Page 43 - Programmable Logic Controllers, Fifth Edition

Input module
terminal strip
PB Bridge LED input
Fuse R1 Filter R2 rectifier status indicator
L1
Threshold Optical
detector isolator

Digital
Z D
Field wiring R3 logic
C circuit

Common
L2
Internal module circuit
Figure 2-14 Simplified diagram for a single input of a discrete AC input module.

• The zener diode (Z ) voltage rating sets the mini- • Sense when a signal is received from a field device.
D
mum threshold level of voltage that can be detected. • Convert the input signal to the correct voltage level
• When light from the LED strikes the phototransistor, for the particular PLC.
it switches into conduction and the status of the push- • Isolate the PLC from fluctuations in the input
button is communicated in logic to the processor. signal’s voltage or current.
• The optical isolator not only separates the higher • Send a signal to the processor indicating which sen-
AC input voltage from the logic circuits but also sor originated the signal.
prevents damage to the processor due to line volt-
age transients. In addition, this isolation also helps Figure 2-15 shows the block diagram for one output of
reduce the effects of electrical noise, common in a typical discrete output module. Like the input module,
the industrial environment, which can cause erratic it is composed of two basic sections: the power section
operation of the processor. and the logic section, coupled by an isolation circuit. The
• For fault diagnosis, an input state LED indicator is output interface can be thought of as an electronic switch
on when the input pushbutton is closed. This indica- that turns the output load device on and off. Logic circuits
tor may be wired on either side of the optical isolator. determine the output status. An output LED indicates the
status of the output signal.
• An AC/DC type of input module is used for both AC A simplified diagram for a single output of a discrete
and DC inputs as the input polarity does not matter. AC output module is shown in Figure 2-16. The operation
• A PLC input module will have either all inputs isolated of the circuits . . . .set can be summarized as follows:
from each other with no common input connections or
groups of inputs that share a common connection. • As part of its normal operation, the digital logic
circuits . . . .set of the processor sets the output
Discrete input modules perform four tasks in the PLC status according to the program.
control system. They:

Logic
Output status Power
indicator
Load
L1
Digital signal Logic Opto-electrical Electronic
from processor circuits isolation switch 120 VAC
L2
Figure 2-15 Discrete AC output module block diagram.

24 Chapter 2 PLC Hardware Components

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