Paper - BSC Engineering
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1. be Ethernet compatible All communications to from this port are seen as Ethernet The designer may need to specify an Ethernet IP address and perhaps little more The second interface port may for example be fiber optic Minor configuration if any is required on the fiber optic port A fast microprocessor or pair of microprocessors handle s the OSI model tasks Data is temporarily stored within the internal memory until it is required on the other side of the gateway A communications user on one side of the gateway is usually not aware it is communicating with a different communications media protocol When communicating data from the field to the host and onwards to control functions it is usually multiplexed Multiplexing enables many channels of data to be digitized and then sent as incorruptible data packets to the host computer or other site By incorruptible is meant that the OSI model firmware hardware automatically enables the Ethernet link to communicate via digital data packets not analog Ethernet has a major advantage over other communications media since it is capable of handling a tremendous amount of data gt 10 Mbits second When collecting data from individual sensors the multiplexer also needs to isolate the data signal levels from each other to prevent data corruption including ground loops and common mode signals Setup for each channel can be on block level all the same setups or channel by c2. pair may be set up at each end of this same twisted pair cable thereby providing hundreds of channels communicating on the original twisted pair As future growth occurs and an Ethernet LAN capability is brought to these sites then the data concentrator s MIM modules may be removed and replaced by EIM Ethernet Interface Modules Connection is removed from the twisted pair and placed onto an n Ethernet LAN cable A simple IEC 61131 compliant software configuration program is written to transfer data back and forth amongst the various data concentrators Figure 4 Data concentrator in a Peer to Peer network Data Concentrator Date Concentrator Distributed Ethernet VO Up to 64 WO Distributed Ethernet O par NCS Staton m E ie ton Ip Wo 32 Siira CT C a ee par Not woe Ethernet Communication Link MODUS TCP OS a x Hi Additional NET Concentmter System WO ker Future E epeneiees Thus the customer receives an immediate growth capability without laying new cables This may be expanded by adding I O modules and even additional data concentrator systems Finally as Ethernet cables are made available in the future the data concentrator communications MIM modules are swapped with EIM modules for communications media upgrade Negligible waste is incurred in this process Site 2 A tank farm application collects sensor data from multiple sources It is not economical to run cables However n
3. Peer to Peer cable pathway allows devices to communicate with other devices without the need for a host computer There is sufficient addressing in the participating devices to provide the timing and direction of data transfers An example of Peer to Peer is a flow meter communicating to an intelligent Distributed with permission of author s by ISA 2010 Presented at ISA Automation Week 2010 http www isa org valve actuator Another is a single or group of instruments connected to an HMI Human Machine Interface panel The Ethernet communications link may also be a Hybrid interconnection This is a combination of Peer to Peer plus Peer to Host cable pathways This enables communications throughout the facility in the most effective manner Ethernet cable connections are effected via Ethernet LAN switches The LAN switch is a microprocessor based piece of hardware and firmware that regenerates Ethernet signals for extending communications ranges LAN switches also provide selective data routing of data packets onto only desired cable paths so unnecessary data packets are not repeated onto all other cable paths LAN switches also provide security user configurable to prevent unauthorized users from sharing the data packet stream A VPN Virtual Private Network consisting of front office network computer another VPN consisting of Production control room computers and a third consisting of selective off site uses IP address
4. advantages of these modern instruments valves alarms and communications The key to success is to use the best sensor valve PLC and communications methods in each respective area To achieve this optimization it is paramount that all of these products use popular open architecture protocol and popular standardized communications In the past various products from different manufacturers used proprietary protocols or communications methods such that the best sensor required special processing conditioning hardware and or software before it could be married with other system data This led to many large systems using products from sensors to valves to host computers and application software from one of the big firms Let us take a look at how this earlier hurdle is now being reduced in magnitude by today s industrial manufacturers The OSI Open System Interconnect Model Figure 1 identifies the various layers necessary for a freshly captured temperature sensor signal to be processed on the Application Layer Level 7 and then progressively handled by software firmware and hardware until it exits out the bottom onto the Physical layerl RJ45 Ethernet cable connector on the an Ethernet cable When the data is collected by another instrument or host computer it would travel upwards through the seven layer pathway However since this second device was different it required some similar and some different additional extensive engin
