The sequencer instructions are the most powerful function block instructions available in programmable controller systems. They normally operate on up to 4 words (64 bits) at a time. This instruction has divided into sub-instructions and they are sequencer output, input, and load. The sequencer instructions are used to transfer data table to discrete output modules for the control of sequential process operations or sequential batch operations (sequencer output). In addition, they are used to compare I/O word data with data stored in tables so that process operating conditions can be examined for control and diagnostic purposes (sequencer input). The instruction also performing I/O word data transferring into the data table (sequencer load).
When the rung containing the sequencer output instruction goes from false to true, the counter increments to the next step in the sequencer table. The data found in that step is output to the output word address specified in the sequencer instruction. The sequencer output instruction normally operates whenever the rung is true. Once the rung is true, the counter is incremented to the next step and the data in that step will be output every processor scan that the rung remains ON. But, the instruction only go to next step when there is a false-to-true transition of the rung.
The sequencer output instruction has mask capability. A mask is a means of selectively screening out data. It allows unused bits of output words specified in the instruction to be used for other purposes.
The sequencer input instruction compares process or machine input data with information stored in memory for equality. It is used alone or in a series and/or parallel combination with other rung condition instructions to determine the status of an output. This instruction normally compares the status of up to 64 input conditions with the data stored in a sequencer table, bit by bit and step by step. The instruction becomes logically true when the state of all input bits are equal to the state of all bits in the current step of the sequencer table.
The sequencer load instruction is used to load information into memory locations such as files or sequencer tables. The instruction receives information from up to 4 independent memory locations specified in the instruction. The input words can represent input, output, or storage words, and the load word addresses do not have to be consecutive. The instruction loads words into a sequencer table or other parts of memory, one step at a time, at the location determined by the step counter. The instruction is started by a false-to-true transition of the rung input condition. Information from the load words will be stored in memory during the processor scan that the instruction has initiated. If the rung stays true for the subsequent scans, the instruction will not repeat. The instruction will step to the next position only on the next false-to-true transition of the rung input bit.