PLC Sequencers Overview 335

actions

A task the PLC performs or tells a connected machine to perform while a step is active. Actions include energizing an output, sending a signal, or setting a flag, each step should have one associated action.

active

Describing a step or instruction that is currently executing. When a step is active, its associated action is being performed and the sequencer is monitoring the transition condition.

address

To give an identifier for a specific memory location in a PLC program. File-based systems address programs in hexadecimal notation, while tag-based systems use tags.

architectures

The underlying structure and organization of a PLC's hardware and software systems. PLC architectures are usually file-based or tag-based.

array elements

An individual entry in an array. In tag-based sequencer programming, array elements are distinguished by their index number and each store a fixed block of data for one output step.

arrays

A collection of data elements that share the same data type. In tag-based PLC systems, arrays are used to store sequencer step data, with each element corresponding to one step.

base-16

Another term for the hexadecimal numbering system, in which each digit represents a value from 0 to 15. Base-16 is used in file-based PLC programming because four hex digits can represent a full 16-bit word.

binary

A base-2 numbering system using only 0 and 1. A single binary value is called a bit.

bits

A single binary value of either 0 or 1. In sequencer programming, individual bits of a word or array element represent the state of individual outputs or status conditions.

Boolean

A data type with only two possible values: true (1) or false (0). Control bits are Boolean because each one is either set or cleared based on the sequencer's current state.

branching

Allowing a logic program to follow different paths based on varying conditions. Branching processes are not well-suited for sequencers, which are best for linear, step-by-step processes that do not change from one cycle to the next.

cam switches

An electromechanical switch actuated by a rotating cam and used to control repetitive industrial processes. Like drum sequencers, cam switches have largely been replaced by PLC sequencers, which eliminate mechanical wear and maintenance requirements.

conditions

A logical state or set of logical states that determines what a PLC program does next. In sequencer programming, conditions define transition requirements and link steps to other rungs in the program.

control bits

A Boolean value stored in Word 0 of the control structure that reports the sequencer's current status or directly affects its behavior. Control bits include Enable, Done, Error, Inhibit, Unload, and Empty.

control instruction

A sequencer command that causes something to happen, such as energizing an output, writing data, or advancing a sequence. Sequencer output and sequencer load are control instructions.

control structure

A dedicated block of memory assigned to each sequencer instruction that stores the sequencer's operational state at runtime. It consists of three registers: status, length, and position.

conveyors

A mechanical system that moves materials or products from one location to another. Conveyors are commonly controlled by sequencers, which can advance and stop them at specific stations in a defined cycle.

counters

A PLC function block that tracks how many times an event has occurred. Counters track a value that increments over time but, unlike sequencers, do not track position within a defined process.

cycles

A repeatable process with defined stages. Sequencers are most effective when they control cycles with steps that occur one at a time and in a specific order, such as pick-and-place or mixing operations.

destination

The output module address where the sequencer writes its data at each step. The same destination must contain all outputs controlled by a single sequencer output instruction.

done

A control bit set after the last step in the sequence is transferred, indicating a completed cycle. Done triggers the position to reset to 1 on the next transition, so the cycle restarts.

drum sequencers

An electromechanical device that uses rotating drums with physical pegs or cams to control repetitive industrial processes. Drum sequencers have largely been replaced by PLC sequencers, which eliminate mechanical wear and maintenance requirements.

electromechanical

Describing devices that use both electrical and mechanical components to perform a function. Drum sequencers and cam switches are electromechanical, while PLC sequencers are purely electronic.

emergency stops

E-stops. A hardware safety device that immediately cuts power or control signals to a machine when activated. Emergency stops are hardwired safety devices that must operate independently of sequencer logic.

empty

A control bit that indicates that a step in the sequencer file is a placeholder and has no defined output data. The Empty bit allows the sequencer to advance through empty steps without writing to the destination, which is useful for future expansion, testing, or intentionally leaving steps inactive.

enable

A control bit that is set when the sequencer's rung is true and the instruction is actively executing. Enable triggers logic that should only run while the sequencer is active.

