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16-4 Basic principles of PLC ladder diagrams
16-4-1 Schematic diagram of PLC ladder diagram program scanning
Output results are
calculated on the basis of
the ladder diagram
configuration
(internal devices will have
real-time output before
results are sent to an
external output point)
16-4-2 Introduction to ladder diagrams
Ladder diagrams comprise a graphic language widely applied in automatic control, and
employs common electrical control circuit symbols. After a ladder diagram editor has been used to
create a ladder pattern, PLC program designed is completed. The use of a graphic format to control
processes is very intuitive, and is readily accepted by personnel who are familiar with electrical
control circuit technology. Many of the basic symbols and actions in a ladder diagram comprise
commonly seen electrical devices in conventional automatic control power distribution panels, such
as buttons, switches, relays, timers, and counters.
Internal PLC devices: The types and quantities of internal PLC devices vary in different
brands of products. Although these internal devices use the same names as conventional electrical
control circuit elements such as relays, coils, and contacts, a PLC does not actually contain these
physical devices, and they instead correspond to basic elements in the PLC's internal memory
(bits). For instance, if a bit is 1, this may indicate that a coil is electrified, and if that bit is 0, it will
indicate that the coil is not electrified. An N.O. contact (Normal Open, or contact a) can be used to
directly read the value of the corresponding bit, and an N.C. contact (Normal Close, or contact b) can
be used to obtain the inverse of the bit's value. Multiple relays occupy multiple bits, and 8 bits
comprise one byte; two bytes comprise one word, and two words comprise a double word. When
multiple relays are processing at the same time (such as addition/ subtraction or displacement, etc.),
a byte, word, or double word can be used. Furthermore, a PLC contains two types of internal devices:
a timer and a counter. It not only has a coil, but can count time and numerical values. Because of this,
when it is necessary to process some numerical values, these values are usually in the form of bytes,
words, or double words.
The various internal devices in a PLC all account for a certain quantity of storage units in the
PLC's storage area. When these devices are used, the content of the corresponding storage area is
red in the form of bits, bytes, or words.
Chapter 16 PLC Function Applications│CFP2000
16-15
Repeated
implementation
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