Digital Logic :
You can start learning digital circuits by learning the operation of logic gates, see an interactive gadget tutor Logic Gates.
Then you can test the basics with these tutor gadgets
Timers and Counters along with Analog Process Controllers formed the front end of Industrial Automation. Even today automation systems and control panels use analog timers and electromechanical counters. These are Indispensable. Analog + Digital combination controllers came next which were little bigger and offered better stability. They had thumbwheel switches as user interface in place of potentiometers.
uP based Systems with chips like 8085 came after that, they consumed more power and were big in size. The coming of Cmos uC like 80C31 made Process Instrumentation Small and SMD made even 48 X 48 DIN sizes possible. Now Analog and uC based designs are used in different applications. Digital Hybrids and uP instruments are not used as they were Transient-Technology Methods.