CONTRARY TO COMMON belief, the underlyingeconomics of FOSS are not totally new. The‘Open Innovation’ concept has long been usedto let different companies – even competitors– jointly develop industrial components. Costreduction is achieved by avoiding unnecessaryparallel development. As such, FOSS can beseen as a variant of ‘Open Innovation’. The mostimportant prerequisite for a company to entersuch a project, however, is to ensure that thecomponents for joint development are notaimed at providing uniqueness to one of theparticipating companies.For machine software, this is illustrated in

the ‘uniqueness pyramid’ (Fig. 1) where thebottom level of the pyramid includes basictechnologies without an exclusive benefit, andthe top area includes unique knowledge thatis differentiating the company from itscompetitors. Somewhere between the base andthe tip of the pyramid is the ‘uniqueness limit’to be individually determined for a specificcompany. If this limit is directly at the baseof the pyramid, there is little or no expertisethat can be exchanged with competitors, andFOSS cannot be used (at least not with acopyleft license – see the FOSS text panel foran explanation). If the uniqueness limit ishigher (yellow and green lines), there is noparticular reason why FOSS with a copyleftlicense cannot be used to develop componentsin the area below the uniqueness limit.

Linux: The new RTOS?In the good old days when most of thecomputer systems on a machine were builtspecifically for the purpose, the traditionalRTOS was also a custom job. In contrast tolarge and demanding operating systems thatwere already available at this time, the machineoperating systems (e.g. OS-9, VxWorks, etc.)were small and fast and could be loadedwithout any problem in 512-KB eproms and runat 8MHz in a single MByte of ram. In addition,they provided the required real-time capabili-ties other operating systems could not.But about ten years ago, with the general

availability of the Internet and the ubiquitous

presence of computers, people were no longerprepared to consider a machine as somethingspecial. It was only natural that a CD/DVD filesystem, USB-based interfaces and unrestrictednetwork capabilities were requested inmachines in the same way as they wereavailable, for example, in laptops. In addition,the advanced features of state-of-the-artprocessors – 64-bit, multi-core, virtualisationetc. – had to be supported in machinecomputers as well.

With this growing number of requirements,it became virtually impossible to implementall of them in the various traditional RTOSes.The Linux operating system, on the other hand,provided such technologies, but possessed noreal-time capabilities. Many people thought itsimply impossible to turn Linux into a real-

22

Technology Open Source industrial software:

more hype or a new, better way?The term ‘Free and Open Source Software’ (FOSS) denotes a type of software license that allows the licensee,among others, to continuously use, adapt and distribute a certain piece of software without fearing itsdiscontinuation. The fact that Open Source software cannot be discontinued is probably the most importantreason why FOSS is quickly gaining uptake in the machine and automation industry. When you ask themanufacturers of industrial products why they prefer FOSS, you frequently hear responses such as the machinebuilder needs complete control of his machine… independence from discontinuations…, or, avoiding vendorlock. The Open Source Automation Development Lab's manager, Carsten Emde, explains

the indust r ia l e thernet book 05.2009

Uniqueness limit

Operating system

Drivers

Operating system libraries

Application libraries

Applications

Testing

GUI

QM

unique

notunique

Fig. 1. Example of a ‘uniqueness limit’ thatshows the different levels of knowledge of acompany. Above the uniqueness limit lies theknowledge that differentiates a company fromits competitors; below this limit are the basictechnologies that do not make any difference inthe market. If this threshold is at the bottom ofthe pyramid for a company (red line), that is,practically all the knowledge is market-relevantand confidential, FOSS should not be used. Thehigher the limit (yellow and green lines), themore (economically) attractive it is to use freeand open software (FOSS).

What is Open Source software?The terms ‘Open Source software’ and ‘FreeSoftware’ represent different ideologicalperspectives, but legally and practically, bothterms are equivalent descriptions of a specifictype of software licensing. The term ‘Free andOpen Source Software’ (FOSS) is, therefore, bestused for this type of license.