5. optimize the best available sensors controls and communications links to meet our own particular process needs We do not need to stay with one vendor for all of our process needs The protocol converters mentioned in this paper emphasize the use of Ethernet based conversion throughout due to vendor s implementation of Ethernet and its Distributed with permission of author s by ISA 2010 Presented at ISA Automation Week 2010 http www isa org inherent OSI stack Custom driver designs are now built in to the instruments and the protocol converters thereby virtually eliminating compatibility issues Design engineers only need to be concerned about their own application top level of the OSI model and the electrical connections such as IP addressing and RJ45 connections bottom level of OSI model Successful vendors are integrating this flexibility when possible This viable flexibility enables us to now have a competitive advantage in designing and revitalizing our own manufacturing processes After reading these pages the reader should have learned at least these three main points about Ethernet T O e that Industrial Ethernet is becoming a de facto and open standard for control system network communications e how Ethernet I O systems can be used as a relatively low cost method of bringing field devices from your plant into a computer SCADA or control system using the Ethernet e how these systems can be configured qu
6. Industrial Ethernet Brad Stephen Carlberg P E Jim McConahay P E Control Systems Engineer Sr Field Applications Engineer BSC Engineering Moore Industries International Inc Richland WA North Hills CA 251 454 1200 818 894 7111 ext 320 brad carlberg bsc engineering com jmcconahay miinet com Abstract Industrial Ethernet is becoming a de facto and open standard for control system network communications Controllers and instruments can communicate with each other via MODBUS Ethernet and via OPC making tag databases and real time process data available anywhere there is a connection to the Ethernet Many instrument and control vendors offer modular Ethernet I O systems that can be used as a relatively low cost method to bring information from plant field devices into a computer SCADA or control system using Ethernet Able to be quickly configured they communicate with one another as well as with other computers over standard Ethernet using off the shelf Ethernet hubs switches and or routers Then data acquisition and control data can be viewed from a browser a spreadsheet or a third party HMI package Keywords 802 3 CIP Common Industrial Protocol COM DCOM Ethernet hubs Ethernet IP Ethernet router Ethernet switch Ethernet IP Fieldbus Interface Module FIM Foundation Fieldbus FTP HMI HMI Human Machine Interface HSE High Speed Ethernet HTTP I O systems IEC 61131 Modbus Modbus RTU Modbus TCP ODVA Open D
7. Manual 00809 0100 4023 Rev DA March 2006 Emerson Process Management 4 ISaGRAF Version 3 4 USER S GUIDE 2000 CJ International Rosemount 1420 Smart Wireless Gateway Reference Manual 00809 0200 4420 Rev BA October 2009 Emerson Process Management 700 Series Managed Industrial Ethernet Switch User Manual amp Installation Guide Revised 2010 7 30 Copyright N Tron Corp 2008 2009 820 S University Blvd Suite 4E Mobile AL 36609 USA 7 NCS EIM Net Concentrator System data sheet Revised 2009 Moore Industries International Inc 8 MNCS Modbus RTU Master and Distributed I O System Installation Manual Revised December 2009 Moore Industries International Inc After The Explosion Jim McConahay Moore Industries International Inc Control Engineering September 2009 Distributed with permission of author s by ISA 2010 Presented at ISA Automation Week 2010 http www isa org
8. an on the maintenance ship announced he was pressing the alarm button About one second Distributed with permission of author s by ISA 2010 Presented at ISA Automation Week 2010 http www isa org later the klaxon was heard from the oil platform control room via the telephone The alarm was successful The alarm switch was cleared and the klaxon immediately silenced Next to simulate a communications fault the Ethernet cable was removed from one of the data concentrators After a few seconds the communications alarms simultaneously activated on both oil platform and maintenance ship Upon reconnecting the cable the communications were successfully restored at both sites within about one second The data concentrator system meets MMS directive for reliable communications for this safety task The system is put into service on schedule The system remains in service and is reliable for all activities and proves to meet all expectations When severe weather briefly obstructs the satellite communications link the data concentrator briefly also goes off line with its appropriate signaling lamps Upon satellite link restoration the data concentrator remote ESD recovers immediately Further application Due to the reliability and simplicity of the data concentrator system ENI engineers have decided to add telemetry capability to each data concentrator The telemetry is bi directional By merely adding a DB25 cable to each data con
