Page 715 - Mechatronics with Experiments

PROGRAMMABLE LOGIC CONTROLLERS 701
needed between the I/O device and the PLC. The network communication I/O module
(i.e., DeviceNet module) handles the data read/write operation between the bus and the
memory of the PLC.
The application programming does not change either way, other than the fact that
there is a real-time network communication driver running in the background, working with
the communication module, which is transparent to the application program developer, and
handles the data between the communication bus and PLC memory. The PLC application
program accesses the memory for I/O data as if the I/O devices were directly wired to the
I/O modules.
Networked PLC control reduces the wiring costs, distributes the intelligence to local
devices, and makes the system I/O expansion easier (Figure 9.3). When a new I/O device is
added, the electrical wires of the I/O device do not need to be run all the way to the physical
location of the PLC rack, but rather, the communication wires of the I/O device simply
need to be connected to the long communication bus cable using a T-type connector.

9.2.2 Opto-isolated Discrete Input and Output Modules
The most common I/O types used in PLC applications are discrete (two state: ON/OFF)
type inputs and outputs. The discrete input can be a conducting or non-conducting state
(ON/OFF) of a DC or AC circuit component. Similarly, the output can be turning ON or
OFF of a DC or AC circuit component. The voltage levels of the input and output circuits
are in the order of 12 V to 120 V either DC or AC. In order to electrically isolate the PLC
hardware from the high voltage levels of the I/O devices, the interface between the PLC
bus and the I/O devices are provided through optically coupled switching devices, namely
LEDs, phototransistors, and phototriacs.
Figure 9.4 shows the four types of opto-isolated I/O modules that are used to interface
two-state input/output (DC or AC circuit) devices to a PLC. Notice that in all cases, the
signal coupling between the PLC side and I/O side is through the light (or optical coupling).
When the conducting/non-conducting state of an AC input circuit is interfaced, a rectifier
circuit is built into the opto I/O module to convert it to DC current and drive an LED.
The resistor in series limits the amount of current flow through the LED. The light emitted
by the LED turns ON a phototransistor on the PLC side. If the input circuit is already a
DC type, then the rectifier would not be necessary. Similarly, the DC and AC outputs are
turned ON/OFF by the PLC by driving an LED (photo-transistor) on the PLC side. For DC
outputs, the LED drives a phototransistor. For AC outputs, the LED drives a phototriac.
Although not shown in the figures, it is important to point out that the solid-state opto-I/O
interface modules also incorporate other functions useful in practical control systems, such
as a switch debouncing circuit or inductive voltage surge protections.
PLC I/O modules provide the same type of I/O interface in groups of 8, 16, or
32 points, that is 8-point DC input module, 16-point AC output module. Figure 9.5 shows
the typical field wiring of the I/O modules. The DC I/O modules can be in current sinking
or current sourcing mode. In current sinking mode, the DC I/O module “sinks” the current
received from the I/O device to the ground. The I/O device is connected between the DC(+)
and the I/O module terminal. In current sourcing mode, the DC I/O module “sources” the
current to the I/O device. The I/O device is connected between the I/O module and the
ground (or DC−) terminal.

9.2.3 Relays, Contactors, Starters
Relays, contactors, and starters are all electromechanical switches. Through an electromag-
netic actuation principle, mechanical motion is obtained. The mechanical motion is used
to open/close a switch which is used to control the continuity of an electric circuit.