PLC and relay control system are complementary. Until now, the relay has never stopped further development. SIEMENS has never promised that ordinary PLC is safe, such as: equipment safety control (power failure, restart, personal protection) is specialized Safety relays are guaranteed, so so far in Europe there are many specialized manufacturers in production, research and development.
A relay control system and a PLC control system are composed of an input section, an output section, and a control section. The ladder diagram of the PLC is very similar to the relay control circuit diagram. The main reason is that the PLC ladder diagram roughly follows the symbols of the circuit components controlled by the relay.