The GNU General Public License (GNU GPL)

Most active FOSS projects use the GNU GeneralPublic License. This license grants the rights:

1. To run the program, for any purpose;

2. To study how the program works, and adaptit to a user’s need;

3. To redistribute copies so one can help his orher neighbour;

4. To improve the program, and release theimprovements to the public, so that the wholecommunity benefits.

The second and fourth points require freeaccess to the source code. This access is, thus,a necessary but not exclusive condition forqualifying a software as Open Source.When thesoftware is passed on to others, the recipientmust also be able to acquire these rightsdepending on the type of FOSS license. If theuser only uses the software himself and doesnot pass it on, all the requirements from theOpen Source license no longer have meaning.Contrary to a frequently-stated belief, moneymay be charged for FOSS, but only forassociated services such as the effort requiredto pass on the FOSS, and not for the licenseitself. Apart from this consideration, a serviceprovider may, of course, also charge customersfor the activities required to develop andmaintain FOSS.

Reprinted from Industrial Ethernet Book 52

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time operating system, yet equipping tradi-tional RTOSes with all the new technologieswas equally impossible…Anyway, making Linux an RTOS was seen as

having more possibilities. In a first step, aseparate microkernel was used to provide thereal-time capabilities, and a modified Linuxkernel was running under the control of themicrokernel to provide the required interfacesand protocols. This principle is known as ‘LinuxReal-Time Extension’ or ‘Dual-kernel approach’(Fig. 2). RTAI, RT-Linux and Xenomai areexamples of such implementations. The disad-vantage with this approach however is that theLinux kernel must be patched in order to beused in this environment, and it was clear fromthe very beginning that these patches wouldnever be accepted in the mainline Linux kernel.In consequence, other developers, namely

Ingo Molnár and Thomas Gleixner, tried topatch the native Linux kernel in such a waythat it becomes an RTOS; this project took thename PREEMPT_RT, as ‘Linux mainline real-time’or as ‘Single-kernel approach’ (Fig. 3). At theoccasion of the Ottawa Linux kernel summit insummer 2006, it was agreed that this is theway to go, and the PREEMPT_RT patches weregradually merged into the Linux kernel sincethen. Today, about 80% of the patches areavailable in mainline, and it is expected thatthe rest of it will follow by the end of the year.A number of commercially available Linux

distributions (e.g. Red Hat’s MRG) alreadycontain the PREEMPT_RT patches in productionquality. The worst-case rule-of-thumb latencyis in the range of 105 multiplied by the CPUcycle duration with these systems withoutspecial optimisation. For example, a 1GHzprocessor has a cycle duration of onenanosecond which results in a worst-caselatency of about 100µs. This is consideredsufficient for the vast majority of real-timeprojects. Using latency optimisation, however,the worst-case latency can be reduced by upto three times.

From theory to practiceIn principle, there are three different ways toinstall Linux on an embedded system:� Use a standard distribution (e.g. Fedora,Novell, Red Hat, Debian etc.) and adapt it tothe requirements of an embedded system.

This is, obviously, restricted to processor andchip sets for server and desktop computers

supported by a particular standard distribu-tion. While there is normally no problem withx86-based architectures, others such as ARMand MIPS are rarely supported.� Use a dedicated distribution for embeddedsystems (Montavista, Timesys, Windriver orELDK/Denx, Elinos/Sysgo, PTXdist/Pengutronix).These distributions support a wide variety of

architectures, chip sets and controllers.� Use the standard (vanilla) Linux kernel andassemble the required tools and librariesdirectly from the related repositories.This method allows the developer to make

maximum use of the Linux kernel but requiresconsiderable knowledge.

