Page 310 - CITS - Electronic Mechanic - TT - 2024

ELECTRONICS MECHANIC - CITS

support ranging from the creation of project structure to the creation of user programs. The OS system is ac-
cessed through a graphical user interface window ( also known as main window). The main window contains all
the functions needed to set up a project , configure the hardware , write and test programs. User program can be
written in any standard PLC programming language like ladder diagram or statement list While processing a PLC
program, the CPU scans and executes the main program cyclically; A program scan cycle consists of sequential
operations that include input scan, program scan, and output scan. In the input scan, the CPU updates the process
image input table, in the output scan; the CPU updates the process image output table. After the completion of
each scan cycle, the CPU returns to the beginning of the next cycle and again repeats the cycle. The time taken
to scan one program is called scan-Cycle time.
Addressing /programming of plc
Addressing:-
Addressing is the most important concept to understand when learning PLCs. Addressing is how we make a cor-
relation between the field devices controlled by the PLC, and the data that is stored in the PLCs memory. If you
cannot address a point or group of points in the PLCs memory, then you cannot control the field device associated
with that point. The easiest way to approach understanding the addressing used in the Allen Bradley PLC-5 is to
view all addresses from the processors point of view. By this, we mean that in order for a real device in the field,
such as a relay, to be operated by the PLC, the PLC must first have a record of that point in its memory. That point
can then be accessed by the PLC.

Chassis
Hardware assembly (physical rack) that houses devices such as I/O modules, adapter modules, processor mod-
ules, and power supplies. Chassis are available in six sizes: 4-, 8-, 12-, and 16-slot.Talk Page

Group
An I/O addressing unit consisting of one input and one output word (16 bits each) of the data table. Depending
on the density of the I/O module and the addressing mode used, some of the bits in a group maybe unused. The
group number is included in I/O addresses in the position represented with agin the format:I:rrg/xx.Talk Page
Rack
An I/O addressing unit that corresponds to eight input image table words and eight output image table words(8
groups). A rack is a logical entity not to be confused with the physical chassis. 8 groups = 1 full rack, 6groups =
rack, 4 groups = rack, 2 groups = rack. The rack number is included in I/O addresses in the position represented
with an “rr” in the format: I:rrg/xx.Talk Page
Slot
A location in a chassis for installing a module. The number of physical slots per group determines the addressing
mode.
Elements
Data files are made up of individual data blocks called “Elements” or “Structures”. Each element is composed of
“words”, and each word is composed of “bits”. Elements are comprised of from 1 to 56 words, depending on the
instruction. Most instructions you will encounter will have from one to three words to an element. An example of an
element level address is T4:0T4:0 indicates the first timer (0) in the default timer file (T4). Each timer is comprised
of three (3) 16 bit words. When you say T4:0, you are referring to all three of these words as a group.
Rack Addressing Rules
Addressing to a rack is just assigning a physical slot in the chassis to a logical word in the processor. Rack ad-
dressing involves only data tables 0 and 1, the output and input data tables. The type of addressing shown is
called Single slot addressing and is the most common type of addressing used in Allen Bradley PLCs. To review
the rules for rack addressing, use the following while referring to the previous figure on PLC 5 Racks.: A rack is a
logical term that defines 8 words in the processor input data table and 8 words in the processor output data table.
These words are numbered 0-7.Each logical rack can store 128 bits of information in its input rack and 128 bits
in its output rack. (8words per rack x 16 bits per word=128 bits of data storage)In single slot addressing, each
physical slot is addressed to one input word and also to one output word. This makes it possible to put either an
Input module or an output module in any slot. The number of logical racks determines how many points a particu-
lar model of PLC can address. This number is fixed for any particular processor and cannot be changed. Rack
addressing is always in the format shown in Figure

295

CITS : E & H - Electronics Mechanic - Lesson 180 - 187