Monday, August 8, 2011
Wednesday, August 3, 2011
What is PLC scan time?
PLC Scan Time:
The PLC program is executed as part of a repetitive process
referred to as a scan. A PLC scan starts with the CPU
reading the status of inputs. Next, the application program is
executed. Then, the CPU performs internal diagnostics and
communication tasks. Finally, the CPU updates the status of
outputs. This process repeats as long as the CPU in the run
mode. The time required to complete a scan depends on the
size of the program, the number of I/Os, and the amount of
communication required.
Programming:
Programming Language:
Ladder logic (LAD) is one programming language used with
PLCs. Ladder logic incorporates programming functions that are
graphically displayed to resemble symbols used in hard-wired
control diagrams.
While ladder logic programs are still the most common, there are many
other ways to program PLCs. Two other common examples are
statement list and function block diagrams.
Statement list (STL) instructions include an operation and an
operand. The operation to be performed is shown on the left.
The operand, the item to be operated on, is shown on the right.
Function block diagrams (FBD) include rectangular functions
with inputs shown on the left side of the rectangle and outputs
shown on the right side.
Ladder logic (LAD) is one programming language used with
PLCs. Ladder logic incorporates programming functions that are
graphically displayed to resemble symbols used in hard-wired
control diagrams.
While ladder logic programs are still the most common, there are many
other ways to program PLCs. Two other common examples are
statement list and function block diagrams.
Statement list (STL) instructions include an operation and an
operand. The operation to be performed is shown on the left.
The operand, the item to be operated on, is shown on the right.
Function block diagrams (FBD) include rectangular functions
with inputs shown on the left side of the rectangle and outputs
shown on the right side.
I/O Types
Types of I/O:
Discrete inputs and outputs, also referred to as digital
inputs and outputs, are either on or off. Pushbuttons, toggle
switches, limit switches, proximity switches, and relay contacts
are examples of devices often connected to PLC discrete
inputs. Solenoids, relay and contactor coils, and indicator lamps
are examples of devices often connected to PLC discrete
outputs.
In the on condition, a discrete input or output is represented
internal to the PLC as a logic 1. In the off condition, a discrete
input or output is represented as a logic 0.
Analog inputs and outputs are continuous, variable signals.
Typical analog signals vary from 0 to 20 milliamps, 4 to
20 milliamps, or 0 to 10 volts.
Discrete inputs and outputs, also referred to as digital
inputs and outputs, are either on or off. Pushbuttons, toggle
switches, limit switches, proximity switches, and relay contacts
are examples of devices often connected to PLC discrete
inputs. Solenoids, relay and contactor coils, and indicator lamps
are examples of devices often connected to PLC discrete
outputs.
In the on condition, a discrete input or output is represented
internal to the PLC as a logic 1. In the off condition, a discrete
input or output is represented as a logic 0.
Analog inputs and outputs are continuous, variable signals.
Typical analog signals vary from 0 to 20 milliamps, 4 to
20 milliamps, or 0 to 10 volts.
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