Solid-State Devices Used in Industrial Logic Circuits: SOLID-STATE DEVICES USED FOR LOGIC: Programmable Array Logic (PAL)

The programmable array logic (PAL) is very similar to the programmable logic array (PLA) in that it has a programmable AND plane, but it's different because it has a fixed OR plane. This arrangement limits the PAL to a specific number of logic functions. For instance, a three-input PAL is limited to four output product terms from the AND logic plane, and other PALs will be needed to provide additional logic functions. This may be a limiting factor, but it provides a specific function that's reliable and low cost.

A similar device called a programmable logic sequencer (PLS) is available to provide sequencer functions. The sequencer is explained in detail in the next section. Its function provides excellent logic control for events that have a definite sequence of steps. The numbers of output that are energized can be controlled in a simplified manner as the sequencer turns each output on or off for a given step.