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 Related HOWTO's re the Serial Port
• 1.4 Feedback
• 1.5 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 Making 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. Configuring the Serial Port

• 6.1 PCI Bus Support Underway
• 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 Devfs (The new Device File System)
• 7.2 Serial Port Device Names & Numbers
• 7.3 Universal Serial Bus Ports
• 7.4 Link ttySN to /dev/modem
• 7.5 Which Connector on the Back of my PC is ttyS1, etc?
• 7.6 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 ?
• 8.6 What is slattach?

9. Speed (Flow Rate)

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

10. Locking Out Others

• 10.1 Introduction
• 10.2 Lock-Files
• 10.3 Change Owners, Groups, and/or Permissions of Device Files

11. Communications Programs And Utilities

• 11.1 List of Software
• 11.2 kermit and zmodem

12. Serial Tips And Miscellany

• 12.1 Serial Module
• 12.2 Serial Console (console on the serial port)
• 12.3 Line Drivers
• 12.4 Stopping the Data Flow when Printing, etc.
• 12.5 Known Defective Hardware

13. Troubleshooting

• 13.1 Serial Electrical Test Equipment
• 13.2 Serial Monitoring/Diagnostics
• 13.3 (The following subsections are in both the Serial and Modem HOWTOs)
• 13.4 My Serial Port is Physically There but Can't be Found
• 13.5 Extremely Slow: Text appears on the screen slowly after long delays
• 13.6 Somewhat Slow: I expected it to be a few times faster
• 13.7 The Startup Screen Show Wrong IRQs for the Serial Ports.
• 13.8 "Cannot open /dev/ttyS?: Permission denied"
• 13.9 "Operation not supported by device" for ttyS?
• 13.10 "Cannot create lockfile. Sorry"
• 13.11 "Device /dev/ttyS? is locked."
• 13.12 "/dev/tty? Device or resource busy"
• 13.13 "Input/output error" from setserial or stty
• 13.14 Overrun errors on serial port
• 13.15 Port get characters only sporadically
• 13.16 Troubleshooting Tools

14. Interrupt Problem Details

• 14.1 Types of interrupt problems
• 14.2 Symptoms of Mis-set or Conflicting Interrupts
• 14.3 Mis-set Interrupts
• 14.4 Interrupt Conflicts
• 14.5 Resolving Interrupt Problems

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

• 15.1 Introduction to UARTS
• 15.2 Two Types of UARTs
• 15.3 FIFOs
• 15.4 Why FIFO Buffers are Small
• 15.5 UART Model Numbers

16. Pinout and Signals

• 16.1 Pinout
• 16.2 Signals May Have No Fixed Meaning
• 16.3 Cabling Between Serial Ports
• 16.4 RTS/CTS and DTR/DSR Flow Control
• 16.5 Preventing a Port From Opening

17. Voltage Waveshapes

• 17.1 Voltage for a Bit
• 17.2 Voltage Sequence for a Byte
• 17.3 Parity Explained
• 17.4 Forming a Byte (Framing)
• 17.5 How "Asynchronous" is Synchronized

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

• 18.1 Successors to EIA-232
• 18.2 EIA-422-A (balanced) and EIA-423-A (unbalanced)
• 18.3 EIA-485
• 18.4 EIA-530
• 18.5 EIA-612/613
• 18.6 The Universal Serial Bus (USB)
• 18.7 Firewire
• 18.8 Synchronization & Synchronous

19. Other Sources of Information

• 19.1 Books
• 19.2 Serial Software
• 19.3 Related Linux Documents
• 19.4 Usenet newsgroups:
• 19.5 Serial Mailing List
• 19.6 Internet

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

• 20.1 Replacing obsolete UARTS