Control whereby the reference variable is provided by a program memory (programmer). Its task is to pick out a point in the respective program and issue corresponding function commands.
In order for a work process to be able to run automatically, the control problem must be converted into some form of description which reflects the kind of signal processing involved. It can operate on a time-dependent basis (time-based control) or a process-dependent basis (process-dependent control).
Time-based controls include those (still used) programmed controls involving mechanical sequencers, as well as controls based on punched cards. Until now, process-dependent controls have been referred to as "sequence controls".
Another distinguishing characteristic is the way that programs are implemented, with distinctions being made between hard-wire programmed controls with fixed program or programmable varieties and memory programmable controls systems with replaceable ROMor programmable types (Figure P 18).
Figure P 18: Types of program implementation