PLC Program Control Instructions 310

always false

A programming instruction used to disable a rung, typically while testing a new program. Any rung in which AFI appears is always false, regardless of the input conditions.

bit

A single numerical unit in the binary number system. Bit is short for BInary digiT.

conditional return

A programming instruction in a subroutine that tells the processor to execute parts of a program only if conditions are true.

edge

The point at which change occurs in a digital signal. Edges typically range from <.5 VDC (low) to >14 VDC (high) on a 24 VDC supply.

edge trigger

A process that stores an event at the start of the event, not during the duration of time that the sensor is on.

examine off

A basic PLC logic instruction symbol that acts like a normally open contact.

examine on

A basic PLC logic instruction symbol that acts like a normally closed contact.

falling edge

The part of a digital signal in which the wave goes from a high value to a low value.

image table

An area in the processor memory where information is temporarily stored and updated until the processor finishes scanning the ladder program.

image table word

A 16-bit or 32-bit unit of data that corresponds to an I/O group and stores I/O values.

immediate input

A programming instruction that interrupts the program scan to examine the state of inputs and immediately update the image table.

immediate output

A programming instruction that interrupts the program scan to examine the state of outputs and immediately update the image table.

input

A device, usually a type of sensor, that sends information into a PLC. Inputs are connected to the PLC by input relays.

internal relay

The part of a PLC where software simulates the physical connections of relay circuits.

jump

A programming instruction that saves program scan time by jumping over portions of a ladder program. JMP is always used with the label instruction (LBL).

jump to subroutine

A programming instruction that temporarily leaves the current ladder program and runs instructions in another program when rung conditions are true. When the 2nd program is finished, the PLC returns to the original program.

label

A program control instruction that is always used in pairs with the jump instruction. LBL is the destination for the JMP instruction.

ladder program

A method for drawing electrical logic schematics. Ladder logic is now most often used to program PLCs.

latching relay

A relay that remains in the last position it was in when the voltage is removed.

master control relay

A hardwired relay that is used to provide a controlled safe stop. The master control relay de-energizes the supply power to the outputs and the machine power to the controlled devices.

master control reset

A programming instruction similar to a master control relay. MCR turns off nonretentive outputs while leaving retentive outputs in their current state.

nested subroutine

A subroutine that is contained within another subroutine.

one shot

A program control instruction that causes an event to happen for only one scan.

one shot falling

A programming instruction that causes an event to happen when the falling edge of a signal goes from a high value to a low value.

one shot rising

A programming instruction that causes an event to happen when the rising edge of a signal goes from a low value to a high value.

output

A device that performs a mechanical action after receiving the electrical signal to do so from the PLC output relays.

output energize

The most common PLC output symbol. The output energize symbol means to energize, or turn on, the output.

output latch

A programming instruction that turns on an output device and allows the device to remain on even if the rung changes to false. OTL can turn a device on, but it cannot turn the device off.

output unlatch

A programming instruction that turns off the bit that was turned on by output latch (OTL). OTU can turn a device off, but it cannot turn the device on.

parameter

A named value or range of values that define or characterize a process. A parameter provides more information than simply signaling whether a conditin is off or on, false or true.

program control instruction

An instruction in a ladder program that modifies the normal sequence of the PLC operating cycle. Program control instructions are used to interrupt processes and change the order in which a ladder program is scanned and executed.

programmable logic controller

A processor-driven device that uses logic-based software to provide electrical control to machines.

real-time value

The actual value of an input or output at the present moment, which may differ from the value stored in memory.

refresh

A programming instruction that interrupts normal ladder rung execution and immediately updates all inputs. REF also resets the watchdog timer.

register

A group of bit data used to store information in a PLC's memory.

retentive device

A device that remembers its previous state and remains in that state after the coil is no longer energized.

return

A programming instruction that marks the end of the subroutine and directs the processor back to the main ladder program.

rising edge

The part of a digital signal in which the wave goes from a low value to a high value.

sequential

Executing one and only one step at a time, followed by the next step in a logical order.

skip

A programming instruction that is similar to JMP. JMP tells the processor to jump over portions of a ladder program.

stack overflow error

An error that occurs when a program exceeds the amount of data that can be stored in temporary memory.

subroutine

A separate program file that stores a set of instructions and returns the PLC scan to an original, main program. Subroutines are used to organize complex programs or repetitive steps within a program.

temporary end

A programming instruction used for testing and debugging programs. TND causes the PLC to run onlycertain parts of a program.

TOF

Timer off delay. A delay timer that immediately closes contacts when the control coil is energized, then waits for a predetermined amount of time to open them after power is removed from the coil.

TON

Timer on delay. A timer that waits to turn on the output after receiving an on signal from the input.

watchdog timer

A timer that monitors how long it takes the CPU to complete a scan. Watchdog timers output an error message if the CPU scan takes too long.

zone

A specific area of a program with distinct boundaries.