end-of-cycle events

An action triggered when a sequencer completes its full cycle, indicated by the Done bit being set. End-of-cycle events include resetting a counter, logging a completed cycle, or starting a downstream process.

error

A control bit set when the processor detects an invalid position or length value in the control structure. Error bits are key for troubleshooting and prevent the sequencer from advancing to the next step.

event

A specific occurrence in a process that triggers a change in program state. An event, such as a sensor activating or a timer completing, is used as a transition condition for sequencer programming.

fail-safes

A design feature that causes a machine to default to a safe state when a fault or power loss occurs. Sequencer programs should be designed with fail-safes in mind to help control hazardous energy during power outages, errors, and other sudden stops.

false

A binary state of 0, indicating that a condition is not met or an output is de-energized. A sequencer will not advance if the transition condition remains false.

false-to-true

A rung transition from off (0) to on (1). Sequencer output instructions advance to the next step only on a false-to-true transition, and rungs that are continuously true or continuously false will not advance the sequencer.

file number

A numeric identifier in file-based PLC systems that distinguishes one file from another of the same type. Together with the file type, the file number forms the complete address of a data location.

file type

A designation in file-based PLC systems that indicates whether a file is binary, integer, timer, counter, or another type of data. The file type is defined in the file address by the letters A through F.

file-based

A PLC memory architecture in which data is stored in fixed, numbered files that are each designated for a specific data type. File-based systems are found on older platforms, which use hexadecimal notation and store sequencer data as words.

filling machine

A piece of automated or semi-automated equipment that dispenses a precise volume of fluid into a container. A filling machine uses valves to control the flow of fluid through nozzles, which are lowered into empty containers.

fixed loops

A process in which the order or number of repetitions does not vary. Fixed loops are well-suited for sequencers, which are best for linear, step-by-step processes that do not change from one cycle to the next.

function blocks

A pre-built instruction set within a PLC software platform that performs a specific control task. Function blocks include sequencers, timers, and counters.

hazardous energy

Any potential or active energy in a machine that could cause injury if released unexpectedly. Hazardous energy includes electrical, mechanical, pneumatic, or hydraulic energy and can be present when sequencers stop unexpectedly.

hex

Hexadecimal. A base-16 numbering system that uses the digits 0-9 and the letters A-F to represent values. In file-based PLC systems, file names use hex notation.

hexadecimal

Hex. A base-16 numbering system that uses the digits 0-9 and the letters A-F to represent values. In file-based PLC systems, file names use hexadecimal notation.

IEC 61131-3

An international electronics standard that defines programming languages and data types for PLCs. IEC 61131-3 defines the standard data types used in tag-based sequencer programming, including Boolean (BOOL), single integer (SINT), integer (INT), double integer (DINT), and long integer (LINT).

index number

A numerical value used to identify, access, or manipulate a specific element within an array. An index number is indicated in a tag by brackets ([#]).

inhibit

A control bit that disables the sequencer regardless of rung conditions when set. Inhibit is useful for maintenance, testing, or temporarily holding a sequence without modifying program logic.

interlocking

A programming technique that prevents one action from occurring unless specific conditions are met. Interlocking multiple conditions across many rungs can be complex and error-prone, while sequencers consolidate transition conditions into a single, verifiable location.

International Electrical Contractors

IEC. A national non-profit association that represents the non-union electrical and systems contracting industry. The Independent Electrical Contractors provide training, advocacy, and safety resources and develop electrical and electronics standards.

ladder logic

A graphical programming language used to write PLC programs, modeled after relay logic diagrams. Sequencer instructions are function blocks that are placed on rungs within a ladder logic program.

length

The control parameter stored in Word 1 of the control structure that defines how many steps are in the sequence. Setting the length too low causes early reset, while setting it too high causes the sequencer to read beyond defined data.

machine guards

A physical barrier that prevents personnel from entering hazardous areas of a machine during operation. Machine guards are hardwired safety devices that must operate independently of sequencer logic.

