Articles and Technical Papers
Instrumentation & Control Systems Magazine
Data-Linc specializes in industrial grade, custom configured, high performance data communications solutions for the industrial automation market. This article addresses the steps taken by Data-Linc to establish the requirements of its Strategic Alliance Partners (Allen Bradley/Rockwell Automation, GE Fanuc Automation, Schneider Automation, Siemens, etc.) for development of a license-free frequency hopping spread spectrum radio modem capable of satisfying the challenging environment of the industrial automation customer. Five case studies are presented that illustrate the specific difficult conditions of high interference, temperature extremes, shock and vibration, high electromagnetic interference as well as large DCS/SCADA installations. Data-Linc manufactures the broadest line in the world of industrial grade modems including wireless, FSK wire, dial-up/leased line, and fiber optic as well as specialty modems.Michel E. Maes and James R. Steffey
2635 151st Place NE
Redmond WA 98052
Wireless data communications for industrial automation applications continues to expand rapidly and a variety of alternatives, both licensed and license free, have been marketed to meet the demand. As the technology develops, higher performance as well as improved features and benefits are making the products increasingly popular for installations that otherwise would have typically required microwave, wire, fiberoptic or dial-up/leased line solutions. Data-Linc Group, located in Redmond, Washington, manufactures the broadest line in the world of industrial grade, custom configured, high performance, modems providing the ultimate in connectivity, interoperability and compatibility with industrial automation equipment. Consistent with their prior reputation for providing an exceptional level of data communications reliability and integrity by utilizing FSK (Frequency Shift Key) technology for data transmission over wire, the company had to be particularly careful in the selection of a wireless technique. Data-Linc consulted with its principal alliance partners, including Allen-Bradley, GE Fanuc and Groupe Schneider (Modicon/Square D/Telemecanique) to obtain their input on the desired performance and characteristics of a radio modem. In order of priority, data communications reliability, license free operation, rapid turnaround delay, dependable 19.2 Kbaud data rate and maximum range capability ranked consistently high. Users also wished to avoid line-of-sight restrictions.
For the purpose of devising a strategy designed to prioritize the most desirable features and benefits, Data-Linc recognized that data communications reliability and integrity, a critical requirement, would force the selection of frequency hopping spread spectrum technology for its radio modem product line. Spread spectrum development had already benefitted from decades of government funded research by DARPA (the Defense Advanced Research Projects Agency) for application in military battlefield communications, a particularly challenging environment, where the maintenance of reliable data communications is clearly paramount. Although implementation of the technology for industrial automation installations would normally involve fewer interference problems, the maintenance of data communications integrity, even under the worst of conditions, remains of undeniable importance to the end user. Two common methods of implementation exist: Direct Sequence and Frequency Hopping. Each offers distinct advantages and disadvantages. In short, direct sequence spread spectrum radio modems are most often supplied where the highest possible data rates are required. The technology is capable of supporting megabaud transmission speeds. Unfortunately, it is possible to lose communications in environments where the received signal power is less than that of a local noise source at the same frequency. For industrial data communications, in which the manufacturers of Programmable Logic Controllers (PLCs) and Remote Telemetry Units (RTUs) require an exceptional level of data communications reliability, frequency hopping techniques can be used, layered on the spread spectrum technology, to avoid interference problems. Data-Linc selected this method, implemented in the license free 902-928 MHz and 2.4-2.4835 GHz bands, to provide worldwide operation (no site license required by the end user), because of its ability to survive even the most adverse interference environments.
