Device servers for Ethernet retrofits

Jason Sprayberry -- Control Engineering, 1/1/2006

Because the majority of today's industrial equipment requires hardware replacement to enable Ethernet connectivity, and typically only top-of-the-line new products offer the newest TCP/IP technology (costing much more than the non-Ethernet equivalent), most industrial users have not switched to Ethernet for their industrial devices. However, advances in third-party Ethernet retrofitting could change this.

One viable option for Ethernet retrofitting is use of device server technology. Device servers are hardware products that connect serial devices to Ethernet. Though most experts agree that serial-over-Ethernet technology can be an important strategy in IP-enabling your site, differences in device servers are as vast as the equipment they plug into. "There are a 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," says Mark Fondl of ICT Global, a provider of industrial communications solutions and services.

Characteristics of industrially focused device servers include: DIN-rail mounting, dc power inputs, terminal block connectors, industrial certifications, and wider operating temperature specifications. A new breed of device server is also now available with intelligence that exceeds the basic, serial-over-TCP/IP feature-set. In these servers, common industrial communications problems are addressed with features like serial port emulation and protocol translation.

Serial port emulation—also called COM port redirection—is use of a software driver installed on a computer to fool the computer into thinking that it has additional COM ports attached. These "virtual" ports are actually logical COM ports pointing to the IP address of the device server (where the serial device is connected). Advantage of COM redirection is that any software application communicating via COM ports can do so with a serial device across the network as if it were plugged directly into the PC (without changes to the software).

A popular industrial application for COM port redirection is when a configuration, or programming software, needs to upload or download to a remote serial device. For this to work, the application chooses one of the COM ports mapped to a device server across the network. The serial device plugged into the device server will be programmable as if it were connected directly to the PC.

Another significant capability of device server technology is called protocol translation, where the device server converts one protocol to another. A few device servers are able to convert Modbus serial (RTU or ASCII) to ModbusTCP. Even fewer allow the conversion of Rockwell Automation Allen-Bradley serial (DF1) to EthernetIP and A-B Ethernet (PCCC). This level of capability allows a networked software application or Ethernet-enabled hardware device to communicate to a serial device using proprietary, industrial TCP/IP protocols.

Two advanced industrial features found on some device servers are multimaster device sharing and low-latency communications.

Multimaster is a differentiating technology because it breaks the rules of point-to-point serial communications by allowing multiple masters (or host devices) to simultaneously communicate to a single serial device. For example, you can use one vendor's software communicating with a PLC while a panel HMI is talking to the same PLC.

Low latency allows the use of special industrial serial-protocols that require quick response time to communicate across TCP/IP networks. Most device servers take 200 to 600 ms to send a serial message across a network and receive a response. However, some serial protocols require much tighter response times. Some device servers with low latency can average less than 6 ms turnaround over the network—well within requirements of virtually any serial protocol.