Page 24 - Programmable Logic Controllers, Fifth Edition Mobile 2

Input Power supply Output
module module module
Processor Module
Input Central
sensing Processing M Output
devices Unit (CPU) load
devices
Memory
Program Data
Optical Optical
isolation isolation

Programming device
(a) Modular type

Power supply

CPU
Input Output
section Memory section
Communications

(b) Fixed type

Figure 1-8 Typical parts of a programmable logic controller.
Source: (a) Courtesy Mitsubishi Automation; (b) Images Courtesy of Rockwell Automation, Inc.
Common power bus
L1 monitoring. The modules plug into a rack. When a mod-
ule is slid into the rack, it makes an electrical connection
with a series of contacts called the backplane, located at
the rear of the rack. The PLC processor is also connected
to the backplane and can communicate with all the mod-
ules in the rack.
The power supply supplies DC power to other modules
Input
connections that plug into the rack (Figure 1-11). For large PLC sys-
tems, this power supply does not normally supply power
Processor PLC
to the field devices. With larger systems, power to field
Output devices is provided by external alternating current (AC)
connections or direct current (DC) supplies. For some small micro
PLC systems, the power supply may be used to power
field devices.
The processor (CPU) is the “brain” of the PLC.
PL
A typical processor (Figure 1-12) usually consists of a mi-
croprocessor for implementing the logic and controlling
L2 the communications among the modules. The processor
Common return bus
requires memory for storing user program instructions,
Figure 1-9 Fixed I/O configuration. numerical values, and I/O devices status.

Programmable Logic Controllers (PLCs) Chapter 1 5

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