MOXA Protocol Converter (MPC) is a new utility that is designed
to make MOXA's embedded computers accessible to a wider
audience. Read on to learn how you can use MPC for your
own industrial automation applications.
Industrial automation and networking
When PLCs were first introduced in the 1970's, the stage
was set for the evolution from analog to fully computerized,
digital control systems for industrial automation applications.
However, we had to wait until the 21st century to see a
significant move to using plant-wide communication networks
and truly centralized control. The downside to this development
was that the stuff that connects to the network—PLCs, meters,
temperature and pressure sensors, different types of actuators,
etc.—use a variety of connection methods and protocols
to transmit and receive data. One of the more challenging
problems faced by solution providers in the industrial
automation industry is how to coordinate the flow of data
between such a wide range of devices and protocols.
The modern industrial control network uses what is often termed "distributed
intelligence" to connect all of the devices in a plant to
one centralized control center. This type of architecture
uses a large number of small, headless computers that connect
directly to localized groups of sensors and actuators. The
headless computers are much smaller and less expensive than
IPCs, and are used for localized data processing and for
converting the format of data streams from one protocol to
another. Since the devices that make up control systems typically
transmit data over a serial line (RS-232, RS-422, or RS-485),
the headless computers have multiple serial ports for connecting
to an array of devices. By also connecting the headless computers
to the factory LAN (either through Ethernet ports or with
a wireless IEEE802.11g/b card), control system designers
can create a plant-wide control system in which many control
decisions are made by computers in the centralized control
center. 
All serial devices have one thing in common. A dedicated device driver for the
serial device must be installed on the computer to which
the device is connected. If the driver is not installed or
configured properly, then the computer and serial device
will not be able to communicate with each other. Data streams
flow through "channels" that connect two ports of the embedded
computer to each other. Data streams can flow between a serial
port and network port, between a LAN port and the same or
a different LAN port, or between two serial ports. As the
data passes through the embedded computer, the data is operated
on by one or more of the installed device drivers.
Inserting this classical "computer – device driver – serial device" setup into
the modern plant-wide control system can create a big headache
for programmers. The three biggest problems are:
| 1. |
A "timer" subroutine that runs on the embedded computer must be created for periodically
querying/controlling each type of serial device. This
sounds easy enough, but
in fact it can be surprisingly difficult to create
suitable timer routines for control devices. |
| 2. |
Data from each serial device must be converted to a format compatible with TCP/IP
transmissions, and if two serial devices need to communicate
directly with each other, additional code must be created
to make this serial-to-serial communication possible. |
| 3. |
In general, you must write new code for each data stream channel to ensure that
data is transmitted successfully through the channel. |
The MPC concept
MOXA Protocol Converter, or MPC for short, is a software
program that runs on any of MOXA's ready-to-run embedded
computers. More specifically, MPC is a software engine
that establishes bi-directional data stream channels
between any two of the embedded computer's ports, and
loads custom-written programs or drivers (written in
C) that audit the data as it flows from one port to
the other.
What you need to use MPC:
| 1. |
MOXA embedded computer attached to an Ethernet network, and one or more serial
devices. |
| 2. |
MPC installed on the embedded computer. |
| 3. |
Drivers (written in C) for each type of serial device |
More
specifically, a channel is a logical path that connects two
physical communication ports, either of which could be a
serial port or a network port. As the data flows from one
port to the other, the data encounters a sequence of user-programmed
drivers. The engine receives a data stream at one communication
port, assembles each data packet in the stream, and then
passes it to the first driver for data processing or protocol
conversion. The driver performs data formatting or conversion
as required on the packet, and the resulting packet is then
passed to the next driver for further processing or conversion.
This process continues until the final packet reaches the
physical port on the other end of the channel.
MPC has three key benefits:
| 1. |
Serial and networking communication is built into the MPC engine—Without any additional programming on the part of the end-user, the MPC engine
manages transparent data transmission along a channel
that connects any combination of two communication
ports.
|
| 2. |
Port-to-port communication is driver programmable—Most field systems are created by making slight changes to existing systems that
are tried and true. By using the MPC engine, these
changes can be realized almost immediately by either
modifying a driver, or adding a driver to a channel.
|
| 3. |
The engine supports multiple drivers in a channel—Allowing multiple drivers in a channel simplifies and modularizes the design,
making it extremely easy to configure applications
that use layers of data conversion and processing.
|
How
to configure MPC
End-users of MOXA's embedded computers have access
to the MPC executable file, which occupies only 50
KB of the embedded computer's memory space. Versions
are available for both Linux and WinCE versions of
MOXA's embedded computers. After copying the MPC file
to a directory on your embedded computer, one of two
methods can be used to configure the "ports" and "channels"
for your application.
Configuration
Method 1: Code your configuration requirements
into a simple xml file.
The
configuration file is a text file in XML format. The entire
contents are enclosed by a document root: <MPC_DOC_ROOT></MPC_DOC_ROOT>.
Two sections of the file are used to configure ports and
channels, respectively, with the relevant text lines enclosed
by element pairs <XPORTS></XPORTS> and <XCHANNELS></XCHANNELS>, respectively.
Configuration
Method 2: Use the "Moxa Device Manger" user interface.
The
second method available for configuring your MPC application
is to use the Moxa Device Manager (MDM) user interface.
NOTE: The Moxa Device Manager utility was introduced in the featured topic articles of the February issue of Moxa Connection: Smart Management of Embedded Computers (click for details). The advantage of using the MDM user interface over simply creating a text file (as described above) is that you do not need to remember the xml syntax. In this case, the user simply enters the appropriate port, channel, and driver information into a series of text input boxes. In addition, you can use MDM's built-in search function to locate all MOXA embedded computers connected to the LAN.
MPC bridges the gap between transparency and programmability
Many of you are probably familiar with MOXA's NPort series of serial device servers. In general, the serial device server is a cost-effective solution for transmitting data over Ethernet between a serial device and a computer. The serial device server is often described as providing "transparent data transmission," since it requires no programming, is easily configured, and the user does not need to understand the complexities of TCP/IP programming.
In contrast, even though the embedded computer comes without a keyboard, mouse, and monitor, it is nevertheless a true computer, and as such can be programmed by the user. In fact, it is fair to say that the embedded computer is a kind of "programmable serial device server," since many of its applications involve transmitting serial data over Ethernet. The advantage to the user is that essentially any application can be programmed into the embedded computer. The disadvantage is that the user must find a way to do the programming.
How does Moxa Protocol Converter (MPC) fit into the picture? The combination of "embedded computer + MPC" gives users a powerful tool for setting up industrial control applications. Only a minimal amount of programming knowledge is required, but by using existing drivers and data processing programs, MPC can be used to create applications that otherwise would require significant programming skill.
Moxa Protocol Converter is scheduled to be launched in early June. For more information
about MPC, please contact Dr. CF Lin by e-mail at cf.lin@moxa.com. » Back to index
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