Delta MS300 Series User Manual page 682

When you use these devices, the content of the corresponding storage area is read in the form of
bits, bytes, or words.
Table 16-5 describes the internal devices in a PLC
Device
Type
An input relay constitutes the basic unit of storage in a PLC's internal memory, and
corresponds to an external input point. It 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 input relay. You cannot use WPLSoft to manually perform ON/OFF actions. You can use
Relay
a relay's contacts (contacts A and B) an unlimited number of times in a program. An
input relay with no input signal must be left idle and cannot be used for some other
purpose.
 Input devices are indicated by X0, X1, X7, X10, X11, and so on. These devices are
indicated with the symbol X, and a device's order is indicated with an octal number.
Input point numbers are indicated in the main PLC and in expansion devices.
An output relay constitutes the basic unit of storage in a PLC's internal memory, and
corresponds to an external output point. It connects with an external load. It can 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 the input contacts, you can use the output relay's contacts an unlimited number of
Output
times in a program. An output relay with no input signal is idle, but can be used by an
Relay
internal relay if needed.
 Output devices are indicated by Y0, Y1, Y7, Y10, Y11, and so on. These devices
are indicated with the symbol Y, and a device's order is indicated with an octal
number. Output point numbers are indicated in the main PLC and in expansion
devices.
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 corresponds to a basic unit of internal
storage; they can be driven by input relay contacts, output relay contacts, and the
Internal contacts of other internal devices. You can use an internal auxiliary relay's contacts an
Relay
unlimited number of times in a program. Internal relays have no outputs to the outside,
and their status must output through an output point.
 Internal relay devices are indicated by: M0, M1–M799, and so on. These devices
are indicated with the symbol M, and the device's order is indicated with a decimal
number.
Counters perform counting operations. The setting value for a counter (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 
ON, this indicates that the counter receives an input pulse, and adds one to its count.
Counter
There are 16 bits available in the counter.
 Counter device are indicated by: C0, C1–C79, and so on. These devices are
indicated by the symbol C, and the device's order is indicated with a decimal
number.
Chapter 16 PLC Function ApplicationsMS300
Description of Function
673