Page 244 - Mechanical Engineers' Handbook (Volume 2)

8 Data Communications 233

Table 9 Open Systems Interconnect Model
Layer Principle Example
7. Application Application Millwide reporting
6. Presentation Display, format, edit Convert ASCII to EBCDIC
5. Session Establish communications Log onto remote computer
4. Transport Virtual circuits Make sure all message parts got there in order
3. Network Route to other networks Talk to Internet
2. Data link Correct errors Send character downline
Synchronize communications Send Ack-Nak
1. Physical Electrical interface Wire and voltages

not providing for specific interface protocols, the OSI model has had a significant impact on
communications because it has provided a framework for compartmentalizing aspects of
communications to allow the handoff of information from one device to another in a standard
way. For instance, the transmission of data from one media type to another (such as copper
wire to fiber to satellite to copper wire and then to wireless) is a result of standards enabling
the seamless transfer of messages in a way that is transparent to the user.

8.5 OPC Standard
A recent standard of use in manufacturing is the OPC (OLE for process control) standard,
which provides for a standard way of communicating with process equipment. It is sponsored
by the OPC Foundation and originated as an extension for process control from the Microsoft
OLE functionality. 4
Functions provided by OPC include ability to browse the variable database of a device
and monitor data on demand or when events occur. The capability of OPC has been expanded
to work with Web communication methods such as XML and cover complex data such as
record structures. The power of OPC is that data from an instrument can be available via a
standard network interface so that any data acquisition program that uses the OPC interface
can gain access to any OPC device. The need to know the protocol of each device or adhere
to the wiring of specialized communications or use custom database access methods is elim-
inated through the use of a standard protocol. Multiple programs can be simultaneously
monitoring the same piece of data, performing different functions at different time intervals,
as events occur. The last point is particularly significant, because much of the work of data
acquisition systems is spent in polling for changes in data or otherwise attempting to deter-
mine when an event has taken place. A program can subscribe to an OPC item and it will
be notified when the item changes value, reducing the complexity of monitoring data dra-
matically.
As an example of the power of this approach, consider the following example (Fig. 8).
A device can collect the identification from a unit of material such as unit number, color,
and manufacturing date and make it available via OPC. As the unit is processed in a man-
ufacturing center, another device collects defect counts and makes the unit number available
via OPC. A human–machine interface program can monitor both sources with the same
interface protocol and software and display it live for an operator to see. Simultaneously,
another program can collect the defect counts and summarize them into totals. Yet another
program can monitor the totals and wait for the unit number to change, triggering a trans-
action to a database or an email if there was a problem. The power of the OPC interface is
that it provides real-time access to the data from each of the sources and multiple programs