Using FireWire for Machine Vision

Transcription

1 Using FireWire for Machine Vision FireWire (IEEE 1394) Architectural Overview IEEE 1394a = 32 MB/sec Bandwidth (practical limit per adapter) Camera Camera Standard Cable FireWire Adapter Card PC Adapter card transfers image over PCI bus into memory, CPU is used to provide image format conversion, if needed. Memory CPU Example: Image Transfer of two FireWire cameras using one adapter card FireWire Advantages?? No traditional frame grabber required. Instead it uses low cost ($100) interface boards?? Vendor neutral camera interoperability thru Cognex generic DCAM plug n play driver?? Camera prices range from $500 - $3000?? Hubs (a.k.a muxes or repeaters) are low cost ($70-$150)?? Inexpensive standard cables ($10-$50) FireWire Issues?? Limited to ~32 MB/sec of reliable bandwidth per adapter card?? Adapter cards do not provide image buffers, meaning images can be lost under high PCI bus traffic. If an image is lost due to bus traffic, there is no ability to send an error message.?? Multi-camera asynchronous acquisition, but may require multiple adapter cards.?? The maximum number of cameras supported is 63 per adapter, but the final number of cameras will be limited by available PCI bandwidth?? 10 meter cable length?? Asynchronous triggering and shutter strobe control must be supported by the camera?? Microsoft Windows XP SP2 has a performance problem with 1394b adapters, a Microsoft hot fix is available Cognex Corporation Page 1 of 24

2 Where do we see this technology??? Applications that do not require the determinism and error detection offered by frame grabbers?? Applications where bandwidth requirements are well understood?? Simple, one camera systems work well?? Systems with two or more cameras on one interface card require the user to calculate and possibly specify bandwidth for each camera Cognex s DCAM Implementation?? Cognex implemented our own generic DCAM driver and interface?? Support multiple camera vendors in same application. This will be a benefit over many competitive implementations.?? Based on the IEEE 1394 DCAM specification version 1.30?? Uses CCF (Camera Configuration File) structure to add camera support without changing source code?? Offered Now in CVL 6.3CR4 and VisionPro 4.1 Cognex FireWire Tested Cameras?? Basler A602fc, A102f, A601f, A311f, A622f?? AVT F046B, F033C, F201B?? PixelLink A741, A781?? Point Grey Flea and Scorpion (multiple models)?? Color support is available using a Bayer pattern (Raw8) color format?? Generic DCAM formats available for customer testing on untested cameras?? Support for other FireWire cameras from Teli, RedLake, Sony, and others may be supported, contact Bashar Mashal or Steve Cruickshank if you need support for other cameras Cognex FireWire Adapters?? Cognex's FireWire implementation requires an OHCI-compliant adapter and is adapter vendor independent?? Experience at Cognex with?? UniBrain FireBoard Blue (1394a) & FireBoard 800 (1394b)?? Integrated in a PC?? Hubs, which are effectively muxes, are transparent to FireWire implementation. They can be used with no additional effort.?? Cognex FireWire kits can be ordered with or without FireWire adapter cards, allowing customers to use their own preferred adapters. Cognex Corporation Page 2 of 24

