Page 55 - Programmable Logic Controllers, Fifth Edition - Mobile version

Battery (battery provides
backup power for the
CMOS RAM)
SLC 5/05 CPU
RUN FORCE
FLT ENET
BATT RS232
RUN REM PROG
Memory
module
Keyswitch Channel 1
ethernet
(10/100Base-T)
Hardware
address
Channel 0
RS232
(DH485, DF1,
or ASCII)

Side view Front view
Figure 2-40 Typical processor module.

• Allows you to perform program entry and editing them. The following static control procedures should be
• Prevents you from using a programmer/operator in- followed when handling and working with static-sensitive
terface device to change the processor mode devices and modules:
• Ground yourself by touching a conductive surface
REM Position before handling static-sensitive components.
• Places the processor in the Remote mode: either the • Wear a wrist strap that provides a path to bleed off
REMote Run, REMote Program, or REMote Test mode any charge that may build up during work.
• Allows you to change the processor mode from a • Be careful not to touch the backplane connector or
programmer/operator interface device connector pins of the PLC system (always handle
• Allows you to perform online program editing the circuit cards by the edge if possible).

The processor module also contains circuitry to com- • Be careful not to touch other circuit components in
municate with the programming device. Somewhere on a module when you configure or replace its internal
the module you will find a connector that allows the PLC components.
to be connected to an external programming device. The • When not in use, store module in its static-shield bag.
decision-making capabilities of PLC processors go far • If available, use a static-safe work station.
beyond simple logic processing. The processor performs
other functions such as timing, counting, latching, com- 2.7 Memory Design
paring, motion control and complex math functions.
PLC processors have changed constantly due to ad- Memory is the element that stores information, pro-
vancements in computer technology and greater demand grams, and data in a PLC. The user memory of a PLC
from applications. Today, processors are faster and have includes space for the user program as well as address-
additional instructions added as new models are intro- able memory locations for storage of data. Data are
duced. Because PLCs are microprocessor based, they can stored in memory locations by a process called writing.
be made to perform tasks that a computer can do. In ad- Data are retrieved from memory by what is referred to
dition to their control functions, PLCs can be networked as reading.
to do supervisory control and data acquisition (SCADA). The complexity of the program determines the amount
Many electronic components found in processors and of memory required. Memory elements store individual
other types of PLC modules are sensitive to electrostatic pieces of information called bits (for binary digits). The
voltages that can degrade their performance or damage amount of memory capacity is specified in increments of

36 Chapter 2 PLC Hardware Components

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