Digital Electronics: Principles, Devices and Applications

638 Digital ElectronicsAB7AB6AB5AB4AB3AB2AB1AB0Address BusA 0 A 1 A 2 A 3 A 0 A 1 A 2 A 3CS16 × 4RAM-1R/WA 0I/OCS16 × 4RAM-2R/WA 0I/OA 0 A 1 A 2 A 3CS16 × 4RAM-3R/WA 0I/OA 0 A 1 A 2 A 3CS16 × 4RAM-4R/WA 0I/OR/WD7D6D0D 4 -D 7(4)D 0 -D 3(4)Data BusD 4 -D 7(4)D 0 -D 3(4)Figure 15.27 Example 15.5.15.8.1 Serial PortsA serial port is a physical communication interface through which the information transfer takes placeone bit at a time. Serial ports are used to connect mouse, keyboard and modems to the computer. Someof the commonly used serial standards include the RS-232C port, PS/2, FireWire and USB.15.8.1.1 RS-232C PortRS-232 is one of the oldest and most well-known standards for serial interfaces approved by theElectronic Industries Association (EIA). It was developed to interface data terminal equipment (DTE)with data communication equipment (DCE). RS-232C, a variant of the RS-232 standard, is the mostrelevant for the computer world. RS-232C is mostly used to connect modem and other communicationdevices to the computer. In this case the computer is referred to as the DTE and the attached deviceas the DCE.The RS-232C standard specifies 25 communication lines between the DTE and the DCE. Hence,the standard RS-232C connector is a 25-pin connector (DB-25). For personal computer applications,