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PROCESSORS, THE POWER SUPPLY,AND PROGRAMMING DEVICES

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CPU block diagram

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Functional interaction of a PLC system

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Multiprocessor Configuration

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PLC Total Scan Representation

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Illustration of a signal that will not be detected by a PLC during a normal scan

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PLC scan with immediate I/O update

These immediate instructions are very useful when the PLC must react instantaneously to a critical input or output.

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Typical PLC subsystem configuration

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Error Detection and Correction

•More sophisticated programmable controllers may have an error detection and correction scheme that provides greater reliability than conventional error detection.

•The key to this type of error correction is the multiple representation of the same value.

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CPU DIAGNOSTICS

•The processor is responsible for detecting communication failures, as well as other failures, that may occur during system operation.•It must alert the operator or system in case of a malfunction.•The processor performs diagnostics, or error checks, during its operation and sends status information to indicators that are normally located on the front of the CPU.•Typical diagnostics include memory OK, processor OK, battery OK, and power supply OK.•Some controllers possess a set of fault relay contacts that can be used in an alarm circuit to signal a failure.

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THE SYSTEM POWER SUPPLY

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A typical Miniprogrammerand a Small PLC

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A Removable Memory Card for a Miniprogrammer

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PLC Ladder Diagram Displayed on a Personal Computer

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THE DISCRETEINPUT/OUTPUT SYSTEM

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Typical discrete input/output system

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Internal switches used to set I/O addresses

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Master racks(a) without I/O modules (b) with I/O modules

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Local Rack Configuration

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Remote Rack Configuration

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Specifications for the I/O Rack Enclosure

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Remote I/O Configurations(a) Daisy chain (b) Star (c) Multidrop

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DISCRETE INPUTS

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Block diagram of an AC/DC input circuit

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Device connections

(a) an AC input module (b) a DC input module

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DC INPUTS (SINK/SOURCE)

•A DC input module can interface with field devices in both sinking and sourcing operations, a capability that AC/DC input modules do not have.

•Sinking and sourcing operations refer to the electrical configuration of the circuits in the module and field input devices.

•If a device provides current when it is ON, it is said to be sourcing current.

•If a device receives current when it is ON, it is said to be sinking current.

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Field device connections for a sink/source DC input module

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Conversion circuit interfacing a sinking output with a sourcing input module

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ISOLATED AC/DC INPUTS

Isolated input interfaces operate like standard AC/DC modules except that each input has a separate return, or common, line.

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TTL INPUTS

Transistor-transistor logic (TTL) input interfaces allow controllers to accept signals from TTL-compatible devices, such as solid-state controls and sensing instruments.

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REGISTER/BCD INPUTS

•Multibit register/BCD input modules enhance input interfacing methods with the programmable controller through the use of standard thumbwheel switches.

•This register, or BCD, configuration allows groups of bits to be input as a unit to accommodate devices requiring that bits be in parallel form.

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DISCRETE OUTPUTS

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AC OUTPUT Circuit Block Diagram

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AC output module connection diagram

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DC OUTPUTS (SINK/SOURCE)

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ISOLATED AC AND DC OUTPUTS

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TTL OUTPUTS

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REGISTER/BCD OUTPUTS

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CONTACT OUTPUTS

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DISCRETE BYPASS/CONTROL STATIONS

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