Transcript Chapter 19 Programming, PLC Interfacing, and Troubleshooting

Chapter 10
Advanced 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
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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 falsetrue 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
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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
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There are two types of field devices that
operate in on/off modes:
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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:
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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:
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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:
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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:
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I/O field wiring
Incorrect wiring of the field devices
Input module
CPU or power supply
Programming error
Output module
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