Delta CT2000 Series User Manual page 456

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.
Introduction to the basic internal devices in a PLC
Device type
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
Input Relay
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.

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 NO 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
Output Relay
relay if needed.

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
Internal Relay
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.

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
Counter
is added to its count. There are 16 bits that can be employed by the user.

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 the main computer and in expansion
devices.
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 the main computer and in expansion
devices.
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.
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.
16-15
Chapter 16 PLC Function CT2000 Series
Description of Function