9. at communicates via MODBUS TCP The MODBUS data concentrator Ethernet communications cable is connected to an Ethernet to fiber optic protocol converter This fiber optic converter operates transparently from Ethernet to fiber optic and then back to Ethernet Fiber optic is preferred when the Ethernet signal needs to be linked from one building to another on the same facility Figure 5 A MODBUS RTU data concentrator expands the number of signals that can be collected and transmitted back to the control room lank Farm Communicating Via Wireless HART oN AN P EEn S fon Data Concentrator Distributed Ethernel MODBUS ATU I O Acts as Network Master Wreless eee HART to MODBUS ATU Converter Control Room Ethernet IMODBUS TCP i I I j 4 PA fam ty ptt Ethernet Display MODBUS TCP Ethernet Fiber Optic to to Fiber Optic Ethernet Fiber Optic Communication Link Converter Converter At this facility the customer has multiple requirements multiple time tables for upgrade levels and tight constraints on budgets Protocol converters bring the applications from dedicated twisted pairs for single channel activities to a growth path providing progressively more and more data per cable This customer facility is adapting with progressive improvements to meet his her interim budget and goals without waste along the way Conclusions Thanks to rapid technology improvements we are now able to
10. centrator ENI is able to add up to 14 additional I O Input Output data concentrator plug in modules for transferring digitized analog and discrete signals temperature pressure flow and even feedback control signals Since the Ethernet packet is burst in just a few hundreds of microseconds for each data concentrator there is plenty of extra time available for the added telemetry on a non interference basis ENI now has a powerful communications method to operate safely at any communications range Distributed with permission of author s by ISA 2010 Presented at ISA Automation Week 2010 http www isa org Figure 3 Two way communications via Ethernet and satellite Ship Control Room Oil Platform Control Room 100 km Distance e Mecharscaiy i satohing i i Pushbarton Concentrator sin Aarm Switch Coolact D et lj i Data aa i Concentrator Distributed i j Gihernet vO Tra l WRaxon Hom mak _ Case Study 3 Wireless HART and 4 20 mA Loops for Use at Multiple Sites Problem An asphalt plant in the mid west is expanding This involves upgrading its sensors and communications links throughout their facility Some control loop tasks were originally performed only locally Other tasks were under the control of a host computer Now tasks are to be simultaneously controlled monitored or updated from the central host computer There are not enough of the original 4 20 cur
11. data was disassembled and placed into variables and memory locations to activate relay contact closures as required A contact closure at one data concentrator activates a relay on the second data concentrator Thus a one way communication was established Each data concentrator has a unique IP address The EIM internal Ethernet communications addressed and directed all levels of the OSI stack so the programmer only needed to provide correct addressing the EIM handled all communications protocol retries collisions dropouts etc The maintenance boat data concentrator has a mechanically latching pushbutton switch The oil platform data concentrator has a klaxon horn plus a set of contact closures connected to the control panel wired in failsafe mode During normal operation of the data concentrator provides operator visual indication audible alarm klaxon and electrical signal connections to the control room panel The MMS directive also requires a reliable communication We need to verify there is continuous communications back and forth between the two data concentrators If not then a communications alarm is automatically initiated Normal communications are set up for bi directional updates of all data within each data concentrator every one second A watchdog subroutine is added to each data concentrator If a successful communication is received from the other data concentrator then the watchdog timer is restarted and a gree
