RS232/422/485 Serial Communications Users Manual (SERIAL) Version 5.1 August 19, 2011 This software is provided as-is. There are no warranties, expressed or implied. Copyright (C) 2011 All rights reserved MarshallSoft Computing, Inc. Post Office Box 4543 Huntsville AL 35815 USA Email:
[email protected] Web: www.marshallsoft.com MARSHALLSOFT is a registered trademark of MarshallSoft Computing. 1 TABLE OF CONTENTS 1 The UART Page 3 1.1 Emulated UARTs Page 3 1.2 UART Types Page 3 1.2.1 National 8250 Page 3 1.2.2 National 16450 Page 3 1.2.3 National 16550 Page 3 1.2.4 StarTech 16650 Page 3 1.2.5 Texas Instruments 16750 Page 3 1.3 UART Operation Page 4 1.4 RS-232 Signals Page 5 1.5 UART Registers Page 6 1.6 Register Summary Page 7 2 Modems Page 10 2.1 Modem Standards Page 10 2.1.1 Speed Page 10 2.1.2 Data Compression Page 10 2.1.3 Error Control Page 10 2.2 Modem AT Command Set Page 11 2.3 Flow Control Page 11 2.4 Modem Initialization Page 12 2.5 Modem CONNECT Page 12 2.6 More Modem Documentation Page 12 3 RS422 and RS485 Page 13 4 Other Serial Devices Page 13 2 1 The UART The heart of serial communications is the "Universal Asynchronous Receiver Transmitter” or UART for short. The UART is responsible for controlling the computer's RS-232/422/485 port. 1.1 Emulated UARTs Some computers, particularly laptops and Handheld / Pocket PC’s, do not contain real UART chips.