC system provides a design environment in the form of
software tools running on a host computer terminal which allows ladder diagrams to be developed,
verified, tested, and diagnosed.

First, the high-level program is written in ladder diagrams. Then, the ladder diagram is converted into
binary instruction codes so that they can be stored in random-access memory (RAM) or erasable
programmable read-only memory (EPROM). Each successive instruction is decoded and executed by the
CPU.
The PLC program uses a cyclic scan in the main program loop such that periodic checks are made to the
input variables. The program loop starts by scanning the inputs to the system and storing their states in
fixed memory locations (input image memory I). The ladder program is then executed rung-by-rung.
Scanning the program and solving the logic of the various ladder rungs determine the output states. The
updated output states are stored in fixed memory locations (output image memory Q). The output values
held in memory are then used to set and reset the physical outputs of the PLC simultaneously at the end
of the program scan.

A PLC resolves the logic of a ladder diagram (program) rung by rung, from the top to the bottom.
Usually, all the outputs are updated based on the status of the internal registers. Then the input states
are checked and the corresponding input registers are updated. Only after the I/Os have been resolved,
is the program then executed. This process is run in a endless cycle. The time it takes to finish one
cycle is called the scan time.
RELAYS

A switch whose operation is activated by an electromagnet is called a "relay"

A Relay consists of two parts, the coil and the contact(s).

COMBINED AND & OR

Major areas of application are as follows

1. Building Management Systems

a) Lighting control systems

b) Air conditioning and heating systems

c) Lifts, hoists and escalators

d) Automatic doors and barriers

2. Industry

a) Packing machines

b) Woodworking machines

c) Conveyors

d) Moulding machines

3. Commercial equipment

a) Automatic washing equipment

b) Vending machines

c) Advertising hoardings

d) Toll barriers

4. Water treatment/Agriculture

a) Farm machinery

b) Irrigation/sprinkler systems

c) Pump management

5. Renewable energies

a) Solar panels b) Wind turbines c) Heat pumps