12. devices the FIM the protocol converter and the PAC were connected to the managed switch Finally the protocol converter was configured in an object oriented configuration language to connect the FIM with the PAC Figure 2 Protocol converters provide interface between devices that use dissimilar communication methods Distributed with permission of author s by ISA 2010 Presented at ISA Automation Week 2010 http www isa org Protocol Converter Converts Modbus TCP to Ethernet IP Foundation Fieldbus Managed Interface Bhen tow O TT Module Switch FOUNDATION Fieldbus H1 Local L HMI Display FOUNDATION Fieldbus Transmitters po dich Programmable Automation Controller PAC Case Study 2 Remote ESD Emergency Shutdown Device for Oil Production Platform Problem ENI Petroleum is the operator of an oil well production platform located just off the coast of Louisiana Part of their operation involves the use of long undersea pipelines plus occasional maintenance ships that perform repair and other services in close proximity to their pipelines For safety the Mineral Management Services MMS has notified the platform owner that a direct real time communications link needs to be installed and a plan in place should any wells be affected by a possible event such as a dropped object in the vicinity of a submerged pipeline Further the communications link needs to be proven reliable with a v
13. earby it is necessary to collect this data convert to readability by an HMI panel and also communicate it to the host computer in the Control Room via a fiber optic communications link Tank farm data is collected via wireless HART using a Wireless Gateway Sensor data is collected from the wireless link and stored in registers within the gateway A MODBUS data concentrator has both MODBUS RTU and MODBUS TCP ports The gateway is wired via its Modbus RTU serial port to one of the MODBUS data concentrator serial ports A Scheduler text file is loaded into the MODBUS data concentrator to enable it to operate as a MODBUS RTU master thereby extracting selected HART data from the gateway Now that the data is being automatically transferred into the MODBUS data concentrator an IEC 61131 compliant software configuration program is loaded into the data concentrator to massage the raw HART data into human readable form scaling binary to decimal conversion etc and organizing for display on a local HMI panel Tank farm data is now usable for humans The MODBUS data concentrator can also Distributed with permission of author s by ISA 2010 Presented at ISA Automation Week 2010 http www isa org use this same collected data to perform control loop functions with sensors and corresponding valves thereby offloading some tasks from previously being controlled by the host computer The MODBUS data concentrator also includes an Ethernet port th
14. eering again To be sure it was very convenient for the system designer to throw up his her hands and concede to having one manufacturer provide all of the system instruments and communications and application programming just to avoid even attempting such a compatibility terror Distributed with permission of author s by ISA 2010 Presented at ISA Automation Week 2010 http www isa org Figure 1 Open Systems Interconnect Model Simplified Customer s MODBUS application layer on computer _______ gt These areas ara automatic to end user Implemented by vendors complying with 802 3x standards Thus negligible concern for customer Customer assigns IP address and cormects LAN cable here gt The popular Ethernet 802 3x standard uses this same seven layer approach Ethernet that we see as designers involves physical hardware RJ45 connectors Ethernet LAN cables LAN switches routers and gateways and some indicator LEDs It also requires us to set a few communications addresses The intermediate addressing and formal communications protocols and their associated timing retries error corrections etc to assure good quality communications are transparent to us The uppermost layer of the OSI is data and our own application program that routinely works with this data In the past there was a major hurdle of inter vendor compatibility This impediment is now essentially gone We can now frequently use sensor
15. erification method Wells and pipelines need to be shut down production stopped should a potential hazard to the pipeline occur or if communications is lost To comply with this directive ENI has been using short range radio links one in their control room with the other on the maintenance boat Now ENI is starting pipeline work which will expand to a distance of greater than 100 km from the platform Normal radios are not reliable for this over the horizon required communications link Considerations Communications solution must accommodate any transmission distance even gt 100 km Reliable communications with built in verification and security Two way voice radios are not appropriate to meet MMC requirements Simple HMI automatic zero to near zero learning curve Suitable for use on platforms as well as on any maintenance ships Installs readily with existing control room electronics on platform and on maintenance ship Solution The conversation initiating EMI s requirement was via telephone VoIP Voice over Internet Protocol This technology compresses voice messages into packets to fit on an Ethernet network The oil platform uses Ethernet communications for their telephone emails weather internet surfing and communications Distributed with permission of author s by ISA 2010 Presented at ISA Automation Week 2010 http www isa org with the mainland The Ethernet cabling is readily available in the contro
