IEEE 1394 and Industrial Automation: A Perfect Blend

IEEE 1394 and Industrial Automation: A Perfect Blend
IEEE 1394 and Industrial Automation: A Perfect Blend
Modern day Industrial Automation Systems control highly complex networks of high performance machine systems executing multi parameter control of variables like Precision Motion, Force, Temperature, Flow-Rate, Pressure, etc.
 
Not only is the control and monitoring of all these parameters important, there needs to be a seamless transfer of Control signals to and from these Distributed Control Systems to central controllers which makes networking a major component in successfully implementing these open systems. The objective of a networking topology in industrial automation systems is threefold:
 
1.      Connectivity to different machines
2.      Data sharing and gathering
3.      Flexibility to incorporate future advances in technology.
 
Industrial automation is commonplace in most conventional manufacturing units like textiles, chemicals, or paper and also in evolving fields of drug discovery, clinical assay and the likes. Increasing competitive pressures insist that these complex controls with extremely high throughputs and miniaturization, be implemented with cost effective electronics and robust software. Though high end machines for advanced industries are built in relatively smaller volumes compared to the consumer goods, the need to curtail capital expense on them is born out of the need for cheaper end products they will produce and hence the need for each successive generation to perform at higher levels and lower cost.
 
IEEE-1394 or Firewire was initially developed as the high speed data bus for consumer technology, but shows immense potential to tackle both higher performance needs and lower system costs as required by industrial automation. Combination of IEEE- 1394 and fast-embedded processors enable new cost effective, high performance architecture for advanced machine design and other real-time automation tasks. The industrial adoption of components originally designed for higher volume consumer applications are a certainty for next generation industrial controls.
 
This paper provides conclusive evidence to the benefits of using 1394 as a data bus for industrial automation using distributed architecture and also compares the benefits of 1394 over the existing Ethernet based distributed networks and other back -plane bus types like VME/VXI and PCI/PXI typically in use for industrial automation with a centralized controller. 

 

 
IEEE 1394 emerged as a serial bus standard in 1995, after being initially defined as Firewire" by Apple. Rapidly it became the bus of choice for the Digital Consumer Electronic manufacturers owing to its extremely high data transfer rates (upto 800Mbps) and support for Real-time data streams. The dream of finally being able to Internetwork all consumer devices in a home became a possibility with 1394 as Peer-Peer communication became a reality. The guaranteed delivery of data coupled with other above mentioned advantages made 1394 a boon for Multimedia applications.
 
IEEE 1394 is now suitably positioned to move into the orbit of Industrial Automation Systems and radically change the approach to automation control design by challenging conventional bus technologies.
 
TRADITIONAL AUTOMATION CONTROL ARCHITECTURE
 
Current solutions in Industrial Automation and Instrumentation can be characterized as centralized and backplane oriented. Backplane based controllers were considered to be natural choices for the designers of yesterday as it was assumed that they could provide high communication speeds needed for Industrial processes like synchronizing motion, synchronizing images and data acquisition. The rack mounted back plane, which is the standard implementation for most Industrial and laboratory automation controllers, uses bus solutions like VME, VXI, and PXI apart from proprietary buses like Modbus. In recent times PCI buses have gained popularity in this market segment owing to penetration of Windows based PCs.
 
In the conventional architecture, all sensors, motors, digital inputs and outputs and analog signals are cabled from the point of use to converge at the centralized controllers with individual backplane cards designed to handle each specialized function. All signals are brought to the physical location of the system controller using multi-wire cable bundles.
 
Diagram 1 is a schematic of a typical Automation Machine system with 6 axes of motion control, machine vision and process control. These types of Machine Systems typically use several specialized backplanes to implement different control functions. Bus to bus communication between these various subsystems is often through traditional RS-232/422/485 serial communication channels or through bus converters. This centralized approach limits reliability and configurability as hundreds of conductors are required to route signals to the central control chassis.  
 
 
Figure 1 – Traditional control architecture diagram
 


 
Overall this traditional approach is cumbersome, physically larger and has higher costs in the range of $10,000 to $30,000 depending upon performance specifics. Another big problem is the software used for controllers. Due to the lack of standard interfaces, different vendors follow different software approaches to develop various subsystems and to integrate them proves to be expensive and time consuming.
 
DISTRIBUTED CONTROL SYSTEMS
 
To avoid the use of a centralized back plane based system, it is important to localize control of devices performing similar functions. This Distributed Control System (DCS) architecture uses some form of serial or parallel cable to link already digitized information from point of use. In DCS analog signals are quickly digitized, and functions that do not need to be centrally supervised are localized.
The advantages of using DCS are as follows:
  1. Greater Signal integrity (S/N) by reducing the distance that Analog signal must travel before they are digitized, important in applications where signal to noise ratio maximization is demanded.
  2. Cabling can be simplified and functional sub systems can be modularized. These subsystems can be then plugged into bigger and more complex networks hence simplifying system configuration.
  3. Remote monitoring of signals or control functions over a corporate or public network is simpler with a DCS architecture as it is naturally packet driven.
 
