Serial HOWTO

David S.Lawyer dave@lafn.org original by Greg Hankins

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

5. Is the Serial Port Obsolete?

• 5.1 Introduction
• 5.2 EIA-232 (RS-232) Cable Is Low Speed & Short Distance
• 5.3 Inefficient PCI Interface to the Computer (in some cases)

6. Multiport Serial Boards/Cards/Adapters

• 6.1 Intro to Multiport Serial
• 6.2 Dumb vs. Smart Cards
• 6.3 Getting/Enabling a Driver
• 6.4 Multiport Devices in the /dev Directory,
• 6.5 Making Legacy Multiport Devices in the /dev Directory
• 6.6 Standard PC Serial Cards
• 6.7 Dumb Multiport Serial Boards (with standard UART chips)
• 6.8 Intelligent Multiport Serial Boards
• 6.9 Unsupported Multiport Boards

7. Servers for Serial Ports

8. Configuring Overview

9. Locating the Serial Port: IO address, IRQs

• 9.1 What Bus is my Serial Port On?
• 9.2 IO & IRQ Overview
• 9.3 PCI Bus Support
• 9.4 Common mistakes made re low-level configuring
• 9.5 IRQ & IO Address Must be Correct
• 9.6 What is the IO Address and IRQ per the driver ?
• 9.7 What is the IO Address & IRQ of my Serial Port Hardware?
• 9.8 Choosing Serial IRQs
• 9.9 Choosing Addresses --Video card conflict with ttyS3
• 9.10 Set IO Address & IRQ in the hardware (mostly for PnP)
• 9.11 Giving the IRQ and IO Address to Setserial

10. Configuring the Serial Driver (high-level) "stty"

• 10.1 Overview
• 10.2 Flow Control

11. Serial Port Devices /dev/tts/2 = /dev/ttyS2, etc.

• 11.1 Serial Port Names: ttyS4, etc
• 11.2 The PCI Bus
• 11.3 Serial Port Device Names & Numbers
• 11.4 More on Serial Port Names
• 11.5 USB (Universal Serial Bus) Serial Ports
• 11.6 Link ttySN to /dev/modem
• 11.7 Devfs (The Improved but Obsolete Device File System)
• 11.8 Which Connector on the Back of my PC is ttyS1, etc?
• 11.9 Creating Devices In the /dev directory

12. Interesting Programs You Should Know About

• 12.1 Serial Monitoring/Diagnostics Programs
• 12.2 Changing Interrupt Priority
• 12.3 What is Setserial ?
• 12.4 Stty
• 12.5 What is isapnp ?
• 12.6 Connecting two PCs together via serial ports
• 12.7 Connect the serial port to a fast network: ser2net

13. Speed (Flow Rate)

• 13.1 Very High Speeds
• 13.2 Higher Serial Throughput

14. Locking Out Others

• 14.1 Introduction
• 14.2 Lock-Files
• 14.3 Lock-Files if you use devfs
• 14.4 Change Owners, Groups, and/or Permissions of Device Files

15. Serial Communications Programs And Utilities

• 15.1 List of Software
• 15.2 kermit and zmodem

16. Serial Tips And Miscellany

• 16.1 Serial Modules
• 16.2 Kernel Configuration
• 16.3 Number of Serial Ports Supported
• 16.4 Serial Console (console on the serial port)
• 16.5 Line Drivers
• 16.6 Stopping the Data Flow when Printing, etc.
• 16.7 Known IO Address Conflicts
• 16.8 Known Defective Hardware

17. Troubleshooting

• 17.1 Serial Electrical Test Equipment
• 17.2 Serial Monitoring/Diagnostics
• 17.3 (The following subsections are in both the Serial and Modem HOWTOs)
• 17.4 Serial Port Can't be Found
• 17.5 Linux Creates an Interrupt Conflict (your PC has an ISA slot)
• 17.6 Extremely Slow: Text appears on the screen slowly after long delays
• 17.7 Somewhat Slow: I expected it to be a few times faster
• 17.8 The Startup Screen Shows Wrong IRQs for the Serial Ports
• 17.9 "Cannot open /dev/ttyS?: Device or resource busy
• 17.10 "Cannot open /dev/ttyS?: Permission denied"
• 17.11 "Cannot open /dev/ttyS?"
• 17.12 "Operation not supported by device" for ttyS?
• 17.13 "Cannot create lockfile. Sorry"
• 17.14 "Device /dev/ttyS? is locked."
• 17.15 "/dev/tty? Device or resource busy"
• 17.16 "Input/output error" from setserial, stty, pppd, etc.
• 17.17 "LSR safety check engaged"
• 17.18 Overrun errors on serial port
• 17.19 Port gets characters only sporadically
• 17.20 Troubleshooting Tools
• 17.21 Almost all characters are wrong; Many missing or many extras

18. Interrupt Problem Details

• 18.1 Types of interrupt problems
• 18.2 Symptoms of Mis-set or Conflicting Interrupts
• 18.3 Mis-set Interrupts
• 18.4 Interrupt Conflicts
• 18.5 Resolving Interrupt Problems

19. What Are UARTs? How Do They Affect Performance?

• 19.1 Introduction to UARTS
• 19.2 Two Types of UARTs
• 19.3 FIFOs
• 19.4 Why FIFO Buffers are Small
• 19.5 UART Model Numbers

20. Pinout and Signals

• 20.1 Pinout of 9-pin and 25-pin serial connectors
• 20.2 Signals May Have No Fixed Meaning
• 20.3 Cabling Between Serial Ports
• 20.4 RTS/CTS and DTR/DSR Flow Control
• 20.5 Preventing a Port From Opening

21. Voltage Waveshapes

• 21.1 Voltage for a Bit
• 21.2 Voltage Sequence for a Byte
• 21.3 Parity Explained
• 21.4 Forming a Byte (Framing)
• 21.5 How "Asynchronous" is Synchronized

22. Other Serial Devices (not async EIA-232)

• 22.1 Successors to EIA-232
• 22.2 EIA-422-A (balanced) and EIA-423-A (unbalanced)
• 22.3 EIA-485
• 22.4 EIA-530
• 22.5 EIA-612/613
• 22.6 The Universal Serial Bus (USB)
• 22.7 Firewire
• 22.8 MIDI
• 22.9 Synchronization & Synchronous

23. Other Sources of Information

• 23.1 Books
• 23.2 Serial Software
• 23.3 Related Linux Documents
• 23.4 Usenet newsgroups:
• 23.5 Serial Mailing List
• 23.6 Internet

24. Appendix A: Obsolete Hardware/Software

• 24.1 Replacing pre 1990 UARTS
• 24.2 Two Ports with the Same IO address
• 24.3 Configuring by modifying source code
• 24.4 Modems on Multiport Cards Obsolete for Sending at 56k