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01 April 2003

Device servers do that

When considering your investment in your installed base of industrial equipment, it is difficult to justify throwing it all away in order to join the Ethernet revolution.

The evidence of Ethernet's role in productivity enhancements, quality control, and cost reduction is overwhelming. It also appeals to our common sense.

"You mean I can get data from any piece of equipment into any of the software applications I run from my office PC? Where do I sign?!"

Then reality sets in. The actual cost of Ethernet enabling so much of your site is prohibitive.

The fact is that a majority of the equipment in use today is not available with Ethernet connectivity without outright replacement of the hardware. In addition, products with the newest transmission control protocol/Internet protocol (TCP/IP) technology are usually available only in top-of-the-line models and cost much more than the non-Ethernet equivalent.

Another reality is that many vendors do not yet offer Ethernet networking as an option and do not offer solutions to the models of equipment you may already have installed.

However, advances in third-party Ethernet retrofitting could allow you to join the revolution without having to pay the price for it.

They make sense because serial ports are present on most industrial devices, and serial communication has always been a dependable means of sharing information. Pairing them with the readily available, inexpensive, ever-reaching Ethernet, users will realize a great alternative for the factory floor.

Serial-over-Ethernet technology can be a very important strategy in IP enabling your site. However, the differences among device servers are as vast as the equipment they plug in to.

"Not all device servers are created equal in the industrial world," said Mark Fondl of ICT Global, an industrial communications solutions and services company. "There are a whole host of serial-to-Ethernet products that are virtually useless on the factory floor. A true industrial device server has a completely unique set of characteristics customized for automation applications."

Tip-offs to industrially focused device servers are DIN-rail mounting, DC power inputs, terminal block connectors, industrial certifications, and wider operating temperature specifications.

A new breed of device server is now available with intelligence that exceeds the basic serial-over-TCP/IP feature set. High-end device servers have anticipated the needs of common industrial communications problems and addressed them with features such as serial port emulation and protocol translation.

In layman's terms, serial port emulation (called component object model, or COM, port redirection) is a software driver that fools a computer into thinking it has additional COM ports attached.

These virtual COM ports are actually logical COM ports pointing to the IP address of the device server, where the serial device is connected. The advantage of COM redirection is that any software application that communicates via COM ports will be able to communicate with a serial device across the network as if it were plugged directly into the PC, without any changes to the software.

A popular use for COM port redirection in industrial applications is when a configuration or programming software needs the ability to upload or download to a remote serial device. For this to work, the application simply chooses one of the COM ports that map to a device server across the network. The serial device plugged in to the device server is programmable, just as if it were connected directly to the PC.

Another significant capability of device server technology is protocol translation, where the device server actually converts one protocol to another. A few device servers are able to convert Modbus serial (remote terminal unit or ASCII) to Modbus TCP.

A few even allow the conversion of Allen-Bradley serial—DF1—to Ethernet IP and AB Ethernet—PCCC. This allows a networked software application or Ethernet-enabled hardware devices to communicate to a serial device using proprietary industrial TCP/IPs.

A common scenario where protocol conversion is used is in the case of human-machine interface (HMI) software—Wonderware, Intellution, RSLinx/RSView, and others—that needs to monitor or collect data from a programmable logic controller (PLC).

Most common industrial applications support ModbusTCP, EtherNet/IP, or AB Ethernet. Therefore, enabling communications from an HMI supporting one of these IPs to a PLC speaking the serial equivalent can be simple.

A DigiOne IA RealPort, for instance, gives the device server an IP address, launches a Web browser, enters the IP address to access the industrial protocol wizard, and finally chooses the correct settings for the appropriate protocol.

Henceforth, the HMI software can communicate with the serial PLC as if it were a native industrial Ethernet-enabled device.

Low latency allows special industrial serial protocols that require a very quick response time to be able to communicate across TCP/IP networks. Most device servers take 200–600 milliseconds to send a serial message across a network and receive a response.

However, solutions now exist that allow serial protocols much tighter response times: less than 6 milliseconds turnaround over the network. This is well within the requirements of virtually any serial protocol.

Industrially focused device servers ably bridge legacy devices and can handle mission-critical serial-to-Ethernet needs. IT

Jason Sprayberry works and develops applications in the industrial automation division at Digi International (www.digi.com).