3 1 Introduction This paper describes Cognex Corporation s support of FireWire (a.k.a. IEEE 1394) digital cameras using CVL and VisionPro software development kits for machine vision applications. A brief history of FireWire is provided, along with the architecture of the digital camera specification called DCAM. It also describes Cognex s own DCAM driver suitable for use with any camera vendor s FireWire digital camera that conforms to DCAM version 1.30 or greater. It explains some of the advantages of selecting the Cognex FireWire solution and outlines some guidelines for what application types are suitable. Special considerations for applications requiring multi-camera, mixed camera vendor, and balanced bus bandwidth are also discussed. 1.1 Introducing FireWire FireWire is a term that refers to the IEEE 1394 specification that defines a standard for high-speed network of digital communication between a computer connected devices, like digital cameras. The popularity of FireWire has been increasing in the industrial marketplace primarily due to the ease of use, plug n play feature, and lower cost than traditional frame grabbers. A wide variety of machine vision cameras are readily available making it a viable choice for many machine vision applications. 2 FireWire Application Considerations One major factor when considering using FireWire in a machine vision application is that the amount of available bandwidth on each IEEE 1394a adapter is only 32 megabytes per second. This is far less total available bandwidth than a traditional frame grabber plugged into a PCI 32-bit 33MHz slot of ~80 megabytes per second. Because of this careful consideration is needed when for applications that require multiple cameras. When machine vision applications require simultaneous image acquisition from multiple FireWire cameras and that image bandwidth exceeds the limits of the bus, one solution is to add additional FireWire adapters. For example, the total bus bandwidth on an adapter for image acquisition with two 640x480 VGA cameras running at their maximum trigger rate of 60 frames/sec per camera would be over 35 MB/sec. This exceeds the limit of 32 MB/sec available on a single FireWire adapter. Adding a FireWire adapter and placing each camera on its own adapter will increase the total available bandwidth. Cognex Corporation Page 3 of 24

4 2.1 FireWire Application Limitations There are several limiting factors to keep in mind when considering FireWire instead of a traditional frame grabber based solution. Trigger Error Detection FireWire cameras do not report a missed trigger (i.e. the camera is being triggered at a rate above what it can reasonably process) to the host PC. A number of FireWire cameras just ignore the missed trigger while others will process the trigger and acquire an image as soon as they are able to. For both scenarios, the desired image is either not captured at all or captured too late and without any notification to the host PC. Image Delivery The cameras also do not guarantee video data integrity between themselves and the adapter card in the host PC. Image data losses due to buffer overrun, dropped data, or packet and data corruption are not currently reported. The danger of data corruption or image data loss is a legitimate concern especially when there is heavy traffic on the host PC s PCI bus. Register Access Time Property changes on the camera such as gain and offset adjustments require read and write access to DCAM-defined registers in the camera. Some FireWire cameras have rather lengthy register read and/or write times. This can be as much as millisecond or more, but less than 200 microseconds is typical. Lengthy register read/write times can reduce the throughput of applications that need to change camera properties frequently. 2.2 Good FireWire Applications The following are some examples of machine vision applications that might be suited for a FireWire solution:?? Applications that require one or two cameras with modest trigger rates and moderate level of PCI or PCI Express bus traffic.?? Applications that require mixing color camera(s) with monochrome camera(s). Analog frame grabbers generally need to use a color digitizer or multiple units of the monochrome digitizer to interface with an analog color camera. This increases the cost of the frame grabber significantly. FireWire color cameras can be connected to the same adapter card as FireWire monochrome cameras.?? Applications that require the use of an additional camera but their current frame grabber can t support the additional channel. Adding a FireWire camera might be a good cost effective solution. Cognex Corporation Page 4 of 24

5 ?? Applications that require a large number of multiplexed cameras operating in nonsimultaneous acquisition mode. IEEE 1394a limits the maximum number of connected devices to 63 per bus.?? Applications that can tolerate missed triggers or over triggering without the need to detect them as error conditions.?? Applications that can tolerate the possibility of image data loss or image corruption on systems that have heavy PCI bus traffic or where the system PCI bus traffic can not be well controlled. 2.3 Poor FireWire Applications The following are some examples of types of machine vision applications that might not be suited for a FireWire solution:?? Applications that require bandwidth for image acquisition greater than 32MB/sec. These applications also typically use 3 or more cameras per system. Adding additional adapters may be useful in increasing the available bandwidth.?? Applications that require the cameras to be triggered for acquisition at close to their maximum rate. Since FireWire cameras do not report missed triggers, the application cannot react to over-triggering predictably and robustly.?? Systems that require camera cables that are more than 10 meters. Upgrading to fiber optic cables will extend the range to 100 meters but it also adds significant cost and more equipment.?? Applications that require a line scan camera. There are very few FireWire line scan cameras currently on the market. The IEEE 1394a bus bandwidth limits line scan cameras to a single unit of the single-tap, 32 MHz max pixel clock rate type. This represents the bottom end of line scan cameras in terms of performance.?? Applications that require image capture synchronization of multiple cameras in a master-slave configuration. Some FireWire cameras may offer special trigger synchronization schemes, but this is non-standard and not part of the DCAM specification.?? Applications that demand high reliability with absolutely no data loss and guaranteed image delivery. Some adapters have limited on board buffer storage and heavy PCI traffic may impact data delivery. 2.4 FireWire Camera Sensor Types Like other industrial cameras, FireWire cameras can come equipped with CCD or CMOS sensors. FireWire cameras themselves don t offer any special advantage when it comes to image sensors compared to analog or other digital industrial cameras that use the same sensor. Cognex Corporation Page 5 of 24

