Sequencer Instructions for PLCs 330

accumulated count

The number of time units or counts that have accrued. When the accumulated value equals the preset value, the done bit is enabled.

bit

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

cam switches

Circular or cylindrical machine components that convert rotational movement into linear movement.

cam timer

A type of mechanical timer that uses cams arranged along a shaft to alternately press down and release switches that control different timing elements of a system.

cascading

The process of linking two or more sequencers together to expand their capability. Also known as chaining.

chaining

The process of linking two or more sequencers together to expand their capability. Also known as cascading.

CONTROL

A sequencer data file composed of three words that tell the processor how the sequencer is structured. CONTROL holds the status bits, the length, and the position of the sequencer.

counter

A device that counts, calculates, or keeps a record of the number of times something happens. PLC counters exist as a set of instructions inside the PLC microprocessor.

data file

A defined area in a PLC that stores bits of information. Some PLCs refer to data files as registers.

DEST

A field in a sequencer instruction that specifies the output image address at which the sequencer instructions will be enabled. DEST is used for SQO only.

DN

Done bit. A bit that activates the output and resets the sequencer program after the last sequencer file is completed. The done bit is one of the most frequently used status bits.

done bit

DN. A bit that activates the output and resets the sequencer program after the last sequencer file is completed. The done bit is one of the most frequently used status bits for PLC applications.

down count

Counting in a downward or decreasing direction. Counting 3, 2, 1, and so on is a down count.

drum cylinder

A circular or cylindrical machine component that uses pegs to convert rotational movement into intermittent electrical contact.

EN

Enable bit. When the enable bit is true (1), the sequencer executes an output or load instruction.

enable bit

EN. A bit that causes the sequencer to execute an output or load instruction when the bit is true (1). The enable bit is one of the most frequently used status bits.

enabled bit

A bit that is ON.

equality

A condition in which the status of all the input bits matches the status of all the bits in the current step of the sequencer. Equality is measured in the SQI instruction.

ER

Error bit. A bit that is set when an error has occurred in the sequence.

error bit

ER. A bit that is set when an error has occurred in the sequence.

event-driven sequencer

A sequencer that controls the order of steps of a process, but not the amount of time the steps take.

FD

Found bit. A bit that is set when all of the non-masked bits in the source word match the corresponding bits in the sequencer word.

field

A placeholder in an instruction that is used for inputting information.

FILE

A field in a sequencer instruction that contains the address of the data file containing the sequencer program. FILE tells the PLC where to find the sequencer instructions.

file identifier

A letter code given to each type of data file to make it easier for the programmer to identify the files.

file number

The specific bit number assigned to each type of data file so that it easily can be found within the ladder logic program.

found bit

FD. A bit that is set when all of the non-masked bits in the source word match the corresponding bits in the sequencer word.

home position

The default position of a robotic arm when it is at rest or ready to begin a new cycle.

LENGTH

A parameter of a sequencer that tells the PLC how many steps must be taken to complete the sequence.

mask

A feature used in sequencer instructions to enable or disable specific bits in a memory word. MASK tells the PLC which inputs or outputs are controlled by the sequencer, and which ones are not.

parameter

A named value or range of values that define or characterize a process.

pick and place

The process of picking up an object or part in one location and placing it in another location. Pick and place robots use sequencers to complete their tasks.

PLC

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

POSITION

An element of a sequencer instruction that tells the PLC what step of the sequencer is currently being run.

preset value

A value set in advance that triggers a timer or counter action. When the accumulated value equals the preset value, the done bit is enabled.

register

A data file in a PLC that stores bits of information. Some PLCs refer to registers as files.

sequence

A connected series of events that occurs in a certain order. In manufacturing, sequential control is required for many applications.

sequencer

A type of program control instruction for PLCs that determines the order in which a group of processes is performed.

sequencer compare

SQC. An instruction in a PLC ladder program in which input data is compared for equality with data that has been stored in memory. Also known as sequencer input.

sequencer input

SQI. An instruction in a PLC ladder program in which input data is compared for equality with data that has been stored in memory. SQI is typically used to monitor or troubleshoot machines.

sequencer instructions

Program instructions for a PLC ladder program that control the order in which a group of processes is performed.

sequencer load

SQL. An instruction in a PLC ladder program that loads the current data into the sequencer step by step. SQL does not use a mask.

sequencer output

SQO. An instruction in a PLC ladder program that determines the output conditions for the next step of the process. SQO cannot be enabled until certain conditions are met.

SOURCE

A field in a sequencer instruction that specifies the address of the input data. SOURCE is used for SQI only.

SQC

Sequencer compare. An instruction in a PLC ladder program in which input data is compared for equality with data that has been stored in memory. Also known as sequencer input.

SQI

Sequencer input. An instruction in a PLC ladder program in which input data is compared for equality with data that has been stored in memory.

SQL

Sequencer load. An instruction in a PLC ladder program that loads the current data into the sequencer step by step.

SQO

Sequencer output. An instruction in a PLC ladder program that determines the output conditions for the next step of the process.

status bit

A bit that controls the condition or state of the PLC sequencer. The two most frequently used status bits are enable (EN) and done (DN).

time-driven sequencer

A sequencer that controls the amount of time of a process, as well as the order of steps.

up count

Counting in an upward or increasing direction. Counting 1, 2, 3, and so on is an up count.

up/down counter

A combination of up counters and down counters that allows you to increase and decrease values by increments of 1.

word

A defined and specific number of digits grouped together. For most PLCs, a word consists of 16 digits.