Devices that convert data between the serial and Ethernet interfaces
allow engineers to take advantage of the best of two worlds. The "serial
world" is the world of sensors, actuators, modems, and basic RS-485
networks that transmit data between devices and computers. The "Ethernet
world" is the world of NICs (Network Interface Cards), the Internet, and
open protocols used to whisk information from host to host.
The transformation between the serial and Ethernet interfaces takes place
at the electronic signal and network protocol levels, such as in the
transformation of data from the RS-232 format into a format suitable for a
TCP/IP network. Carrying out such a transformation requires
Terminal Server technology , in which a
Serial Terminal Server is a smart, standalone device
with an embedded operating system and CPU that is large enough to contain
its own operating system and the requisite software protocols, such as the
TCP/IP stack. A
Serial Terminal Server also comes
equipped with the required hardware interfaces, such as RS-232, RS-422,
and RS-485 ports. The
Serial Terminal Server can
transfer, and even process data between the serial and Ethernet interfaces
to carry out pre-defined tasks.
Ethernet, and in a broader context, the Internet, have been adopted by the
automation industry as its main communication backbone. By using open
technologies, such as TCP/IP and Ethernet, control devices can be
networked more quickly and with greater ease. Since most LANs are also
connected to the Internet, this type of serial-to-Ethernet solution gives
devices a data transmission outlet to any Internet host in the world.
Emerging Ethernet technology is providing new ways of connecting legacy
serial devices to a Local Area Network (LAN) or Wide Area Network (WAN).
It does not matter what type of serial interface- RS-232, 422 or 485- the
devices in the applications are using;
Serial Terminal Servers allow all three
interfaces access to a LAN.
Serial Terminal Servers connect a single serial
device or many serial devices to one LAN connection.
Serial Terminal Servers are engineered to accept
serial data, convert it to Ethernet packets, transmit that information
onto the LAN and reconvert it to serial at the other end. The most common
Ethernet servers have 1, 2, 4, 8 or 16 serial ports.
Communications with the connected serial devices can be handled without
changing existing serial programs. The older legacy DOS programs that
communicate via Com ports 1, 2, 3, or 4 on a computer can also be used in
Ethernet applications. The
Serial Terminal Servers can accommodate this by
using Virtual Com port software or a feature called Serial Tunneling
(sometimes called Nailed Down) configuration.
Serial Tunneling or Nailed Down configuration can be set up in the
Serial Terminal Server using a built-in
configuration menu. Serial Tunneling basically means that two
Serial Terminal Servers are used to make two
different serial to LAN connections. This can be on the same or different
subnets on the LAN.
Let's use a computer and a Serial Data Acquisition device in this example.
One
Serial Terminal Server will be connected to the
serial port of the computer running the program. The second
Serial Terminal Server will be connected to the
serial connection on the Data Acquisition device. When each
Serial Terminal Servers has a LAN connection and
power is applied, communication between the two dedicated
Serial Terminal Servers automatically start a
session.
Simply put, once the two Ethernet devices are used in the Serial tunneling
mode they automatically transfer serial data across the LAN as if it was
transparent. The program only opens the Com port and sends data out.
Remember when I said that the LAN is transparent to the serially connected
devices? That feature opens the door to countless applications. Computers
or similar intelligent devices are not necessary to setup communication
between two serial devices using
Serial Terminal Servers and the Serial Tunneling
feature.
For example, let's say a scale needs to send data to a display or a
positioning camera needs to send information to a controller on a
manufacturing line. The fact that the Serial Tunneling feature makes the
LAN transparent, the mentioned applications are feasible. Data received by
one Ethernet serial port will be transmitted out the other.
A second way of communicating to
Serial Terminal Servers is with the use of
Virtual COM Port software on a Computer. When using this type of software
only one
Serial Terminal Server is needed to make the
connection at the serial device. The computer is connected using the
Network Interface Card (NIC). This type of software loads in the operating
system on the computer as another Com port.
Running a Windows operating system it can be found in the device manager
under Com ports. Each Com port is linked with an IP address to a specific
Serial Terminal Server. Once again the LAN is
transparent to the software program running the application on the
computer. This type of Virtual Com port can be used with most Windows
programs. However, DOS programs will not be able to open the virtual Com
port.