16. es and security password Such precautions are intended to keep all Production activities away from the curious eyes and fingers of unauthorized personnel yet facilitate control and monitoring of all facility activities A facility with thousands of 4 20 mA current loops may be multiplexed onto a single Ethernet cable with actual improvement in data transfer speed resolution and accuracy Case Study 1 FOUNDATION Fieldbus Transmitters into Programmable Automation Controller Problem A client wanted to monitor and control the fluid flow of a process containing several FOUNDATION Fieldbus flow pressure and level transmitters and variable frequency drives on pumps with a Programmable Automation Controller PAC The PAC communicated with the variable frequency drives via a DeviceNet card in the PAC backplane but could not communicate with the FOUNDATION Fieldbus transmitters since they couldn t communicate via MODBUS TCP and an Ethernet TCP IP card in the PAC backplane Solution A MODBUS TCP to Ethernet TCP IP protocol converter and a managed Ethernet switch were installed between the FOUNDATION Fieldbus Interface Module FIM and the PAC The MODBUS IP registers for each of the process variables for the FOUNDATION Fieldbus transmitters were mapped in the FIM using browser based configuration software Then corresponding global tagnames were built for each transmitter in the PAC with its configuration software Next all three
17. eviceNet Vendor Association OPC OLE for Process Control OSI Open System Interconnect PAC PLC PPP Profibus PA Protocol Converter RJ45 SCADA SMTP SNMP TCP TCP IP frames TCPIP UDP VPN WAN Introduction Electronic communications from one instrument to the next is rapidly becoming requisite best engineering practice in the design and upgrade of any industrial sites In the past it was acceptable for sensors valves machines and processes to perform their routine tasks with only occasional settings updates and maintenance by multiple process operators and or technicians each carrying clip boards This is no longer practical and economical for the efficiencies required by today s industry The rapid emergence of international manufacturers that use the latest in modern technology and low labor costs is rapidly antiquating earlier designs processes and instrumentation approaches Further increased awareness of safety profit point and real time monitoring required to tweak the greatest productivity at lowest cost and least risk to personnel the environment and the stockholders is further pushing the performance peaking requirements of virtually every industrial facility To achieve this improvement to be competitive and peak in profitability we now need to redesign our processes and adapt The most successful method is to review our current process method and compare it with the latest methodology then adapt All new facilit
18. hannel preferred At the other end of the communications link may be the host computer By retaining the Ethernet communicated digital packets in digital format on the full length of the Ethernet cable there is no loss of resolution accuracy of the original signal data Some legacy host computers may only accept analog input signals If so an Ethernet based demultiplexer is used to re create the signals from their digital packets back into analog voltage or current signals There is a slight loss of data resolution at this D A interface immediately followed by the host computer own A D Consider the cost savings and complexity reduction by replacing costly legacy A D and D A I O cards with a single Ethernet I O card on the host computer Signal pathways in legacy systems such as 4 20 mA current loops are single direction Ethernet and other digital communication systems share the cable pathway permitting communications in both directions The advantage is reducing cable count immediately The OSI model incorporated into the electronics of sensors actuators and other communications equipment automatically handles the Ethernet communication signal timing handshakes data collisions resends and verification of data integrity It is not a single pathway There are multiple methods to route the Ethernet cables throughout the facility A simple Peer to Host computer cable path connects a single sensor or multiplexer to the host computer A
19. ickly and communicate with each other AND other computers over standard Ethernet using off the shelf Ethernet devices It is the authors hope to have also educated the readers about Ethernet hardware and software tools Specifically e Commercial off the shelf hardware Adapter cards switches Category 5 6 cable high power wireless LAN e Industrially hardened hardware Redundant switches self healing topologies industrial connectors e Freely available software troubleshooting tools Packet sniffers traffic monitoring SNMP management e Knowledge base of today s engineers Educated on and familiar with Ethernet technology at all levels Look forward to the benefits of even more compatibility to come Acknowledgements We must acknowledge Walt Brunner of Columbia Electric Pasco for expertise and support with ControlLogix and FactoryTalk View Steve Frate and Jesse Benefield of Red Lion Controls for their expertise and support with the DSPLE Protocol Converter and Russ Muller of Emerson Process Management for his expertise and support with the Fieldbus Interface Module Without their help the implementation of Case Study 1 would have been much more difficult References ControlLogix via Red Lion DSP Jesse Benefiel Red Lion Controls amp Russ Muller Emerson CRIMSON 3 USER MANUAL Mike Granby and Jesse Benefiel Copyright 2003 2010 Red Lion Controls Inc Rosemount 3420 Fieldbus Interface Module Reference