masks

A bit pattern that filters sequencer data before it reaches the destination output module. A 1 in the mask allows the sequencer's data to pass to that output, while a 0 blocks it.

memory

The storage space a PLC uses to hold program instructions, data, and variable values. Sequencers are more memory-efficient than equivalent ladder logic because they store output patterns in a compact table rather than as individual rung instructions.

mixing

An industrial process in which materials are combined. Mixing applications are commonly controlled by sequencers because they involve repeatable cycles like adding ingredients in a specific order and running at set speeds for defined times.

motor

An electromechanical device that converts electrical energy into rotational motion. Motor startup can be a common output controlled by a sequencer.

move

A PLC instruction that copies a value from one memory location to another. Move instructions can control complex output patterns over several rungs of logic, while sequencers consolidate that logic into a single structured block.

multi-axis machine

A computer-numerical controlled (CNC) machining center that has four or more linear and rotational axes. Multi-axis machines, or multitasking machines, combine the functions of a mill and a lathe.

networking

Connecting PLCs and other devices to share data across a facility or enterprise. Networking is more effective with tag-based PLC systems, which are easier to map to HMI screens, SCADA systems, and networked devices.

non-fixed loops

A process in which the order or number of repetitions varies based on conditions. Non-fixed loops are not well-suited for sequencers, which are best for linear, step-by-step processes that do not change from one cycle to the next.

output module

A hardware component in a PLC rack that connects the PLC to physical devices like motor starters, solenoids, and lights. An output module is the destination to which a sequencer output instruction writes its bit pattern.

parameters

An essential data point. Sequencer parameters include length and position.

pick-and-place

An automated operation in which a machine, usually a robot, picks up a part from one location and places it at another. Pick-and-place operations are one of the most common sequencer applications because they involve a repeating cycle of extend, grip, retract, and release steps.

PLC

Programmable logic controller. An electronic device used to automate industrial machinery and processes by executing programmed logic. A PLC uses ladder logic, including function blocks like sequencers.

position

The control parameter stored in Word 2 of the control structure that holds the real-time step number. Position is especially important during troubleshooting stalled sequencers because it will display the step at which the sequencer stops.

programmable logic controllers

PLCs. An electronic device used to automate industrial machinery and processes by executing programmed logic. Programmable logic controllers use ladder logic, including function blocks like sequencers.

proximity sensor

A sensor that detects the presence of an object without physical contact. A proximity sensor signal is a common trigger for transition conditions in sequencer applications.

pumps

A device that moves fluids through a system. Pumps care commonly controlled by sequencers, especially during mixing, filling, or transfer processes.

quality control

QC. The process of verifying that a product or process meets defined standards. Sequencer load data serves as a quality control reference that compares originally loaded values against real-time conditions.

reads

To retrieve data from a memory location or input address. Sequencer input and compare instructions read live conditions from a source address at each step.

real time

The actual time in which a process or event occurs. Sequencers rely on real time for data related to sequencer load, sequencer compare, position parameters, and other functions.

registers

A named memory location within a PLC that holds a specific value, such as a step position or sequence length. Registers each contain one string, or word, of binary bits.

robot assembly

The use of robotic systems to join parts or products automatically. Robot assembly cycles are commonly controlled by sequencers.

rung

A single horizontal line of logic in a ladder logic program, containing input conditions on the left and output instructions on the right. Sequencer output instructions are placed on the right side of rungs, while load instructions are placed on the left.

scalability

The ability of a system to grow or adapt to increasing complexity without requiring a complete redesign. Scalability improves with tag-based PLC systems because programmers can define data structures to match the application.

sensor

A device that detects a physical condition like position, temperature, pressure, proximity and sends a signal to connected equipment or PLCs. Sensor signals are among the most common transition conditions in sequencer programming.

sequencer compare

A verification instruction that compares real-time input conditions against the expected values stored in the sequencer file at each step. Sequencer compare sits on the right side of a rung and is used as a diagnostic tool to advance the sequencer if equipment is operating as expected or to trigger alarms if it is not.

