Software and hardware have a “chicken and egg” relationship – one really can’t work without the other. In Steve Leibsen’s blog, he talks about getting more capacity through multi-core based systems. That’s the hardware side of the equation, now let’s look at software to make realtime industrial control function. Ethernet was born of a PhD Dissertation by Robert Metcalf in the early 1970s. Xerox PARC, center of many new computing technologies, developed the underpinnings of Ethernet. By 1979 Metcalf left PARC to promote the use of networking in what would become personal computers. Ethernet's carrier sense multiple access with collision detection (CSMA/CD) promotion through inclusion in various international standards eventually won the marketplace competition with IBM’s token ring configuration.
Ethernet is a packet-based communications technology which improves the efficiency of sending and receiving messages by grouping information together and wrapping it in header information. This reduces the information overhead associated with every packet. There are more important implications of the impact of Collision Detection. Every packet has a finite probability of interfering with another packet’s reception. The solution was to detect a collision between packets, and randomly assign a delay to the conflicting packets for retransmission. The important aspect of retransmission is that the “clients” calculate the amount of delay before retransmission. Independent retransmission leads to the probability that one or more of the retransmitted packets will collide again.
Considering all of these issues, it’s possible to create a hierarchy of realtime control based on native Ethernet and an extended superset of Ethernet-typr hardware.
The following explicit channels are created by using additional capabilities to solve some shortcomings of basic Ethernet in realtime control:
- Standard channel, which is the Non Real Time (NRT) channel, uses Internet Protocol (IP) Transmission Control Protocol (TCP/IP), or User Datagram Protocol (UDP/IP) communication. Ordinary Ethernet configurations serve simple devices with typical reaction times in the range of 100 ms. It is typically used for configuring and reading diagnostic data.
- A Real Time (RT) channel supports typical cycle times of less than 10 ms but generally not faster than 1 ms - with the performance class of today's industrial control “fieldbus” systems. It can be implemented through software supporting the higher performance transmission of alarms, process data, and event-driven messages.
- Highly dynamic drive applications are covered by the Isochronous Real Time (IRT) channel with cycle times of well below 1 ms and a jitter of less than 1 µs. This channel requires a special kind of hardware support for realizing a Time Division Multiplex Access (TDMA) process with better controlled bandwidth utilization.
Programmable Logic Controllers (PLCs) are one of the foundations of process automation. Early PLCs employed electro-mechanical relays to create the required functions, which in turn defined systematic response times of milliseconds due to the actuation time of relays. PLCs have been defined using RS-232 communications ports, which are substantially slower than Ethernet speeds, which are replacing these communications ports.
KW-Software makes software-defined PLC controllers and a software stack of communications protocols designed to operate machinery automation equipment. Called Profinet IO, the industrial Ethernet protocol is a well-established industrial standard. KW-Software offers a software stack that implements the standard. Profinet is a "switched" network operating at a transmission rate of 100 Mbps on a full duplex connection. Using bridge and switch gear, it’s possible to create many different topologies including tree, ring and linear structures. Using these configuration options, the network topology can be adapted to the unique communication demands of the application. Fiber optics (FO) or wireless connections can be used in addition to copper wire connections. While FO technology has advantages in electromagnetically noisy environments, wireless solutions are preferred for connecting mobile, temporary installations and difficult to access systems.
KW-Software’s Profinet software stack operates under Wind River (1) VxWorks and Linux. Wind River Partner Validation Program for the industrial and medical markets includes initial partners Acontis Technologies, Esterel Technologies, KW-Software, and Softing AG. The Wind River industrial middleware offering includes selections from:
- Security Libraries,
- Mobile IP,
- Media library and more.
KW-Software also operates under INtime® for Windows real-time operating system (RTOS) from TenAsys® Corporation (2) to support hard real-time operation.
Configuring an industrial system is contingent on what operating system software you select, and then the specific essential software used to support the industrial standards. Pre-configured and validated software from vendors like Wind River and Green Hills Software (3) can simplify your development task. In some cases, the operating software may be better chosen by first selecting the industrial operating applications package.
Building an application can be an involved process. Apart from completely understanding the application, developers must know the RTOS, development tools, and the APIs for the library. The question then is what library elements are required to support the application. Most commercial libraries are modular in organization with minimal span of control. In most cases, the limited span of control simplifies selection of modules since there is at most a linear relationship of dependent modules. Even though the modules have well-defined functions, you still must spend the time to learn what modules are present and the functions that are provided. Although this may seem like a daunting task, it’s really no more conceptual work than what is already required based on your application. The additional work is attributable to the particulars of the library operation, and in some cases may be the determining factor in selecting a library.
There are a number of libraries that may fill a particular purpose. How would you select an industrial library and set of applications?
1. Wind River Systems is an Associate Member of the Intel Embedded Alliance
2. TenAsys Corporation is an Affliate Member of the Intel Embedded Alliance
3. Green Hills Software is Affliate Member of the Intel Embedded Alliance
Roving Reporter (Intel Contractor)
Intel(r) Embedded Alliance