6 CCD is a mature technology that has served the machine vision market well for many years. On the other hand, state-of-the-art CMOS sensors can operate at a higher pixel clock rate than CCD sensors of equivalent resolution. This speed advantage translates into a shorter frame time ( i.e. higher maximum frame rate 96 frames/sec versus 60 frames/sec for 640x480) that may be important to certain applications. CMOS sensors offer a further speed advantage over CCD when acquiring a Region-Of- Interest (ROI) image that is smaller than the sensor s full resolution. For example, the time to acquire a 320x240 ROI (1/4-sized image from VGA sensor) would drop by about 70% for a CMOS sensor but only about 50% for a CCD sensor. This speed advantage may be important to certain applications. However, when considering a FireWire camera with a CMOS sensor for an application, one would be well advised to take into account the present-day shortcomings of these sensor s generally lower sensitivity and lower signal-to-noise ratio. A sensor should not be chosen solely based on speed and/or cost. One would want to do the same for an analog/digital camera with a CMOS sensor. 2.5 FireWire Trigger Support In order to meet the demands of most machine vision applications, a camera needs to be able to initiate image capture in response to an external input within a reasonably short interval. This electronic triggering capability needs to be complemented by functionality that allows the application to change the shutter duration entirely via software. Most, if not all, DCAM v.1.30-compliant FireWire cameras meet both demands, as do most modern analog and digital machine vision cameras. One interesting difference between the technologies is that the external input ( trigger ) is connected directly to FireWire cameras. With an analog/digital camera and frame grabbers, it is often that the trigger gets connected to the frame grabber and the frame grabber in turn fires the camera via a TTL shutter control line that runs between the frame grabber and the camera. Though both arrangements are functionally equivalent, a user of FireWire cameras needs to make sure that the signal carried by the trigger line meets the specifications stipulated by the camera s manufacturer. The tolerance of the on-camera trigger line to over-voltage and spikes is generally less than that for the opto-isolated trigger inputs found on frame grabbers. 2.6 FireWire Strobe The DCAM v.1.30 specification does not define any strobe functionality, but it was added in version For cameras that are only DCAM v.1.30-compliant, a constant illumination light source may be the only lighting option. However, this arrangement can be problematic if a very short exposure duration is required to freeze motion for reliable image capture, especially under high magnification. Cognex Corporation Page 6 of 24