It was important to implement a significant number of frequency hops to ensure a high level of confidence that interference problems could be overcome. For most applications, Data-Linc radio modems cycle repeatedly through 112 frequencies (96 frequencies for most countries outside North America), all of which are within the license free band. The bandwidth of each frequency utilized for data transmission is 630 KHz for units operating in the 902-928 MHz band. Any segment of the full operating band, such as 917-928 MHz for example, can even be supplied where cellular or paging transmissions would otherwise result in contention problems. The hopping pattern is established in accordance with a pre-set algorithm for the purpose of maintaining synchronization of the Master and its Remote (or family of Remotes). An important additional benefit, as a result, is the ability to operate multiple pairs or Master/Remotes SCADA systems in close proximity, without risk of interference, by setting individual algorithms for each set. Any data entering the buffer of a Data-Linc SRM6000 frequency hopping spread spectrum radio modem is immediately packetized and transmitted (no RTS signal required). A single packet is sent at each frequency hop and is subjected to 32 bit error checking by the Remote. Good data packets are released from the buffer of the Remote to the attached device while invalid packets are rejected and retransmitted at the next frequency hop. All Remote transceivers and their Master continue to hop after transmission of each individual data packet regardless of the validity or invalidity of data, thereby maintaining synchronization of all units. Error checking every data packet, including the retransmission of any packets determined to be invalid, will obviously result in a decline in net data throughput. In other words, prioritizing data communications reliabilty and integrity will potentially jeopardize the required PLC line speed of 19.2 Kbaud where interference is encountered if the radio modems themselves are designed to operate at 19.2 Kbaud. Data-Linc modems operate, therefore, at 115.2 Kbaud in order to assure that required PLC line speeds do not suffer even in worst case adverse environments. Furthermore, the radio modems are also capable of a 5 to 15 millisecond turnaround delay between the Master and successive Remote units. This combination of high data rate and rapid response time has established a new performance standard for frequency hopping spread spectrum radio modems, capable of providing an exceptional level of data communications reliability without sacrificing required net data rates for RS-232/422/485 communications. Distance capability, as a result of continuing enhancements in the sophistication of the software for data recovery, has steadily improved and currently ranges from 20 to 30 miles (1/3 watt to 1 watt transmitted power output) with an installed 3 inch whip antenna. At these distances, there is a strict line of site requirement which can be achieved with any approprite combination of external antennas or Repeaters (up to two Repeaters in any Master/Remote combination, thereby supporting a maximum range of 90 miles). For most applications, the line of site requirement does not apply at distances of less than one-half mile but solid metal walls or earth barriers will always prove an insurmountable obstacle to successful data transmission making an external antenna necessary in such circumstances. It should also be noted that operation of the modems in the 2.4-2.4835 GHz band results in a significant reduction in range. LEDs are provided for easy diagnosis of both radio and data flow functions. Configuration flexibility allows point-to-point, point-to-multipoint (polling), addressing or address transparency, dial-up mode, random response and two tier multi-drop capability. The SRM6000 is being employed in many different applications from low speed monitoring and control in Water/Waste Water plants to high speed control of stacker cranes. With a wide temperature operating band of -40C to +75C, the SRM6000 is at home in virtually any outdoor location. Its rugged, completely solid state construction makes it impervious to vibration and shock. It will interface with any asynchronous serial data stream at TTL, RS-232, RS-422 or RS-485 signal level. The SRM6000 can be operated from 120/240VAC or 10 to 35VDC. With a receive current draw of only 100mA, solar cell/battery operation is very practical.
It is unusual to find the convenience of wireless operation combined with exceptional radio modem performance. The Data-Linc SRM6000 has succeeded in providing such a combination. It can be found in many facilities, providing simple installation and reliable, trouble free service. Several examples of such service are described below.
1. High Interference Environments
2. Temperature Extremes
3. High EMI Environment
4. SCADA Installation
5. High Vibration
Because of its particular focus on the industrial automation market, Data-Linc has had to deal with a wide variety of the most challenging operating environments and conditions. In the process of selecting a viable technology to address the requirements of its automation alliance partners, Data-Linc has succeeded in providing a license free radio modem exceeding the desired performance characteristics specified in its original study. Freedom from line of site restrictions could not be achieved, however, at the longer distances the Data-Linc SRM6000 is capable of reaching. At up to one-half mile, the line of site restriction usually is unnecessary but exceptions to this rule of thumb are when solid barriers such as earth berms or metal walls are encountered. Line of site may not be required at even up to one mile but beyond that range Repeaters or external antennas will normally be required (up to two Repeaters per Master/Remote combination can be employed in each leg of a multipoint system). Finally, Data-Linc often provides special features and capabilities for its industrial automation partners available from no other manufacturer. For example, the SRM6000 Radio Modems support remote programming and register down load on the programming port using the SNP/SNPX protocol unique to GE Fanuc Automation. In addition, the company factory configures its modems and connecting cable for each location within an installation to assure a "no brainer" installation (no modem field settings, programming or adapters to hassle with). This attention to detail and level of service and support should be expected of any manufacturer servicing high value-added markets such as the industrial automation customer.
Michel E. Maes, President, and James R. Steffey, Vice President, both graduated in Physics from the University of Washington in the late 1950s. They founded Data-Linc Group in 1988 following many years of research and development efforts devoted to modem communications.
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