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2. CPU block diagram
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3. Functional interaction of a PLC system
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4. Multiprocessor Configuration
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5. PLC Total Scan Representation
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6. Illustration of a signal that will not be
detected by a PLC during a normal scan
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7. 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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8. Typical PLC subsystem configuration
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9. 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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10. 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. 10

11. THE SYSTEM POWER SUPPLY
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12. A typical Miniprogrammer
and a Small PLC
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13. A Removable Memory Card
for a Miniprogrammer
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14. PLC Ladder Diagram Displayed on a
Personal Computer
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17. Typical discrete input/output system
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18. Internal switches used to set I/O addresses
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19. Master racks
(a) without I/O modules (b) with I/O modules
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20. Local Rack Configuration
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21. Remote Rack Configuration
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22. Specifications for the I/O Rack Enclosure
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24. Remote I/O Configurations
(a) Daisy chain (b) Star (c) Multidrop
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25. DISCRETE INPUTS
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26. Block diagram of an AC/DC input circuit
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27. Device connections
(a) an AC input module (b) a DC input module
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28. 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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29. Field device connections for a sink/source
DC input module
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30. Conversion circuit interfacing a sinking output
with a sourcing input module
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31. 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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32. 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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33. 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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34. DISCRETE OUTPUTS
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35. AC OUTPUT Circuit Block Diagram
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36. AC output module connection diagram
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37. DC OUTPUTS (SINK/SOURCE)
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38. ISOLATED AC AND DC OUTPUTS
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39. TTL OUTPUTS
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40. REGISTER/BCD OUTPUTS
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41. CONTACT OUTPUTS
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42. DISCRETE BYPASS/CONTROL STATIONS
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