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مواقع
مفضله
The
PLC mainly consists of a CPU, memory areas, and appropriate circuits to receive
input and output data. We can consider the PLC to be a box full of hundreds or
thousands of separate relays, counters, timers and data storage locations. These
counters, timers, etc. don't "physically" exist but instead are
simulated and can be considered software counters, timers, etc. These internal
relays are simulated through bit locations in Registers
(more on that later). Figure
10. The Contents of the Box
The
Central Processing Unit (CPU) is the most important part of the PLC. It holds
the processor that defines what the PLC can and cannot do. The Processor's
functions are preset so that the PLC has certain fixed limits. These limits are
usually the maximum number of inputs and/or outputs (I/O)
available, but they can also include the maximum number of timers, counters, and
registers, as well as type of functions the PLC can perform. INPUT
RELAYS are connected to the outside world. They
physically exist and receive signals from switches, sensors, etc. Typically they
are not relays, but are transistors. INTERNAL
UTILITY RELAYS do not receive signals from
the outside world, nor do they physically exist. They are
simulated relays and are what enables a PLC to eliminate external relays. COUNTERS
do not physically exist. They are simulated counters and
they can be programmed to count pulses. What does the term
"pulse" mean in this context? Well, one example of a pulse would be a
bottle passing by a sensor. Typically these counters can count up, down, or both
up and down. Because they are simulated they are limited in their counting
speed. Some manufacturers also include high-speed hardware based counters. TIMERS
do not physically exist. They come in many varieties and increments. The most
common type is an On-Delay
Timer. Others include Off-Delay
Timers, Retentive
and Non-retentive.
Increments vary from 1ms (millisecond) through 1s (second). OUTPUT
RELAYS are connected to the outside world. They
physically exist and send on/off signals to Solenoids,
lights, etc. They can be Transistors,
relays, or Triacs,
depending upon the model chosen. DATA
STORAGE. They are typically
registers assigned simply store to data. They are usually used as
temporary storage for math or data manipulation. They are also often used for
retentive data storage.
Now
that we have discussed the inner workings of the PLC, let's take a look at the
outward appearance of the device. There are two basic forms that a PLC comes in:
"block" and "rack mounted" I/O.
The
block I/O is a design more common to PLCs that communicate with small amounts
I/O. ("Small amount" refers to a quantity less than 60 I/O.) The input
and output terminals are where the user would hard wire the devices to be
controlled by the PLC. Each terminal has a unique "address." (We will
discuss addressing in greater detail in the next section.) The
CPU is located inside of the block. The communication ports allow the PLC to be
connected to a computer or hand held programmer. They may also be used to
connect special modules or Expanders.
Expander blocks do not contain a CPU. They merely "expand" the number
of I/O controlled by the CPU. Based on the manufacturer, each expander could
allow a different type of input or output to be used. For example, the base unit
could control Digital
I/O and the first expander may control analog outputs only. Figure
11. Block I/O Figure
12. Block I/O with Expanders
Rack
mounted I/O is composed of several printed circuit board I/O cards that are
mounted on a "rack" or metal back plate. Generally, the
"rack" is designed to hold 4, 6, 8, or more cards. Hundreds of inputs
and output devices can be controlled with rack I/O. Like the block I/O, each
terminal on each card has a specific PLC address. Unlike the block I/O, based on
the end user's needs, each card can control different types of I/O. For example,
a digital, an analog input, and a triac output can all be mounted on the same
rack. It is also possible for many rack mounted PLC products to support
additional racks of I/O modules located as much as hundreds or thousands of feet
from the CPU. In this configuration, there is a "master" CPU attached
to "remote" I/O. (A brief discussion of this set-up is covered in the
last section of the module.)
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تصميم المهندس : محمد صبري محمود فهيم إستعداد تام لتصميم المواقع إستعداد تام للعمل داخل أو خارج مصر E-Mail: Mohamedmsm@Masrawy.com |
PLC
components 
What
Each Part Does
