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Chapter 16 PLC Function Applications
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CFP2000
16-16
Introduction to the basic internal devices in a PLC
Device type Description of Function
Input Relay
An input relay constitutes the basic unit of storage in a PLC's internal memory
corresponding to an external input point (which serves as a terminal connecting
with an external input switch and receiving external input signals). It is driven by
external input signals, to which it assigns values of 0 or 1. A program design
method cannot change the input relay status, and therefore cannot rewrite the
corresponding basic units of an input relay, and WPLSoft cannot be used to
perform compulsory On/Off actions. A relay's contacts (contacts a and b) can be
used an unlimited number of times. An input relay with no input signal must be left
idle and cannot be used for some other purpose.

Device indicated as: X0, X1, X7, X10, X11, etc. This device is expressed
with the symbol "X," and a device's order is indicated with an octal number.
Input point numbers are indicated in Section 16-8 I/O devices explanation.
Output Relay
An output relay constitutes the basic unit of storage in a PLC's internal memory
corresponding to an external output point (which connects with an external load).
It may be driven by an input relay contact, a contact on another internal device, or
its own contacts. It uses one N.O. contact to connect with external loads or other
contacts, and, like input contacts, can use the contact an unlimited number of
times. An output relay with no input signal will be idle, but may be used an internal
relay if needed.

Device indicated as: Y0, Y1, Y7, Y10, Y11, etc. This device is expressed
with the symbol "Y," and a device's order is indicated with an octal number.
Output point numbers are indicated in Section 16-8 I/O devices explanation.
Internal Relay
Internal relays have no direct connection with the outside. These relays are
auxiliary relays inside a PLC. Their function is the same as that of an auxiliary
(central) relay in an electrical control circuit: Each auxiliary relay corresponding to
a basic unit of internal storage; they can be driven by input relay contacts, output
relay contacts, and the contacts of other internal devices. An internal auxiliary
relay's contact can also be used an unlimited number of times. Internal relays
have no outputs to outside, and must output via an output point.

Device indicated as: M0, M1 to M799, etc. This device is expressed as the
symbol "M," expressed, and its order is expressed as a decimal number.
Counter
A counter is used to perform counting operations. A count setting value (such as
the number of pulses to be counted) must be assigned when a counter is used. A
counter contains a coil, contact, and a counting storage device. When the coil
goes from Off

to On, this indicates that the counter has an input pulse, and one
is added to its count. There are 16 bits that can be employed by the user.

Device indicated as: C0, C1 to C79, etc. This device is expressed as the
symbol "C," expressed, and its order is expressed as a decimal number.
Timer
A timer is used to complete control of timing. The timer contains a coil, contact,
and a time value register. When the coil is electrified, if the preset time is reached,
the contact will be actuated (contact a will close, contact b will open), and the
timer's fixed value be given by the set value. Timer has a regulated clock cycle
(timing units: 100 ms). As soon as power to the coil is cut off, the contact will no
longer be actuated (contact a will open, contact b will close), and the original
timing value will return to zero.