Basic Operating Principles of PLCs
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229
instructions in assembly. The software often contains instructions that require data from
other parts of the memory. In this case, the operating system is driving the microprocessor
to collect these data for further processing. For example, if one instruction refers to a digital
input state, the microprocessor is responsible for tuning the operation to get this value from
the memory, specifically from the “input image table”.
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Every time that the operating system requests the microprocessor to scan the current states
of the inputs, it stores this information in the input mapping memory, also called the input
image table. Subsequently, as the microprocessor executes the automation program, various
state updates for the output devices are produced. These states are stored from the microproces-
sor in the output mapping memory, also called the output image table. When the execution
of the automation program’s instructions is finished, the output modules are updated, which
means that the stored states in the output mapping memory are transferred to the outputs.
◾
As the operating system executes the various operations through the microprocessor, it is
normally required to temporarily store some information or current results. For this pur-
pose, a specific memory sector from the RAM is dedicated, where the user has no access.
◾
When the automation program contains instructions that are related with counters, timers,
auxiliary bits, and data functions, then another specific sector of the memory is needed for
storing the corresponding parameters. For example, the CPU has to store the number and
type of time units (e.g., ms, sec, min, etc.) in the case of timers, or the limits and the count-
ing step for the case of counters. When an automation program’s instruction is executed that
concerns a counter or a timer, the operating system drives the microprocessor to seek the
corresponding data in this memory sector.
At this point, the role of the auxiliary memory bits should also be explained. From the design of
classical automation circuits, it has been indicated that the implementation of the logic according
to which the controlled system operates, requires the utilization of auxiliary relays multiple times.
The main role of these auxiliary relays is to represent an operational state of the automated system,
and to provide necessary NC and NO contacts, which are inserted properly in the automation
circuit. In programmable automation, the role of an auxiliary relay takes a simple memory location
of a single bit. The activation and deactivation of an auxiliary relay is equivalent to the storage of a
logical “1” or “0” respectively, in the memory sector dedicated for this purpose. The utilization of
the relay’s auxiliary contacts is equivalent to calling functions of an auxiliary memory bit through
the automation program instructions. Since this call can take place as many times as necessary in
an automation program, this is referred to in programmable automation as an “infinite number of
contacts” situation. The memory locations with the auxiliary bits are sometimes mentioned also
as “internal coils” or “logical coils” in correspondence to the coils of the auxiliary relays. Every
auxiliary memory bit in the corresponding memory location has its own address, so that it can be
defined uniquely through the related instructions.
After the description of the I/O modules, CPU structure, memory, and communication and
data exchange buses, the overall functionality and construction of one PLC can be addressed. This
component-based approach is indicated in the block diagram of Figure 6.32. In short, the overall
operation of a PLC involves the following operations:
1. The CPU sequentially defines the addresses of the inputs through the address bus, transfers
the states of the inputs through the data bus, and stores them in the input mapping memory.
2. The CPU sequentially executes the automation program instructions, stored in the corre-
sponding memory.
230
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Introduction to Industrial Automation
3. The results from the execution of the output activation or deactivation instructions are
stored in the output mapping memory.
4. When the automation program instructions refer to auxiliary bits, timers, counters, and
other internal units, then the microprocessor is referring to corresponding memory locations
through the data and address buses.
5. The CPU, by sequentially defining the addresses of the outputs through the address bus,
transfers the output states from the output mapping memory to the corresponding outputs
through the data bus. This output update creates a corresponding change in the operational
states of the output devices.
6. All the previous actions are driven from the operating system stored in the ROM.
I1.0
I3.1
RAM
...
Timers
Instruction
scanner
Memory
ROM
Operating
system
Counters
Data
memory
Battery
Auxiliary bits
(logic coils)
Input
mapping
memory
Output
mapping
memory
Addres
s
decoder
Addres
s
decoder
Input module
Output module
I1.0
I1.1
I1.2
I1.3
Optional
EEPROM
Automation
program
Clock
Microprocessor
Program
counter
Decoder
Instruction
register
Address
bus
Data
bus
0002
0000
0001
0003
Automation
program
PLC memory
I1.3
I2.1
Q1.5
...
Register
Address
Q0.0
Q0.2
Q0.3
Q0.1
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