Many existing CTC systems are relay based. Modern installations are microprocessor
based with solid-state support circuits and advanced communication links. For this
discussion, we will consider a relay-based system. A later section of this chapter will
introduce solid-state systems.
Control and indication codes rely on step-by-step operation of relays and mechanisms
at the field location, working in synchronism with step-by-step operation of relays at
the control office.