PLC Sequencer instructions

Programmed sequencers can perform the same specifi c on or off patterns of outputs that are continuously repeated. Sequencer instructions simplify your ladder program by allowing you to use a single instruction or pair of instructions to perform complex operations.

For the Allen- Bradley line of controllers, sequencer commands may include the following:

SQO (Sequencer Output):

Is an output instruction that uses a fi le to control various output devices.

SQI (Sequencer Input):

Is an input instruction that compares bits from an input file to corresponding bits from a source address. The instruction is true if all pairs of bits are the same.

SQC (Sequencer Compare):

Is an output instruction that compares bits from an input source file to corresponding bits from data words in a sequence fi le. If all pairs of bits are the same, then a bit in the control register is set to 1.

SQL (Sequencer Load):

Is an output instruction used to capture reference conditions by manually stepping the machine through its operating sequences. It transfers data from the input source module to the sequencer file. The instruction functions much like a file-to-word transfer instruction.

Parameters that may be required to be entered in sequencer instructions can be summarized as follows:


Is the starting address for the registers in the sequencer file and you must use the indexed fi le indicator (#) for this address. The fi le contains the data that will be transferred to the destination address when the instruction undergoes a false-to-true transition’


Is the bit pattern through which the sequencer instruction moves source data to the destination address. Recall that in the mask bit pattern, a 1 passes values while a 0 blocks the data fl ow. You use a mask register or fi le name when you want to change the mask pattern under program control. An h is placed behind the parameter to indicate that the mask is a hexadecimal number or a B to indicate binary notation. Decimal notation is entered without any indicator.


Is the address of the input word or fi le from which the SQC and SQL instruction obtains data for comparison or input to its sequencer file.


Is the address of the output word or file to which the SQO moves the data from its sequencer file.


The address that contains the parameters with control information for the instruction. The control register stores the status byte of the instruction.

To program a sequencer, binary information is fi rst entered into the sequencer fi le or register made up of a series of consecutive memory words.