The Linux Serial HOWTO

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

1. Introduction

• 1.1 Copyright, Disclaimer, & Credits
• 1.2 Release Notes
• 1.3 New Versions of this Serial-HOWTO
• 1.4 Related HOWTO's re the Serial Port
• 1.5 Feedback
• 1.6 What is a Serial Port?

2. How the Hardware Transfers Bytes

• 2.1 Transmitting
• 2.2 Receiving
• 2.3 The Large Serial Buffers

3. Serial Port Basics

• 3.1 What is a Serial Port ?
• 3.2 IO Address & IRQ
• 3.3 Names: ttyS0, ttyS1, etc.
• 3.4 Interrupts
• 3.5 Data Flow (Speeds)
• 3.6 Flow Control
• 3.7 Data Flow Path; Buffers
• 3.8 Complex Flow Control Example
• 3.9 Serial Software: Device Driver Module

4. Is the Serial Port Obsolete?

• 4.1 Introduction
• 4.2 EIA-232 Cable Is Low Speed & Short Distance
• 4.3 Inefficient Interface to the Computer

5. Multiport Serial Boards/Cards/Adapters

• 5.1 Intro to Multiport Serial
• 5.2 Making "devices" in the /dev directory
• 5.3 Standard PC Serial Boards
• 5.4 Dumb Multiport Serial Boards (with 8250/16450/16550A UART's)
• 5.5 Intelligent Multiport Serial Boards

6. Configuring the Serial Port

• 6.1 PCI Bus Not Yet Supported
• 6.2 Configuring Overview
• 6.3 Common mistakes made re low-level configuring
• 6.4 I/O Address & IRQ: Boot-time messages
• 6.5 What is the current IO address and IRQ of my Serial Port ?
• 6.6 Choosing Serial IRQs
• 6.7 Choosing Addresses --Video card conflict with ttyS3
• 6.8 Set IO Address & IRQ in the hardware (mostly for PnP)
• 6.9 Giving the IRQ and IO Address to Setserial
• 6.10 High-level Configuring: stty, etc.

7. Serial Port Devices /dev/ttyS2, etc.

• 7.1 Serial Port Device Names & Numbers
• 7.2 Link ttySN to /dev/modem ?
• 7.3 Notes For Multiport Boards
• 7.4 Creating Devices In the /dev directory

8. Interesting Programs You Should Know About

• 8.1 Serial Monitoring/Diagnostics Programs
• 8.2 Changing Interrupt Priority
• 8.3 What is Setserial ?
• 8.4 Stty
• 8.5 What is isapnp ?

9. Speed (Flow Rate)

• 9.1 Can't Set a High Enough Speed
• 9.2 Higher Serial Throughput

10. Communications Programs And Utilities

• 10.1 List of Software
• 10.2 kermit and zmodem

11. Serial Tips And Miscellany

• 11.1 Line Drivers
• 11.2 Known Defective Hardware
• 11.3 What Are Lock Files ?

12. Troubleshooting

• 12.1 Serial Electrical Test Equipment
• 12.2 Serial Monitoring/Diagnostics
• 12.3 The following subsections are in both the Serial and Modem HOWTOs:
• 12.4 My Serial Port is Physically There but Can't be Found
• 12.5 Extremely Slow: Text appears on the screen slowly after long delays
• 12.6 Somewhat Slow: I expected it to be a few times faster
• 12.7 The Startup Screen Show Wrong IRQs for the Serial Ports.
• 12.8 "Cannot open /dev/ttyS?: Permission denied"
• 12.9 "Operation not supported by device" for ttyS?
• 12.10 "Cannot create lockfile. Sorry"
• 12.11 "Device /dev/ttyS? is locked."
• 12.12 "/dev/ttyS?: Device or resource busy"
• 12.13 Troubleshooting Tools

13. Interrupt Problem Details

• 13.1 Types of interrupt problems
• 13.2 Symptoms of Mis-set or Conflicting Interrupts
• 13.3 Mis-set Interrupts
• 13.4 Interrupt Conflicts
• 13.5 Resolving Interrupt Problems

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

• 14.1 Introduction to UARTS
• 14.2 Two Types of UARTs
• 14.3 FIFOs
• 14.4 UART Model Numbers

15. Pinout and Signals

• 15.1 Pinout
• 15.2 Signals May Have No Fixed Meaning
• 15.3 Cabling Between Serial Ports
• 15.4 RTS/CTS and DTR/DSR Flow Control
• 15.5 Preventing a Port From Opening

16. Voltage Waveshapes

• 16.1 Voltage for a Bit
• 16.2 Voltage Sequence for a Byte
• 16.3 Parity Explained
• 16.4 Forming a Byte (Framing)
• 16.5 How "Asynchronous" is Synchronized

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

• 17.1 Successors to EIA-232
• 17.2 EIA-422-A (balanced) and EIA-423-A (unbalanced)
• 17.3 EIA-485
• 17.4 EIA-530
• 17.5 EIA-612/613
• 17.6 The Universal Serial Bus (USB)
• 17.7 Synchronization & Synchronous

18. Other Sources of Information

• 18.1 Books
• 18.2 Serial Software
• 18.3 Linux Documents
• 18.4 Usenet newsgroups:
• 18.5 Serial Mailing List
• 18.6 Internet