RTU as an Intelligent Remote Gateway

Summary

By Kevin L. Finnan, CSE-Semaphore

The trend toward intelligent devices in remote SCADA installations would seem to put remote terminal units (RTUs) on the path to obsolescence. Instead, it illustrates the evolving value of RTUs in contemporary systems.

The proliferation of intelligent, remote devices has changed the landscape at remote installations in SCADA systems.  Analyzers, drives, IP video cameras, smart meters, valve actuators, wireless I/O modules, and wireless sensor networks are now commonplace.

In the past, a remote site installation would include a remote terminal unit (RTU) interfaced to discrete sensors and control elements via analog and digital I/O points.  Now, such an installation will include intelligent devices with Ethernet and serial interfaces.  Products such as actuators and drives interface directly to process equipment. Wireless I/O modules provide analog and digital I/O points. With this combination of intelligence and I/O on-site, do we even need an RTU?

An RTU could still be necessary because most intelligent devices lack a wide area SCADA network interface such as a cellular modem. But a new product class, an “intelligent gateway,” less expensively combines a modem with Ethernet and serial interfaces. Has the gateway finally obsoleted the RTU?

To answer that question, consider a list of typical requirements for a remote SCADA site:

• Wide area SCADA network interface
• Protocol conversion
• IP forwarding
• Programmable automation
• Alarm management
• Data base management and data logging

Next, we will explore each of the requirements, with regard to their potential for eliminating the RTU.

Remote terminal units have become very compact and cost-effective for gateway installations in conjunction with intelligent devices. Even without analog and digital I/O, this RTU provides local communications via Ethernet and RS-485. The integral cellular modem links to a SCADA network.

Wide Area SCADA Network Interface

The key function of a gateway is to provide an interface to the wide area SCADA network. Most remote, intelligent devices lack the communication capabilities that are necessary to interface with a SCADA network.  The simplest gateway products on today’s market connect to local devices via Ethernet, RS-232 or RS-485 and use cellular modems to get onto wide area networks. Some gateways offer a choice of modems, including public switched telephone (PSTN). However, if the SCADA network uses a technology other than cellular or PSTN, the traditional gateway will not provide the wide area interface. Only RTUs offer a choice of wide area network physical interfaces that includes cellular, PSTN, licensed radio, spread spectrum radio and satellite.

Protocol Conversion

The simplest gateways use only a single, communication protocol such as Modbus and, thus, provide no conversion.  For example, a gateway will collect information from drives and smart meters via RS-485 Modbus or Modbus IP on Ethernet.  The gateway buffers the information and transmits it on the SCADA network using Modbus or Modbus IP through the cellular or PSTN modem.

More sophisticated products address a reality of existing installations. These systems have grown over many years and use a mix of protocols.  Common protocols for SCADA networks include DNP3, IEC 60870-5 and Modbus. Fastest growing are IP implementations but many, proprietary protocols continue to be used. While Modbus is also popular for remote devices, this area is diverging with growth in Ethernet, HART, 802.15.4 protocols and a surprising number of proprietary, wireless networks.  For a gateway, the offering in terms of protocols scope, including proprietary interfaces, will be a key consideration. Since most gateways lack such a broad range of protocols, this consideration turns engineers to RTUs.

IP Forwarding

With an IP network that extends to the end device, shouldn’t you be able to configure it without traveling to the site?

Although it would appear to be a natural capability for gateways that support Ethernet or IP networking, IP forwarding remains a differentiator for a small number of RTU products. Implementation of network address translation (NAT) is a key consideration. NAT allows the RTU to listen on a specific TCP/IP port number and forward requests to the appropriate IP address. This ability to listen and forward the messages without interpreting or storing is important. It allows not only uploading of process information; it also provides the ability to download configuration settings to the remote device. In many cases, this means that a trip to the site is not necessary.

As a remote gateway, an RTU uses Ethernet and serial interfaces for networking with intelligent devices, IP cameras and wireless sensor networks. Integral communications such as a cellular modem are used to interface with a SCADA network, serve web pages and deliver multimedia messages to users anywhere.

Programmable Automation

Practically by definition, gateways do not include programmable automation—nor do common, remote devices such as drives, video cameras, wireless I/O modules and wireless sensor network hubs. RTU products with programming capabilities ranging from ladder logic through the full, IEC 61131-3 suite and beyond continue to fulfill all monitoring and automation requirements at remote sites.

While it is possible to add a PLC to a gateway, the combined cost exceeds that of an RTU. In addition, the systems integrator must deal with two configuration environments—gateway and PLC—rather than the single environment afforded by the RTU.

Alarm Management

Most intelligent, remote devices are very good at reporting alarms via Ethernet or a serial port.  Some provide extensive arrays of alarm function codes.  But it is left to a device “up the network” to manage the alarms. While gateways offer limited capabilities, alarm management in an RTU is perfect for handling intelligent devices.  Rather than simply passing alarm reports through to the SCADA network, an RTU can provide significant processing.

Group or zone management allows the RTU to consolidate alarm reports when a large number arise from the same device or process.  This can occur during start-up or when power is restored.  Instead of passing through a high volume of irrelevant messages, the RTU will very efficiently indicate a single, overall condition.

An RTU can also manage recipients.  In many systems, today, alarm messages are transmitted to multiple recipients in maintenance, management and operations. An RTU can be readily configured to route messages only to appropriate personnel, even to the point of knowing which shifts they work during the day. The RTU also manages alarm acknowledgement and escalation when an alarm is not acknowledged.

Finally, by taking advantage of programmable automation capabilities, the RTU can execute logic operations in response to specified alarms.

Data Base Management and Data Logging

Over Ethernet and serial interfaces, today’s intelligent devices are able to produce large volumes of information.  Whereas a discrete device would output only a single, process variable, intelligent devices can deliver multiple process variables plus hundreds of alarms, configurable parameters, diagnostic indications and status flags.

While a gateway includes significant memory for buffering in order to prevent loss of information, an RTU offers much more.

An RTU will be able to process information from intelligent, remote devices in a manner that is useful to the diverse requirements of accounting and asset management as well as operations and maintenance.  Many utilities are employing smart systems for analytics, modeling and optimization. Diagnostic and status information from intelligent devices feeds these top-end, smart systems. An RTU performs the filtering to allow the smart system to focus on relevant or significant information.

An RTU can combine statistical operations such as averaging and totalizing with historical data management to maintain long-term information on assets and processes. These can be further combined with calculations such as corrected flow from meters to provide a full commodity accounting. Standard calculations are fundamental to industries such as oil & gas. RTU data base management also includes an audit trail, which is a necessity for any accounting functions.

Conclusion

While a gateway-class product fills the need to link intelligent devices in remote installations to SCADA networks, an RTU in a gateway role offers considerably more. In addition to basic gateway functionality, the RTU supports protocol conversion, IP forwarding, programmable automation, alarm management, and data base management. This functionality complements the capabilities of intelligent devices and enhances the value of the remote installation.

About the Author

Kevin Finnan is Vice President of Marketing for CSE-Semaphore, a manufacturer of Internet-enabled automation, SCADA and telemetry products.  Prior to joining CSE-Semaphore, he held various positions from Systems Engineer through Director of Marketing at Bristol Babcock.  He has 30 years of experience with flow computers, measurement instrumentation, process automation, security, and SCADA systems over a broad scope of vertical markets including oil & gas. He has been active on boards, presenting papers or teaching classes in numerous industry organizations such as ISA, Entelec and natural gas short courses.