Serial HOWTO

Next Previous Contents Serial HOWTO David S.Lawyer dave@lafn.org original by Greg Hankins v2.27 February 2011 This document is for the UART serial port. This port has mostly disappeared from desktops and laptops is still used elsewhere such as for embedded systems. It covers information other than that which should be covered by Modem-HOWTO, PPP-HOWTO, Serial-Programming-HOWTO, or Text-Terminal-HOWTO. It lists info on multiport serial cards. It contains technical info about the serial port itself in more detail than found in the above HOWTOs and should be best for troubleshooting when the problem is the serial port itself. If you are dealing with a Modem, PPP (used for Internet access on a phone line), or a Text-Terminal, those HOWTOs should be consulted first. 1. Introduction 1.1 Copyright, Disclaimer, & Credits 1.2 New Versions of this Serial-HOWTO 1.3 New in Recent Versions 1.4 Related HOWTO's, etc. about the Serial Port 1.5 Feedback 1.6 What is a Serial Port? 2. Quick Help 3. How the Hardware Transfers Bytes 3.1 Transmitting 3.2 Receiving 3.3 The Large Serial Buffers 4. Serial Port Basics 4.1 What is a Serial Port ? 4.2 IO Address & IRQ 4.3 Names: ttyS0, ttyS1, etc. 4.4 Interrupts 4.5 Data Flow (Speeds) 4.6 Flow Control 4.7 Data Flow Path; Buffers 4.8 Complex Flow Control Example 4.9 Serial Driver Module 4.10 The Serial Port is Now Obsolete on PCs 4.11 RS-232 Cable Is Low Speed & Short Distance 4.12 Inefficient PCI Interface to the Computer (in some cases) 5. Multiport Serial Boards/Cards/Adapters 5.1 Intro to Multiport Serial 5.2 Dumb vs. Smart Cards 5.3 Getting/Enabling a Driver 5.4 Multiport Devices in the /dev Directory, 5.5 Making Legacy Multiport Devices in the /dev Directory 5.6 Standard PC Serial Cards 5.7 Dumb Multiport Serial Boards (with standard UART chips) 5.8 Intelligent Multiport Serial Boards 5.9 Unsupported Multiport Boards 6. Servers for Serial Ports 7. Configuring Overview 8. Locating the Serial Port: IO address, IRQs 8.1 What Bus is my Serial Port On? 8.2 IO & IRQ Overview 8.3 PCI Bus Support 8.4 Common mistakes made re low-level configuring 8.5 IRQ & IO Address Must be Correct 8.6 What is the IO Address and IRQ per the driver ? 8.7 What is the IO Address & IRQ of my Serial Port Hardware? 8.8 Choosing Serial IRQs 8.9 Choosing Addresses --Video card conflict with ttyS3 8.10 Set IO Address & IRQ in the hardware (mostly for PnP) 8.11 Giving the IRQ and IO Address to Setserial 9. Configuring the Serial Driver (high-level) "stty" 9.1 Overview 9.2 Flow Control 10. Serial Port Devices /dev/ttyS2, etc. 10.1 Serial Port Names: ttyS4, etc 10.2 The PCI Bus 10.3 Serial Port Device Names & Numbers 10.4 More on Serial Port Names 10.5 USB (Universal Serial Bus) Serial Ports 10.6 Link ttySN to /dev/modem 10.7 Which Connector on the Back of my PC is ttyS1, etc? 10.8 Creating Devices In the /dev directory 11. Interesting Programs You Should Know About 11.1 Serial Monitoring/Diagnostics Programs 11.2 Changing Interrupt Priority 11.3 What is Setserial ? 11.4 Stty 11.5 What is isapnp ? 11.6 Connecting two PCs together via serial ports 11.7 Connect the serial port to a fast network: ser2net 12. Speed (Flow Rate) 12.1 Very High Speeds 12.2 Higher Serial Throughput 13. Locking Out Others 13.1 Introduction 13.2 Lock-Files 13.3 Change Owners, Groups, and/or Permissions of Device Files 14. Serial Communications Programs And Utilities 14.1 List of Software 14.2 kermit and zmodem 15. Serial Tips And Miscellany 15.1 Serial Modules 15.2 Kernel Configuration 15.3 Number of Serial Ports Supported 15.4 Serial Console (console on the serial port) 15.5 Line Drivers 15.6 Stopping the Data Flow when Printing, etc. 15.7 Known IO Address Conflicts 15.8 Known Defective Hardware 16. Troubleshooting 16.1 Serial Electrical Test Equipment 16.2 Serial Monitoring/Diagnostics 16.3 (The following subsections are in both the Serial and Modem HOWTOs) 16.4 Serial Port Can't be Found 16.5 Linux Creates an Interrupt Conflict (your PC has an ISA slot) 16.6 Extremely Slow: Text appears on the screen slowly after long delays 16.7 Somewhat Slow: I expected it to be a few times faster 16.8 The Startup Screen Shows Wrong IRQs for the Serial Ports 16.9 "Cannot open /dev/ttyS?: Device or resource busy 16.10 "Cannot open /dev/ttyS?: Permission denied" 16.11 "Cannot open /dev/ttyS?" 16.12 "Operation not supported by device" for ttyS? 16.13 "Cannot create lockfile. Sorry" 16.14 "Device /dev/ttyS? is locked." 16.15 "/dev/tty? Device or resource busy" 16.16 "Input/output error" from setserial, stty, pppd, etc. 16.17 "LSR safety check engaged" 16.18 Overrun errors on serial port 16.19 Port gets characters only sporadically 16.20 Troubleshooting Tools 16.21 Almost all characters are wrong; Many missing or many extras 17. Interrupt Problem Details 17.1 Types of interrupt problems 17.2 Symptoms of Mis-set or Conflicting Interrupts 17.3 Mis-set Interrupts 17.4 Interrupt Conflicts 17.5 Resolving Interrupt Problems 18. What Are UARTs? How Do They Affect Performance? 18.1 Introduction to UARTS 18.2 Two Types of UARTs 18.3 FIFOs 18.4 Why FIFO Buffers are Small 18.5 UART Model Numbers 19. Pinout and Signals 19.1 Pinout of 9-pin and 25-pin serial connectors 19.2 Signals May Have No Fixed Meaning 19.3 Cabling Between Serial Ports 19.4 RTS/CTS and DTR/DSR Flow Control 19.5 Preventing a Port From Opening 20. Voltage Waveshapes 20.1 Voltage for a Bit 20.2 Voltage Sequence for a Byte 20.3 Parity Explained 20.4 Forming a Byte (Framing) 20.5 How "Asynchronous" is Synchronized 21. Other Serial Devices (not async RS-232) 21.1 Successors to RS-232 21.2 EIA-422-A (balanced) and EIA-423-A (unbalanced) 21.3 EIA-485 21.4 EIA-530 21.5 EIA-612/613 21.6 The Universal Serial Bus (USB) 21.7 Firewire 21.8 MIDI 21.9 Synchronization & Synchronous 22. Other Sources of Information 22.1 Books 22.2 Serial Software 22.3 Related Linux Documents 22.4 Serial Mailing List 22.5 Internet 23. Appendix A: Very Obsolete Hardware/Software 23.1 Replacing pre 1990 UARTS 23.2 Two