User`s manual

Watlow Controls Communications Guide 5
Chapter 2 -
Definition of Terms
Elements of Machine-to-
Machine Communication
In human communication there are basic
words and sentences used to get a message
across. Likewise, with computers and
controllers. They need a code called a
character format or character set. They
need rules called protocol to govern their
conversation and prevent confusion and
errors. Computers need a connecting
interface over which to 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). Or they may use one pair to send
in both directions (half duplex).
Character Format
The code or character formats for Watlow
data communication is shared by virtually
everyone in the electronics business
everywhere. This code defines a computer
stream of 1’s and 0’s, that are created by
varying a voltage signal in a regular
manner. The code is the American
Standard Code for Information
Interchange, called ASCII (asky).
Bits and Bytes
The word bit is simply the contraction of
the words binary digit
. A bit is the basic
unit in ASCII. It is either a “1” or a “0”.
A byte is a string of seven or eight bits
that a computer treats as a single
character. ASCII requires seven bits to
represent each letter of the alphabet, each
digit and each punctuation mark we use.
ASCII
The ASCII code defines 128 separate
seven- or eight-bit characters (see chart
below), one for each letter, digit and
punctuation mark. ASCII also includes
control characters similar to those we find
on computer keys, such as backspace, shift
and return. It also has nine
communications control characters for
identification, enquiry (inquiry), start of
text, end of text, end of transmission,
acknowledge, negative acknowledge and
escape.
The ASCII code is sometimes written in a
base-16 number system that is called
hexadecimal or “hex” for short. The
numbers 0 through 9 represents the first
ten digits of this system, and the letters A
through F represents the final six digits.
The 128 ASCII character code with the
decimal and hexadecimal equivalents is
listed below.