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Chapter 10Advanced Programming,

PLC Interfacing, and Troubleshooting

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Jump Commands

• PLCs have instructions that allow the normal sequential execution of the program to be altered if certain conditions exist

• Output instructions that perform this function are known as override commands

• An example of an override command would be the jump instruction

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Implementing Jump Instructions• The jump instruction allows the

program to skip certain parts of a program, effectively allowing several programs

• A set of rung conditions precede the jump instruction. When rung conditions are true, the jump will take place

• The jump will be always be associated with a label, which defines the jump to location

• Several jump instructions can use the same label as the jump location

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Jump to Subroutine• Another jump operation is

the subroutine operation (JSR)

• A JSR label tells the program where to jump to

• A return (RET) instruction is used at the end of the subroutine to tell the program to continue executing where it left off

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Data Manipulation• Data manipulation instructions

allow data to be manipulated by shifting bits left or right in a register permitting more efficient programming of the PLC

• Shift registers are used for most data manipulation instructions

• An example of forward shift registers in an application is demonstrated on the right

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Programming Implementation of Shift

Register Example

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Shift Register Operations• Bit Shift Left (BSL) will

shift data left within a register for each false-true rung transition

• Bit Shift Right (BSR) will shift data right within a register for each false-true rung transition

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Sequencers• Before PLCs, the control

of an automatic machine was often performed by a drum sequencer

• As the drum rotated, pegs and cams determined what outputs and the dwell of the outputs during a cycle

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PLC Sequencers• PLCs can perform sequencer

functions on entire words within the PLC

• The words are transferred from memory to output modules in consecutive order

• Parameters necessary for sequencer operations include:

– File - designated memory location that forms the 16-bit pattern of the outputs during the sequence

– Mask - some bits in a word need not be controlled during a sequence. These bits are masked out

– Dest - the output destination to

which the data in the sequencer is to be transferred to

– Control - this parameter indicates the address of where the control bits of the instruction are located

– Length - number of steps of the sequencer file starting at position 1

– Position - this parameter shows the actual step in the sequence that’s being performed

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Architecture of Sequencer Output Instructions

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Discrete Input/Output Modules• Discreet I/O modules contain the necessary

connections and interfacing between field devices and the internal process unit

• Many types of I/O modules are available to accommodate different requirements for power, interfacing, and logic

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Types of Field Devices Connected to I/O Modules

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Discrete Input Modules• There are two types of field devices that

operate in on/off modes:– Relay contacts– Solid-state relays

• PLC produce several types of discrete input modules for connection to field devices

• Specifications for input modules should take into consideration the following requirements:

– Type of current (AC or DC)– Voltage level– Number of inputs– Active-high or Active-low inputs

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Relay Contact Input Modules

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Discrete Output Modules• Several types of discrete output modules are

available to handle a variety of applications• Requirements for output modules should take into

consideration:– AC or DC current– Voltage level– Active-high or Active-low outputs– Relay or Solid-state relay switching– Load capability

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Types of DC Output Modules

Relay output Solid-state active-high Sinking active-low

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Troubleshooting I/O Interfaces• Three steps are used to test the proper

operation of discrete input interfacing:– Input Testing

• Observe indicator light on the PLC module, it should turn off and on in response to the push button

• If the light doesn’t turn on, a voltmeter should be used to test the signal at the input terminal

• In an improper reading is measured, the next step is to disconnect from the terminal of the module and the power source, then perform a continuity test

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Troubleshooting Discrete Output Modules• The following steps

should be used to troubleshoot output modules:– Activate the output terminal

of the module– An indicator lamp

connected should energize– If the lamp fails to light and

the field device fails to energize, the problem may lie within the program

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Analog Input and Output Modules• These modules are fed analog signals and then convert

them into equivalent digital signals for the PLC to work with.

• Analog input modules are sometimes used with sensors and transducers to perform process control functions such as PID control

• The most common analog signal used is the 4-20 mA level associated with process control, however other ranges are available or selectable

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Special Purpose Modules• Several types of special purpose modules are

available for a variety of applications, including:– Bar Code modules– Radio Frequency (RF) modules– Vision System modules– PID Control modules– Fuzzy Logic modules– Stepper Motor modules– Thermocouple modules

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Troubleshooting Programmable Controllers• The most effective approach to troubleshooting PLC faults

is to use a logical procedure• Typically, six areas of a PLC are likely to be the source of

faults:– I/O field wiring– Incorrect wiring of the field devices– Input module– CPU or power supply– Programming error– Output module