The concept of serial communications is discussed in this chapter, which is most commonly used in the industrial automation. The serial communications concepts for the physical standards and as well as the application-level protocols such as modbus are discussed. The physical signal information for the RS232 and the functional characteristics and an overview of EIA-485 are provided in simplex and duplex modes. The modbus serial communication is dealt in terms of the fundamental, OSI models, master and slave modes, and cyclic redundancy check (CRC) codes. The error checking methods were provided for the user to diagnose the problems in the modbus in existing implementations.