20. ies need to be designed from the ground up to compete with offshore competitors Existing facilities need to progressively upgrade If well planned the increase in productivity and profitability will promptly pay for the improvements at each step If it is a municipality there may be little competition from abroad However the real need is the same for increased productivity in the face of diminished taxpayer funds and the need to minimize the installed cost to customers for the product Distributed with permission of author s by ISA 2010 Presented at ISA Automation Week 2010 http www isa org To assist us today we have modern instruments with built in intelligence that perform single or multiple tasks have integral communication ports Ethernet serial fiber optic wireless etc and use open sourced communications protocols MODBUS TCP MODBUS RTU HART FOUNDATION Fieldbus PROFIBUS PA OPC HSE Profinet CIP to name a few Modbus TCP an open protocol in which Modbus packets are encapsulated in TCP IP frames is used because Modbus is widely known and supported worldwide OPC OLE for Process Control uses Microsoft s COM DCOM NET standards for open comm OPC can run over any physical medium and is an open free of charge protocol that both hardware vendors and software vendors have access to Many supported development tools OPC has a strong backing by the OPC Foundation HSE High Speed Ethernet is a Foundation Fieldbus pr
21. l room and critical areas of the platform The maintenance boat is large enough that it also uses satellite communications for the same important tasks The system consisted of a simple NET Concentrator System NCS data concentrator hardware system that mounts on DIN rails The data concentrator system consists of two EIM Ethernet Communications Modules The first EIM is matched with a Discrete Input Module DIM and is mounted in the Ship Control Room The second EIM is matched with a Relay Output Module ROM mounted in the Oil Platform Control Room The control programs run on the kernel level of the data concentrator Two data concentrator hardware systems are assembled Communications in between the two data concentrator systems are via Ethernet cable Addressing is via IP addresses The system used a IEC 61131 compliant with the five standard languages ladder function blocks structured text sequential function charts and instruction lists software configuration program to set up the two data concentrator systems to collect contact switch input signals timers and communicate back and forth with each other EMI then developed their own application software to scan the input switch contacts and combine these into digital memory locations The data concentrator s EIM module then converted these contact closures and packaged them for transfer via Ethernet cable to the other data concentrator At this second data concentrator the Ethernet
22. n color communication status lamp is illuminated for this data concentrator Should several communications dropouts occur then the watchdog timer times out and the communications status changes color and a separate set of contacts is sent to the control panel Should further watchdog time delay occur then another set of contacts is closed and sent to the control panel signaling end of grace period for down communications The system successfully tested well on the bench The data concentrator was secured via password protection Next the Ethernet port assignments needed to be activated to communicate all data via the Ethernet cable This information was forwarded to the satellite link Internet providers The ports were opened for bi directional access between the two data concentrators It was also opened for the laptop used by the service technicians so the technicians could monitor any activity between the two data concentrator by tapping into the Ethernet cable at select locations on the oil platform and also on the Maintenance ship The satellite link and on board Ethernet technicians thus provided added security that this critical information was protected from be shared by any unauthorized Ethernet visitors Upon activation of the data concentrator system both data concentrators were powered up Within one minute the communications lamps on both data concentrator were glowing green color as they should The technici
