IAI America SSEL - Manual (Page 295)

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Part 2 Programs
3. Difference from a Sequencer
The parallel processing method has evolved from the traditional method of using a sequence control
circuit consisting of relays to a more recent one using a sequencer equipped with a microcomputer.
Since a microcomputer basically allows one process for each clock, a sequence control circuit with a
microcomputer must scan the entire program to achieve apparent parallel processing. For this reason, a
scan time is required, which adds to overhead (dead time).
The microcomputer scans the enter program and outputs only where the condition is satisfied.
On the other hand, a system consisting of a microcomputer and a real-time operating system no longer
uses parallel processing scan (by always scanning the entire program), but adopts an event-driven
method instead (whereby the system operates only when an event occurs, such as upon receipt of an
input signal). Since no extra scan is necessary, the system can operate at high speed. In addition, each
program to be processed in parallel is programmed in steps, so the program is easy to understand and
maintain.
The programmer need not worry about running all programs in parallel, which is controlled by the real-time
operating system.
Real-time OS
Program 1 Program 2 Program n
Programmed
in steps