RS-232: Serial Ports © 2002 Lava Computer MFG Inc. www.lavalink.com Table of Contents Introduction............................................................................................................................................1 What is a serial port? .............................................................................................................................1 What is RS-232? ....................................................................................................................................2 DCE and DTE........................................................................................................................... 2 Signalling .................................................................................................................................. 2 Electrical ................................................................................................................................... 2 Mechanical................................................................................................................................ 3 Synchronous vs. asynchronous RS-232.................................................................................................3 UARTs ...................................................................................................................................................3 Definition .................................................................................................................................. 3 Stop and start bits...................................................................................................................... 4 Parity bits .................................................................................................................................. 4 Data bits .................................................................................................................................... 5 Buffers and triggers .................................................................................................................. 5 Windows port configuration screens ........................................................................................ 5 UART history ........................................................................................................................... 6 How the UART structures serial communication ..................................................................................7 UART registers.......................................................................................................................................8 REGISTER 0: Data Register .................................................................................................... 9 REGISTER 1: Interrupt Enable Register (IER)...................................................................... 10 REGISTER 2: Interrupt Identification Register (IIR) ............................................................ 10 REGISTER 2: Interrupt Identification Register (IIR) ............................................................ 10 REGISTER 2: Enhanced Feature Register (EFR) [16650 ONLY] ........................................ 10 REGISTER 3: Line Control Register (LCR).......................................................................... 11 REGISTER 4: Modem Control Register (MCR).................................................................... 12 REGISTER 5: Line Status Register (LSR)............................................................................. 12 REGISTER 6: Modem Status Register (MSR)....................................................................... 13 REGISTER 7: Scratch Register.............................................................................................. 13 RS-232 signal descriptions ..................................................................................................................13 DTR ........................................................................................................................................ 13 DSR......................................................................................................................................... 13 CTS ......................................................................................................................................... 13 RTS ......................................................................................................................................... 13 DCD ........................................................................................................................................ 13 RI ............................................................................................................................................ 14 TD ........................................................................................................................................... 14 RD........................................................................................................................................... 14 GND........................................................................................................................................ 14 Connectors and cables .........................................................................................................................14 Cable lengths........................................................................................................................... 14 Number of wires ..................................................................................................................... 14 ii Serial port resource requirements ........................................................................................................18 Addresses ................................................................................................................................ 19 Interrupt request levels............................................................................................................ 19 Serial port line settings ........................................................................................................................20 Flow control: CST/RTS and Xon/Xoff................................................................................... 20 How fast are serial ports?.....................................................................................................................21 Serial UART speeds................................................................................................................ 21 Other speed factors ................................................................................................................. 22 Operating system support ....................................................................................................................22 Lava: the source for serial ports...........................................................................................................24 iii List of Figures FIGURE 1. Transmitting serial data.....................................................................................................7 FIGURE 2. Receiving serial data .........................................................................................................8 FIGURE 3. DB-9 pin locations ..........................................................................................................15 FIGURE 4. DB-25 pin locations ........................................................................................................15 FIGURE 5. Serial port Resources dialog box ....................................................................................20 FIGURE 6. Serial port Settings dialog box........................................................................................21 FIGURE 7. Serial port Advanced Settings dialog box.......................................................................21 iv v List of Tables TABLE 1. Summary of UART registers.............................................................................................9 TABLE 2. Data register......................................................................................................................9 TABLE 3. Interrupt Enable Register (IER)......................................................................................10 TABLE 4. Interrupt Identification Register (IIR)—read only..........................................................10 TABLE 5. Interrupt Identification Register (IIR)—write only ........................................................10 TABLE 6. Line Control Register (LCR) ..........................................................................................11 TABLE 7. Parity definitions.............................................................................................................11 TABLE 8. Common baud rates ........................................................................................................11 TABLE 9. Modem Control Register (MCR)....................................................................................12 TABLE 10. Line Status Register (LSR) .............................................................................................12 TABLE 11. Modem Status Register (MSR).......................................................................................13 TABLE 12. DTE-to-DCE DB-9 connection (Straight cable).............................................................16