23. otocol runs over Ethernet and is an open protocol and has a large organized support base by both vendors and the Foundation Fieldbus Organization Profinet is the Profibus Trade Organization s answer to open communications It is NOT Profibus over Ethernet Reason Strong backing by PTO Can use 802 3 layer but not restricted to this layer and can run over modems WAN s VPN s as long as it uses TCP IP addressing CIP Common Industrial Protocol is being promoted by ODVA Open Device Vendors Association uses a Producer consumer network philosophy for real time control applications With IP and TCP or UDP the use of Ethernet among other technologies such as wireless or PPP works Same at the top many protocols are available based on the task that needs to be done Better yet those application protocols can be used simultaneously Many of these communications protocols have built in error reporting and self diagnostics for use as a preventive warning prior to performance degradation In the case of valves some include internal routines to verify responsiveness as to actual positioning There is a select group of instruments specifically designed for the highest reliability to be used in key safety related positions complete with appropriate SIL certifications When modernizing industrial designs for profitability and safety we will want to replace earlier vintage instruments controls and methodology to capitalize on the significant
24. provide universal products that provide for simple communication of process and control data and most important communicate seamlessly with one another Distributed with permission of author s by ISA 2010 Presented at ISA Automation Week 2010 http www isa org The opposite of open connectivity is proprietary Proprietary is fine for unique applications where no other vendor product can provide the extra level of precision or other parameters critical to the task We pay a premium since proprietary requires the customer engineer to invest extra dollars for special communications unique setups and custom data conversion algorithms to make the proprietary data be able to interact with the rest of the system There is no ideal communications media Twisted pair fiber optic wireless and Ethernet each has its merits Each is found in virtually every facility Each has advantages of cost speed plant wiring infrastructure data switching and power to instruments and actuators No one media or protocol solution should be forced onto a given site We should design for the optimized communications media and protocol for the application So what if our ideal communications and protocols for one area of the plant do not match Introduce the Protocol Converter also known as a gateway A gateway is both hardware and firmware It has two interface ports plus a shared memory in one overall package One interface may for example
25. rent loop communications links to handle all of the sensors and control pathways Considerations e Use existing cable runs 4 20 mA current loop cables when practical to save money e Use fiber optic cable for long distance e Multiple contractors and vendors involved e Allow flexibilities for future growth as additional parts of the facility are upgraded e Be compatible with host computer MODBUS plus I O channels Solution There is no one solution Site 1 One application uses multiple 4 20 mA sensors via a large multi pair cable Customer seeks additional 4 20 mA channels now but is no budget to run any new cable However when a future portion of the facility grows then an Ethernet or fiber optic cable will be installed Distributed with permission of author s by ISA 2010 Presented at ISA Automation Week 2010 http www isa org A data concentrator is installed at each end of the existing 4 20 mA cable in a Peer to Peer network Each data concentrator consists of up to 16 analog I O modules thereby supporting scanning up to 64 input or output channels Added to each data concentrator is an MIM Modbus Interface Module that communicates MODBUS RTU communications over RS485 twisted pair By borrowing one pair of wires from the 4 20 cable bundle the customer now has 64 channels of analog data being transferred on only one pair After this initial success the customer realizes that the multiple data concentrator systems not just one
26. s instruments and control elements from many different vendors Setup for the instruments for communications and data compatibility are greatly simplified Essentially no engineering time is invested in the details of middle portions of the 7 layer OSI model The designer may now use the optimum device in each application with this advantage realized several times in the overall plant instrumentation strategy Note that multiple vendors may manufacture a similar type of product with similar communications and data protocol The OSI model does not imply that these are drop in replacements for each other However for similar products it is likely that many of the same features are available This simplifies the design architecture of any system by permitting a variety of suitable products to be connected at each location Open connectivity is desirable so that the hardware cables and connectors and protocols OPC HTTP MODBUS TCP MODBUS RTU Ethernet IP FTP FOUNDATION Fieldbus PROFIBUS PA etc that interconnect your overall system can communicate This may be sensor to valve sensor to multiplexer sensor to host computer etc Open connectivity enables us to work more freely to collect input data process data and provide output to controls and HMI Human Machine Interface panels As designers we are being paid great sums of money to concentrate on the details of optimizing the process for our customers We want to select vendors that
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