Watlow Series 988 Family - Manual (Page 5)

Introduction to Data Communications, Chapter 1 Data Communications with the Watlow Series 988 Family
1.1
Chapter 1 Introduction to Data Communications
NOTE:
This manual applies
only to controllers
with the data
communications
option (9___-____-
_R__ or 9___-____-
_S__ or 9___-____-
_U__). Please use it
in conjunction with
the user's manuals.
Machine-to-Machine Communication
Humans use basic components to exchange messages. Computers and controllers
also use certain elements in order to communicate: a character set; a common
data link, or interface; and a protocol, to prevent confusion and errors.
Serial communication is the exchange of data one bit at a time on a single data
line or channel. Serial contrasts with "parallel" communication, which sends several
bits of information simultaneously over multiple lines or channels. Not only is serial
data communication typically simpler than parallel, it generally costs less.
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).
Bit is simply the contraction of "binary digit," either a "1" or a "0." A byte is a string
of seven or eight bits, which a computer treats as a single "character." The ASCII
(pronounced "asky") character set uses a unique, seven-bit byte to represent each
letter, digit and punctuation mark.
Protocol
Now we need a few rules to "talk" by. Protocol determines who gets to talk when. A
protocol is a set of standards for formatting and timing information exchange
between electronic systems.
Protocol describes how to initiate an exchange. It also prevents two machines from
attempting to send data at the same time. There are a number of different data
communications protocols, just as there are different human cultural protocols that
vary according to the situation.
A Protocol Example
Let's assume that we have a computer and controllers linked together. They all use
ASCII and are connected via a common interface. In process control applications,
one device often has greater function and memory capability than the devices it is
communicating with. This "master" device always initiates exchanges between it
and the connected "remote" devices.
Here's what happens: Imagine "PC-1," the master computer, sitting at the end of a
long hallway with nine doors in it. Each door has a remote device behind it. PC-1
has a telephone line to all the devices. The remote devices are busy controlling
heaters to specific set points. PC-1 monitors and changes the instructions that
each remote device uses to control its heaters.
Interfaces