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 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 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 Modem Limitations
• 5.3 Dumb vs. Smart Cards
• 5.4 Getting/Enabling a Driver
• 5.5 Multiport Devices in the /dev Directory,
• 5.6 Making Legacy Multiport Devices in the /dev Directory
• 5.7 Standard PC Serial Cards
• 5.8 Dumb Multiport Serial Boards (with standard UART chips)
• 5.9 Intelligent Multiport Serial Boards
• 5.10 Unsupported Multiport Boards

6. Servers for Serial Ports

7. Configuring Overview

8. Locating the Serial Port: IO address, IRQs

• 8.1 IO & IRQ Overview
• 8.2 PCI Bus Support
• 8.3 Common mistakes made re low-level configuring
• 8.4 IRQ & IO Address Must be Correct
• 8.5 What is the IO Address and IRQ per the driver ?
• 8.6 What is the IO Address & IRQ of my Serial Port Hardware?
• 8.7 Choosing Serial IRQs
• 8.8 Choosing Addresses --Video card conflict with ttyS3
• 8.9 Set IO Address & IRQ in the hardware (mostly for PnP)
• 8.10 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/tts/2 = /dev/ttyS2, etc.

• 10.1 Serial Port Names: ttyS2, tts/1, etc.
• 10.2 Devfs (The Device File System)
• 10.3 Legacy 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 What is slattach?

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 Lock-Files and devfs Problems
• 13.4 Change Owners, Groups, and/or Permissions of Device Files

14. Communications Programs And Utilities

• 14.1 List of Software
• 14.2 kermit and zmodem

15. Serial Tips And Miscellany

• 15.1 Serial Module
• 15.2 Serial Console (console on the serial port)
• 15.3 Line Drivers
• 15.4 Stopping the Data Flow when Printing, etc.
• 15.5 Known IO Address Conflicts
• 15.6 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 My Serial Port is Physically There but Can't be Found
• 16.5 Extremely Slow: Text appears on the screen slowly after long delays
• 16.6 Somewhat Slow: I expected it to be a few times faster
• 16.7 The Startup Screen Show Wrong IRQs for the Serial Ports.
• 16.8 "Cannot open /dev/ttyS?: Permission denied"
• 16.9 "Operation not supported by device" for ttyS?
• 16.10 "Cannot create lockfile. Sorry"
• 16.11 "Device /dev/ttyS? is locked."
• 16.12 "/dev/tty? Device or resource busy"
• 16.13 "Input/output error" from setserial, stty, pppd, etc.
• 16.14 "LSR safety check engaged"
• 16.15 Overrun errors on serial port
• 16.16 Port gets characters only sporadically
• 16.17 Troubleshooting Tools
• 16.18 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 EIA-232)

• 21.1 Successors to EIA-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 Usenet newsgroups:
• 22.5 Serial Mailing List
• 22.6 Internet

23. Appendix: Obsolete Hardware (prior to 1990) Info

• 23.1 Replacing obsolete UARTS