Diagram 2 is a schematic of a Distributed Control System showing localized control and linkage of digitized signals from point of use.
 
Many distributed control schemes have been developed and implemented for industrial applications over the past three decades. The oldest were based on Field Bus and its derivatives with newer technologies like Device Net, CanBus or ProfiBus taking over. These buses had data transfer rates in the range of only a few megabits/second which was far lower than backplane buses like VME or PCI. This hampered the adoption of DCS based architecture by a majority of system designers even though distributed control systems were far more efficient as compared to back plane based industrial automation systems.  
 


Diagram 2 – Distributed Control System
 

 
DISTRIBUTED CONTROL SYSTEMS USING IEEE-1394
 
With the evolution of IEEE 1394 specifications, since it's inception in 1995, 1394 has emerged as the only optimum solution for Distributed control systems. With 1394 the concept of all signals converging on the backplane is inverted, since 1394 can bring an adequately fast bus to the signals and the point of control. Though 1394 was conceived and developed as a bus for digital consumer electronic devices, many of its technical features are well suited to advanced control systems.
The features differentiating 1394 from other competing bus technologies for distributed control systems are:
  1. High Speed1394.a supports speeds of upto 400Mbps which is faster than nearly all DCS serial buses by 3 orders of magnitude. As compared to the widely used Fieldbus derived technologies, 1394 is 1000 times faster. On the other hand, 1394.b with speeds of 800 Mbps and a roadmap with speeds of 3200 Mbps already under study, competes with the parallel back plane bus solutions. Since the nature of control messages is such that though short in data length but numerous and frequent, 1394 provides a bandwidth option that satisfies all the requirements for advanced automation control.
  2. Isochronous ModeIEEE-1394 guarantees time-based delivery of data packets which most other serial communication schemes do not. This feature is critical in closed loop servo control applications, data acquisition from analog sources and machine vision using digital video. In addition to the need for guaranteed delivery time, guaranteed delivery order is also equally important for these applications, as each message represents the state of a machine or instrument function at a given point in time which may be part of a closed loop system wherein the order of the data must be sequential. Ethernet based networks do not support ordered and timely data sequences which is critical for all industrial automation networks.
  3. Asynchronous ModeEfficient Control systems should have the ability to respond instantaneously to events. The 1394.a clocking scheme allows for Asynchronous event messages to be generated every 125 microseconds and for 1394.b this is further reduced to 62.5 microseconds. This time window is adequate for most control applications allowing a high priority message to propagate through the system with a known latency. For Control systems having intelligent nodes, asynchronous mode also provides a way to change control parameters on the fly in parallel loop operations. This kind of control system is usually dynamic in nature where initial parameters need to be modified with changing system conditions.
  4. Peer to Peer Mode: This feature of 1394 allows individual nodes to directly communicate with each other without a host, decreasing latency associated with a host centric Ethernet based Network. This is a significant advantage for systems where change in state of one node needs to be deterministically passed onto another node with minimum latency delay. This also eliminates the need for a PC in low end systems by introducing the possibility of embedded solutions.
  5. Broadcast Mode: In a typical distributed Control system environment, many nodes might need to start or stop a process in synchronization with an event or a trigger generated by a central processor. Broadcast mode of operation is also useful in cases where safety violation conditions affecting the whole system have to be intimated to all the nodes, which can be done at once using this mode. For OHCI chipsets using direct memory access transfers, data transfer speed is not dependent upon operating systems interrupt latency.
 
With the 1394.b specifications being frozen, problems with the 1394.a specifications are further resolved, making the choice simpler for designers. The bottlenecks faced with the 1394.a specifications, suitably addressed by the 1394.b specifications are:
1.      Galvanic IsolationThe earlier version of 1394 was susceptible to influence of system level ground fluctuations that could produce unintended data corruption due to limited electrical isolation.
2.      Distance between nodesThe 4.3 meter distance limitation as imposed by 1394.a specifications is no longer a bottleneck with distances to the range of 100 meters possible by 1394.b.
3.      RFI Interference1394.b specs. are far more immune to electrical noise fluctuations commonplace in factory environments.
 
These enhancements over other bus technologies and especially the predominant Ethernet network makes 1394 a robust and pervasive technology for advanced automation control systems.
 
