Page 253 - Sensors and Control Systems in Manufacturing

Networking of Sensors and Contr ol Systems in Manufacturing
4.9 Understanding Networks in Manufacturing 213
A seemingly simple problem in the development of computer networks
is to establish the ability to interconnect any two computer system ele-
ments. This might involve a computer terminal, a modem, a barcode
scanner, a printer, sensors, and other system elements that must exchange
information in manufacturing. It might seem reasonable that such one-
to-one interconnection would follow a well-defined strategy and make
maximum use of standards. Unfortunately, for historical reasons and
because of the wide diversity of the equipment units that are available
today, this situation does not hold. In fact, achieving interconnection
between typical system elements can be a disconcerting experience.
4.9.1 RS-232-Based Networks
One of the most common approaches used to interconnect computer
system elements in manufacturing is associated with a strategy that
was never really intended for this purpose. As noted by Campbell
(1984), “In 1969, the EIA (Electronic Industries Association), Bell Lab-
oratories, and manufacturers of communication equipment coopera-
tively formulated and issued ‘EIA RS-232,’ which almost immediately
underwent minor revisions to become ‘RS-232-C.’ The RS-232 inter-
face was developed to allow data equipment terminals to be con-
nected to modems so that data could be transmitted over the tele-
phone network. The entire purpose of this standard was to assure
that the use of telephone lines for computer communications would
be handled in a way acceptable to the telephone company.
Thus, in its general application today, RS-232 is not a standard. It
is more a guideline for addressing some of the issues involved in
interfacing equipment. Many issues must be resolved on an individ-
ual basis, which leads to the potential for difficulties. Essentially, a
vendor’s statement that a computer system element is RS-232-com-
patible provides a starting point to consider how the equipment unit
might be interconnected. However, the detailed aspects of the inter-
connection require further understanding of the ways in which equip-
ment units are intended to communicate. Campbell (1984) is a helpful
introduction to applying RS-232 concepts.
In a sense, the history of the RS-232 interface illustrates the diffi-
culties associated with creating a well-defined means for allowing
the various elements of a computer network to interface. Past experi-
ence also indicates how difficult it is to develop standards that will
apply in the future to all the difficult situations that will be encoun-
tered. As it has evolved, the RS-232 approach to the communications
interface is an improvement over the “total anarchy” at position A in
Fig. 4.11, but it still leads to a wide range of problems.
An entire computer network can be configured by using combi-
nations of point-to-point RS-232 connections. In fact, a number of