Distributed Control / Industrial Ethernet / PLCs & PACs

PLCs and PACs can solve the protocol translation problem

Using PLCs or PACs and Ethernet as gateways to digital communication protocols can be a low-cost alternative

By Dan Hebert

Most process plants use many different digital communication protocols, so conversion is required. A DCS can often perform protocol conversion, but generally at a very high cost. A better alternative in many cases is using PLCs or PACs as protocol conversion gateways, and then connecting the PLC or PAC to the DCS via Ethernet. For plants controlled by one or more PLCs or PACs instead of by a DCS, the PLC or PAC becomes the gateway by default.

"One use of our plug-in communication modules is to migrate an old DCS to the new Rockwell Automation PlantPAx," says Ken Roslan, vice president of global marketing for ProSoft Technology. "This allows continued use of the old DCS I/O and a phased migration, minimizing risk and downtime."

In a related example, the United Space Alliance (USA) assembly and refurbishment facility at NASA's Kennedy Space Center in Florida used ProSoft's Modbus communication modules and Rockwell Automation's ControlLogix platform to control the batch mixing process for the insulation used on the space shuttle.

A ControlLogix PAC and an Allen-Bradley SLC-500 controlled each of two mixers. USA tried using 4-20 mA feedback between the controllers and the flow meters, but found they were unable to obtain the needed level of accuracy and precision. "We contacted Micro Motion and they pointed us to ProSoft's Modbus communication modules, which integrate directly into the ControlLogix and SLC-500 platforms," explains Dan Dermody, control systems engineer at USA.

See Also: PLCs Taking Some Excellent Adventures

"We tested them out and quickly discovered they provided the accuracy and precision we needed. The module collects flow data and feeds it directly into the ControlLogix data tables," Dermody continues. "This type of flow control system maintains all the process parameters, so nothing goes out of specification during mixing."

In wastewater plants, Profibus is a much used protocol. The Changi Water Reclamation Plant in Singapore selected Profibus DP V1 as its communication protocol, requiring conversion to talk to Schneider Electric PLCs.

A better alternative in many cases is using PLCs or PACs as protocol conversion gateways.

The plant uses 160 ProSoft Profibus DP V1 Quantum modules to communicate between the PLC and a long list of field devices including magnetic flowmeters, thermal mass flowmeters, pressure and differential pressure level transmitters, radar/ultrasonic transmitters, dissolved oxygen analyzers, temperature transmitters and electric actuators.

Plants often need to convert from Profibus DP to Profibus PA. "In many applications, Profibus DP is advantageous to span long distances, and it also allows creation of fiber redundant rings for increased reliability," notes Andres Suazo Wildt, the serial and process fieldbus product specialist at Phoenix Contact. To connect to Profibus PA, Profibus DP/PA couplers are available from Phoenix Contact.

IO-Link is a popular low-cost, point-to-point communication protocol often supplied with many types of sensors. "Unfortunately most PLC I/O cards available today do not yet support IO-Link," points out Dr. Helge Hornis, the intelligent systems group manager at Pepperl+Fuchs.

The Pepperl+Fuchs solution is its SmartBridge, connected between the PLC's I/O card and IO-Link sensors to intercept IO-Link data and send it to the app. Using a smartphone or tablet running iOS or Android, the SmartBridge establishes a wireless interface to the mobile device.

In cases where the I/O card is IO-Link-enabled, SmartBridge listens to the communication and sends a copy of the sensor data to the mobile device. When a conventional I/O card is used, SmartBridge assumes the role of the IO-Link master and separates the diagnostics data from the switch state, with all diagnostics-related information visualized on the app, while the sensor switch state is passed on to the I/O cards.

Turck provides a specialized PLC communication device called an Ethernet spanner/scanner module. "Our Ethernet spanner/scanner module provides an easy way to link multiple subnets together for data exchange, even ones speaking different protocols," says Chris Vitale, the senior product manager for networks at Turck.

"The device reduces the need for multiple PLC origination points, greatly reducing costs. By linking subnets with a spanner/scanner, networks can be less complex because the device allows Ethernet IP addresses to be reused. The Ethernet spanner/scanner specifically acts as an I/O connection aggregator, reducing the number of connections to the PLC, simplifying installation and lowering system costs," adds Vitale.