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Sections: Parts: | Input | Processor | Output | Selection

Programmable Controllers

Definition. A complete system of assembled necessary control components and sold as one unit.

Parts.
1. Input/Output Section
2. Processor
3. Programming Device

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Sections: Parts: | Input | Processor | Output | Selection

Input/Output Sections

Definition. The section which interfaces the high-power industrial devices to the low-power electronic circuitry that stores and executes the control program.

Program. The entire sequence of coded instructions that control the system's performance.

Input Modules [IM]. The sub-part of the section which receives high-power signal and converts them into low-power digital signal compatible with the electronic circuity of the processor.

Output Modules [OM]. The sub-part of the section which receives low-power digital signal and converts them into high-power signal capable of driving an industrial load.

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Sections: Parts: | Input | Processor | Output | Selection

Processor

Definition. The main part of a PC which holds and executes the user program. This section consists of the following:

1. Input Image Table [IIT]. A portion of the processor memory [RAM] for storing input conditions.

2. Output Image Table [OIT]. A portion of the processor memory [RAM] for storing and retrieving output conditions.

3. Central Processing Unit. The subsection of the processor that actually performs the program execution, executes the user program, continuously and immediately updates the Output Image Table.

4. User Program Memory [UPM]. A portion of the processor that hold the User Program, a coded instructions to control the machine or process.

The user program execution involves three types of scan:

a. Input Scan, reading the input conditions, which could last up to a few hundred microseconds.
b. Program Scan, the execution of the first program instruction to the last, which could consume about some milliseconds, and
c. Output Scan, reading the output conditions, which could last up to a few hundred microseconds.

Capabilities of the User Program Memory.
a. introduce time delay
b. count events
c. act as a computer, i.e., perform arithmetic: add, subtract, divide, multiply
d. perform mathematical comparisons: <, >, =, ¹, º, etc.

5. Variable Data Memory [VDM]. The memory set aside for keeping track of variable numbers or data that are involved in the user program. These variable numbers and data can be the following:
a. The Preset Value of the Timer. The number of seconds that the On-Delay Timer must remain energized in order to give a "timed-out" signal.
b. The Accumulated Value of the Timer. The current number of seconds that have elapsed since the timer was energized.
c. The Preset Value of the Counter. The number of an Up-Counter must count up in order to give a "Count Complete" signal. For a Down-Counter, it is the starting number that the counter must count down from.
d. The Accumulated Value of the Counter. The current number of counts recorded by the Up-Counter, or the current number of counts remaining before zero for the Down-Counter.
e. The Value of a Physical Variable. The measured physical variable of a transducer, e.g., temperature, pressure, through a analog-digital converter.

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Sections: Parts: | Input | Processor | Output | Selection

Output Devices

Definition. Generally, called the Programming Devices, usually consisting of a keyboard, a monitor and others such as a pointing device like a mouse or an electronic pen, where the user can do:
1. Program Entry / Encoding
2. Editing / Revision
3. Testing
4. Monitoring

Considerations

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