sequencer input

A verification instruction that compares live equipment conditions against stored reference data. Sequencer input is located on the right side of the rung and is often paired with sequencer output to record machine states in real time.

sequencer instructions

The editable parameters, bits, and other code within a PLC sequencer function block, including verification and control commands like sequencer compare and sequencer output. Sequencer instructions tell the sequencer how to step through a process, and PLC programmers may use "sequencer instructions" and "sequencers" interchangeably when referring to code.

sequencer load

A control instruction that captures the real-time status of equipment and stores them as reference data in the sequencer file. Sequencer load is often used during setup tasks like step-through-programming to create reference data that sequencer output and sequencer input will follow or use for quality control later.

sequencer output

A sequencer control instruction that writes a pre-defined bit pattern to a destination output module at each step in the sequence. Sequencer output is the most common sequencer instruction and sits on the righthand side of ladder logic rungs.

sequencers

A function block that controls a process through a defined series of steps, one at a time, in a specific order by following programmed sequencer instructions. Sequencers can be used to streamline ladder logic and save memory to reliably repeat cycles of actions.

shifts

To move from one position to another through a register. Shifts in data can be preserved by the Unload bit.

source address

A memory location from which the PLC reads data. Sequencer input and compare instructions read data from a source address and compare it against stored reference data.

status

The operational state or progress of a PLC program. The status of a sequencer is stored in Word 0 and updates automatically as the program executes or can be changed manually by setting control bits like empty, inhibit, or unload.

steps

A discrete stage that makes up a sequence, representing a single point in the machine's cycle. Steps occur one at a time in most sequencer processes.

step-through programming

A method of programming a sequencer by physically moving a connected machine to each position and capturing the input state at that position using the sequencer load instruction. Step-through programming is particularly useful for robot teaching.

tags

A user-defined name assigned to memory locations in tag-based PLC systems. Tags replace the numeric file addresses of older systems, making programs easier to read and maintain.

teaching

The process of programming a robot or machine by manually guiding it through its intended motions and recording those positions. Sequencer load supports teaching by capturing input states at each step rather than requiring manual data entry.

timer

A PLC function block that measures elapsed time and sets a bit when a preset value is reached. A timer is a common trigger for sequencer transition conditions, especially when a step must last for a defined period of time.

transitions

A condition or set of conditions that must be met for the sequencer to move from one step to the next. Transitions can be based on sensor inputs, timers, operator actions, or any other PLC condition.

troubleshooting

The process of diagnosing and correcting faults in a machine or program. Troubleshooting sequencers can be more difficult than troubleshooting standard ladder logic because the function block is highlighted as a whole rather than showing individual input and output states.

true

A binary state of 1, indicating that a condition is met or an output is energized. In ladder logic, a rung is true when all its input conditions are satisfied.

unload

A control bit associated with shift-based sequencer operations. Unload captures and stores data that shifts out of a register position, so it is not lost.

verification instruction

A PLC sequencer command that checks conditions and reports a result rather than directly controlling equipment. Sequencer compare and sequencer input are verification instructions.

wear

The gradual degradation of mechanical components from repeated use. Eliminating wear was one of the primary advantages of replacing mechanical sequencers like cam switches with PLC function blocks.

Word 0

The first word, or register, of the sequencer's control structure. Word 0 holds the control bits that report and manage the sequencer's status and is updated automatically by the PLC as the sequencer runs.

Word 1

The second word, or register, of the sequencer's control structure. Word 1 holds the length parameter for the sequencer, which is input manually by the programmer.

Word 2

The third word, or register, of the sequencer's control structure. Word 2 holds the position parameter for the sequencer, which updates in real time as the sequencer runs.

words

A fixed pattern of 16-bit binary values stored in a single register in file-based PLC systems. Words in a sequencer's data file each represent the output pattern that controls the actions for one step.

write

To send data from the PLC to a memory location or output address. The sequencer output instruction writes a bit pattern to the destination at each step.