Part 2: Introduction To Digital Communication; Overview; Definition Of Terms - Omega i-SERIES User Manual

PART 2
INTRODUCTION TO DIGITAL COMMUNICATION

2.1 Overview

This manual describes how to use a digital communication link and i-SERIES or
MODBUS communication protocols to operate the i-Series controllers. It has been
assumed that the user has some experience of communication protocols and some
familiarity with i-Series controllers.
2.2 Definitions of terms
This guide is intended to help the user to become familiar with digital communication
between a computer (or other controlling instrument) and one or more devices.
User of this manual should be familiar with following definitions:
• Serial Communication is the exchange of the data one bit at a time on a single data
line. Serial compares with parallel communication, which sends several bits of
information simultaneously over multiple lines or channels.
• Interface are connections over which computers communicate. They may use one pair
of wires to send information in one direction and another pair to send in the opposite
direction (full duplex). They may also use one pair to send the information in both
directions (half duplex).
• Bit is a unit of digital data (binary digit) either a "1" or "0".
• Byte is a string of seven or eight bits, which represents a single character.
• ASCII (American Standard Code for Information Interchange) – is a 7-bit code defines
128 characters, which include digits, upper and lowercase letters, punctuation symbols,
and control codes such as backspace, line feed, carriage return and so on. The ASCII
code can be written in a base – 16 number system, called hexadecimal ("hex"). The first
10 digits of this system are represented by the numbers 0 through 9, and the other six
digits are represented by the letters A through F. The 128 ASCII character code with the
decimal, hexadecimal and binary equivalents is listed in Appendix B.
• Synchronous and Asynchronous Communications
There are two basic types of serial communications, synchronous and asynchronous.
With synchronous communications, the two devices initially synchronize themselves to
each other, and then continually send characters to stay in sync. Asynchronous means
"no synchronization", and thus does not require sending and receiving idle characters.
However, the beginning and end of each byte of data must be identified by start and stop
bits. The serial ports on IBM-style PCs are asynchronous devices and therefore only
support asynchronous serial communications.
• Start and Stop Bits
The start and stop bits identify the beginning and end of each character and permit a
receiver to resynchronize a local clock to each new character. The start bit indicates
when the data byte is about to begin and the stop bit signals when it ends. The start bit is
always a 0. The stop bit is always a 1.
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