INDUSTRIAL AUTOMATION NETWORK CHARACTERISTICS:  1394 vs. ETHERNET
 
A structured approach is helpful when considering networking infrastructure for Industrial Automation. Networking is critical for driving any Automation system and networking technology itself must have truly enabling capabilities like the ones provided by 1394. A point by point analysis of all characteristics of a good network clearly indicates this as below:
  1. HierarchyThis defines the three levels of communication present in a typical network of automation control systems. The first level of communication is the networking of I/O devices. A typical I/O network requires deterministic, daisy chain, real time responses in the range of 10-50 milliseconds. The next higher level is PLC to PLC, PLC to HMI and PLC to SCADA. PLC to PLC and PLC to SCADA communications are Real time and hence need Isochronous Mode. PLC to HMI is usually non real time but there is a lot of Asynchronous data transfer that is required between these two.
  2. Response Time and VarianceThis is used to address issues like the typical response time of a network, categorization of messages as high priority and low priority, real time or non real time, amount of latency delays etc. 1394 has the fastest response time for any other competing bus technology and supports Real time data transfer.
  3. Bandwidth1394 has the highest bandwidth of upto 800 Mbps with speeds of upto 3200 Mbps on the roadmap. Ethernet on the other hand has a bandwidth of 100Mbps and further only 30-40% of this available bandwidth can be utilized.
  4. EfficiencyThis is a measure of the amount of additional work required to be done to send a data packet across the network. For an Ethernet based network the minimum Data length is 64 bytes. Hence even if 10bytes of data have to be sent, the payload has to be padded which increases the overhead on the data. 1394 has a header bit specifying the Data Length which is user programmable and data can be sent in multiples of quadlets without having to pad it first. Efficiency of a network is also dictated by the number of messages exchanged to Read or write data to a device. For example a PLC that needs to read Inputs and write outputs to a block of I/O on the network will take 6 bus cycles to complete this with certain protocols. In the first bus cycle a Request will be posted from the PLC to the I/O, in the second bus cycle the I/O will send an ACK acknowledging the request, in the third bus cycle the I/O will send the input data, in the fourth cycle the PLC will send an ACK, in the fifth the PLC will send the output data and finally in the sixth the I/O will send a ACK saying that it received the data. In case of 1394 this is handled in hardware with the PHY sending an ACK in the same bus cycle in which it receives a request. This makes the access much faster as no software protocols are required to handle Read/Write operations.
  5. Access MethodsThese can be split into two types; Deterministic and Collision Detection. 1394 implements deterministic access which allows the designer to calculate the maximum response time of a network based on the number of nodes in it. Ethernet uses Collision detection for access which makes access inherently non-deterministic. In this access scheme a node listens before it starts talking, i.e. each node makes sure that no one else is sending data when it needs to. In a network with multiple nodes this leads to wastage of bandwidth and decreases efficiency of the network. Hence in the case of Ethernet, only 30-40% of actual bandwidth is utilized.
  6. TopologyThis describes how cables are run between various nodes in a network. Ethernet uses a Star or a Hub topology wherein cables from each node are individually run to the central hub. 1394 uses a Daisy Chain topology where the cable is run from node to node thereby greatly reducing the cabling involved and the time taken to identify the faulty cable incase of a physical damage.
  7. DistanceThis is an indicator of the distance a signal can travel on a bus before it loses its strength and gets corrupted. Ethernet supports a distance of 75 meters compared to a distance of 100 meters supported by 1394.
  8. Protocols supportedIt is important to consider the protocols running on top of the physical bus to ensure maximum efficiency. 1394 supports all 6 std.protocols including TCP/IP whereas Ethernet supports only TCP/IP and UDP.
  9. Media SupportedThe physical medium for communication is also an important consideration for network designers. 1394 supports Glass, Optic, and CAT-5 whereas Ethernet supports only glass and CAT-5.
  10. Universal Plug and Play1394 is a plug-n-play technology and Ethernet does not support Plug-n-play.
 
SUMMARY
 
Adoption of 1394 technology will provide the Industrial Automation industry to also extend functionality to a broader level. Large corporations continually struggle with managing the plethora of critical business assets across their organizations. Typically the plant-floor machines are separated and disconnected from the rest of the corporation's assets including information technology assets. But managing all these assets together allows the business to function as a unit. The network implementation using 1394 outlined above can bridge the management of mission-critical assets across the organization.
 
The adoption of the 1394 Multimedia Standard Protocol for Instrument and
Control Applications in February, 2000 laid the foundation for the movement of
Control applications to 1394. Continuing innovations will bring further
convergence of PC based automation software with DSP control modules
running on 1394 networks.
 
The industrial Automation.community can benefit in many ways by taking a close look at how 1394 has benefited the world of consumer electronic devices. The impact of 1394 on the industrial control field is likely to be as disruptive and revolutionary and will enable new applications to be developed due to the flexibility and improved economics of this new control architecture.
 
GLOSSARY OF TERMS
 
DCS: Distributed Control Systems
TCP/IP: Transmission Control Protocol/Internet Protocol
OHCI: Open Host Controller Interface
PLC: Programmable Logic Controller
HMI: Human Machine Interface
SCADA: Supervisory Control and Data Acquisition (a computer system for gathering and analyzing real time data)
 

About The Author


Gaurav Sareen is the Product Manager for Wireline Technologies at Wipro. He manages Intellectual Property portfolios for IEEE 1394, USB and Ethernet. He works closely with the Engineering team and is responsible for Product roadmap development and maintenance.

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