Technology

05.2009 the indust r ia l e thernet book23

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Fig. 2. The ‘Dual-kernel approach’ to achieve real-time capabilities with Linux

A4_version:Layout 1 4/6/09 11:32 Page 2

When a standard distribution is used, it maybe difficult to obtain support and training fromthe software distributor, since these packagesare not intended to be used in embeddedsystems and the distributor may not have therequired expertise. Manufacturers of dedicateddistributions, however, provide support andtraining which may be purchased on a per-timebasis. In addition, the development of specialdrivers and extensions may be commissioned.The same holds for the third way when a

standard vanilla kernel is used. A number ofLinux software service providers such asLinutronix, Germany, offer support to managethe native Linux kernel and to build tool chains

and other software packages individually.Overall, these ways to install the operatingsystem on an embedded system are not verydifferent from the ways of traditional RTOSes,

except that Linux makes it possible to changeeither the distribution or the software serviceprovider more easily, since the Open Sourcenature of the license forbids the addition ofproprietary components to the kernel.

The development processHow to develop system software and user appli-cations? Developing for an embedded system ingeneral is done either on a cross-developmentsystem or in a self-hosted environment.Traditional RTOSes are either self-hosted, e.g.

OS-9/68k, or cross-development systems, e.g.VxWorks, and cannot be changed easily. As anadvantage of cross-development, the fastest

24

Tech

nolo

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the indust r ia l e thernet book 05.2009

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What is a copyleft?The word ‘copyleft’ is an artificial word todenote the ‘opposite of copyright’. This word-play employs the term ‘left’ as the opposite of‘right’ and also in the sense of the past tenseof ‘to leave’ (somebody, something). Whereasthe normal copyright forbids the passing on ofa copyright work, the copyleft allows it andforces the identical license to be used as for theoriginal work [in the manner of a legalcovenant – Ed]. This requirement applies notonly to the original version of the work, but alsoto all changes and additions. This constitutes aunique regulation since normal copyrightregulations leave it up to the license holder tochoose the type of license that will govern thechanges and additions. However, when alicense holder acquires a license containing acopyleft, he assumes this particular requirementwhich takes effect when the work is passed on.

The presently-known different Open Sourcelicenses differ according to the strength of thiscopyleft effect. The BSD (Berkeley SoftwareDistribution) license does not dictate to thelicense holder which license will govern thesubject software along with the changes andadditions; it is therefore an Open Source licensewithout copyleft.An example of an Open Sourcelicense with full copyleft is the above-cited GNUGPL; it forces the license holder to retain theoriginal license in its identical form for thesubject software and any changes andadditions.

For examples of the requirements of the GPLwhen passing on the Linux kernel, see also theFAQ box.

Fig. 4. Example of a real-time project in the Eclipse Open Source integrated developmentenvironment

Fig. 5. The Open Source CyclictestoSCOPE developed by Arnaldo Carvalho de Melo is used toregister and detect latencies on a real-time system

A4_version:Layout 1 4/6/09 11:32 Page 3

available system can be used for the develop-ment in order to reduce the compile time; theadvantage of a self-hosted system is thatdebugging and tracing is more efficient andstraight-forward. With Linux it is often possibleto combine cross and self-hosted developmentwhich makes it possible to use the best of bothworlds.Many traditional RTOSes come with a propri-

etary work bench that integrates a number oftools under a common GUI to develop, debugand optimise a given system. Under Linux,there is a wide variety of such tools, but notall of them are integrated into a common userinterface. Neither are they always easy to findand to use. Overall, however, similarly powerfultools are available. The most frequently usedOpen Source integrated development environ-ment, for example, is Eclipse (Fig. 4). Itprovides everything needed during code devel-opment.Another example that may be very helpful

during optimisation of a real-time system isthe CyclictestoSCOPE developed by ArnaldoCarvalho de Melo (Fig. 5). Again, Open Sourcesoftware service providers may help to findtools which are best suitable for a givensituation and provide training to use them.Linux is now an RTOS and can be used in

similar ways to traditional RTOSes. There arevarious options to use either standard distribu-tions, dedicated distributions or to build localexpertise in a particular company. Theadvantage of Open Source software and, inparticular, Linux relates to the flexibility andunrestricted availability which it affords – noother operating system supports as many archi-tectures and controllers as Linux does.The disadvantage is that a particular tool may

be difficult to find and less elegant in usecompared to its commercial counterpart. Onthe other hand, Open Source software serviceproviders may be able to help in cases wheresomething does not work. Normally, the savingsin the license costs far exceed those of hiringa Linux service provider for a limited time.

OSADLThe Open Source Automation Development Lab(OSADL) was founded in December 2005 andregistered as a cooperative in summer 2006with eleven founding members. Today, OSADLcomprises some 30 members including semi-conductor companies, software manufacturersand distributors, manufacturers of industrialcomputer boards and other hardware, Linux andOpen Source software service providers and,most importantly, machine builders andindustrial automation companies.The goal of the Open Source Automation

Development Lab (OSADL) is to promote andsupport the usage of Open Source software inthe industry. The membership fees are used todelegate the development of Open Sourcesoftware requested by the majority of members,

or at least agreeing to do so. Other benefitsof the OSADL membership include participa-tion at exhibitions where OSADL is representedwith a booth, free access to OSADL-organisedconferences and seminars, free legal advice onquestions of general interest and much more.Details about OSADL’s various projects and

activities are available at www.osadl.org

Carsten Emde has spent more than 20 years asa software developer, system integrator andsoftware consultant for industrial computersystems. Among others, he is specialised onreal time, video and image processing. Sincefounding the Open Source AutomationDevelopment Lab (OSADL) in 2005, he isserving the organisation as general manager.

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Technology

05.2009 the indust r ia l e thernet book

Open Source software Frequently Asked QuestionsQ: I have used the Open Source GNU C compiler (gcc) to generate a machine control application. DoI have to make the gcc source code available to a customer when I am selling the machine?

A: No.

Q: I have used the Open Source GNU C compiler (gcc) to generate a machine control application. DoI have to make the source code of the application available to a customer when I am selling the machine?

A: No.

Q: The control computer of a machine that I have developed runs under the Linux Open Source operatingsystem. Do I have to make the schematics of my machine available to a customer when I am sellingthe machine?

A: No.

Q: I have developed a machine control application for the Linux Open Source operating system. Do Ihave to make the source code of the application available to a customer when I am selling the machinealong with the operating system and the application to a customer?

A: No.

Q: I have discovered a bug on the gcc and fixed it. I am using the gcc solely within my company. Do Ihave to publish the bug fix anywhere?

A: No (but you will make a point of doing so if you wish the bug to be fixed in the next version).

Q: I am using the Linux Open Source operating system solely within my own company. Are thereimportant obligations of the Open Source license that I have to follow.

A: No.

Q: I am using the Linux Open Source operating system solely within my own company and do notdistribute it to anybody. I have discovered and fixed a bug in the network driver. Do I have to publishthe bug fix anywhere?

A: No (but you will make a point of doing so if you wish the bug to be fixed in the next version).

Q: The control computer of a machine that I have developed is running under the Linux Open Sourceoperating system. Do I have to follow any obligations of the Open Source licence when I am sellingthe machine along with the operating system to a customer?

A:Yes.A number of information and disclosure obligations must be followed: Information that the productuses Open Source software and the license text of the Open Source license, i.e. the GNU General PublicLicense version 2, must accompany the product. This is best done in a dedicated section of the User’sManual. The source code of the Linux operating system must be made available in one of the variousways described in the GNU General Public License.

Q: The control computer of a machine that I have developed is running under the Open Source operatingsystem Linux. I have discovered and fixed a bug in the network driver. Do I have to publish the bug fixanywhere, when I am selling the machine along with the operating system to a customer?

A: Yes. The bug fix must be licensed under the same license as the entire operating system and, thus,made available to the customer.This mechanism is called copyleft and defined in the GNU General PublicLicense.

Q: I am very enthusiastic about using Open Source software in the machine and automation industryand in embedded systems. I have a lot more questions with respect to legal and other issues.Who hasthe answers?

A:To use Open Source software effectively in a commercial environment, a number of common interestgroups have been founded. One of them is the Linux Foundation where Linux creator Linus Torvaldsand other kernel developers are employed. A similar organisation, the Open Source AutomationDevelopment Lab (OSADL), is taking care of the special requirements of the machine and automationindustry and of using Open Source software in embedded systems (see the section headed ‘OSADL’to the left of this panel).

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