7 With this scenario, upgrading to a DCAM v.1.31-compliant FireWire may be the only reliable and sensible solution. DCAM v.1.31-compliant FireWire cameras can fire a strobe via an output pin on the camera itself. There is control for signal polarity, duration of strobe pulse, and the delay from the start of exposure until the strobe signal is sent, all accessible via software. It should be noted that not all FireWire cameras in the marketplace support strobe functionality that is compliant with the DCAM 1.31 specification. 2.7 FireWire Multi-Camera Considerations As mentioned at the beginning of this document, running multiple FireWire cameras simultaneously is limited by the total available bandwidth per adapter. The bandwidth for image acquisition using IEEE 1394a adapters and cameras is 32 MB/sec while the bandwidth using IEEE 1394b adapters and cameras is 64 MB/sec. Note that Cognex software does not currently support FireWire cameras operating at IEEE 1394b speeds, but the use of IEEE 1394b adapters running cameras at IEEE 1394a speeds is supported. Generally speaking, multiple FireWire cameras cannot be synchronized so that all cameras on the same bus share the same video timing (assuming the same type of camera is used). This may be less of a concern if each of the cameras is capable of responding to an external trigger and acquire an image with minimum latency (2 microseconds or less). If the cameras have such capability, then a single trigger source for all the cameras may be a viable alternative to a master/slave configuration involving all the cameras. A notable exception - one that requires absolute synchronization of video timing among cameras - is applications that require accurate 3-dimensional imaging. 2.8 Introducing the FireWire Standard The term FireWire is a trademarked word coined by Apple Computer engineers to denote a new digital communication standard for transporting digital image and audio data. This standard was the 1394th IEEE standard developed in 1995, hence the term IEEE and is titled Standard for a High Performance Serial Bus. Subsequent amendments and supplements to the standard specification have occurred, namely IEEE 1394a in 2000, and IEEE 1394b in Collectively, these standards are commonly referred to as IEEE 1394, or simply FireWire. The IEEE 1394 standard is a high-speed digital network interface standard that uses a serial bus to transmit uncompressed digital data (i.e. image or audio) between a computer and other connected devices (like digital cameras) in a point-to-point transmission. This standard, adopted primarily by multi-media devices, ensures a fixed bandwidth per device that is sharing the network bus. It can operate in what is known as isochronous data transport mode to guarantee timely delivery with no retries of multiple real-time Cognex Corporation Page 7 of 24

8 image data streams. It can also operate in asynchronous data transport mode ensuring guaranteed delivery through the use of retry events. The standard was designed for the consumer market and has been gaining popularity in the industrial marketplace for use in machine vision due to it s high-speed, low cost, simplicity in cabling, and wide availability of cameras with many choices in image sensors and image sizes. The IEEE 1394 standard has native support in the Microsoft Windows operating systems, a common platform for machine vision applications. The standard for industrial machine vision digital cameras that is based upon the IEEE 1394 standard suitable for any platform is called the IIDC Digital Camera Specification, or DCAM. 2.9 IEEE 1394b The IEEE 1394a-2000 standard had some limitations in a few technical areas like data transfer rates and cabling lengths. A supplement to the standard called IEEE 1394b was developed in 2002 to improve these areas, as shown in Table 1. The theoretical data transfer rates for IEEE 1394a supported three different speeds: 12.5 MB/sec, 25 MB/sec, and 50 MB/sec. Each device can select which transfer mode it was operating in. The data transfer rates for IEEE 1394b were increased to add 3 more speeds: 100 MB/sec, 200 MB/sec and 400 MB/sec. A fiber optic cable must be used when using the highest data transfer rate. Theoretical Data Rate (MB/sec) Cable Length (meters) Cable Type IEEE 1394a 12.5 or 25 or official 10 practical Copper IEEE 1394b 100 or 200* or 400* 100 (fiber optic) Copper Plastic Fiber Optic Glass Fiber Optic CAT-5 Table 1. IEEE 1394a versus IEEE 1394b (* not yet supported) The cable length was increased from 4.5 meters using copper wire to 100 meters using optical fiber. Testing using IEEE 1394a adapters and 10 meter cable lengths has been conducted at Cognex and 10 meter cables are supported. In the marketplace today, there is limited support for IEEE 1394b. There are very few industrial digital cameras designs supporting it and only a few host adapters that can Cognex Corporation Page 8 of 24

9 support the increased data rates. It remains to be seen how the industry adopts IEEE 1394b. Many are still waiting for Microsoft to offer native driver support IEEE 1394 vs. Other Standards IEEE 1394 was developed as a pure digital connectivity standard. In comparison to other connectivity standards, as shown in Table 2, IEEE 1394 offers some advantages in a few technical areas like extensibility, fast data rates, easy software implementation, and wide adoption of the standard with high availability of industrial digital cameras in the marketplace. The lack of a separate frame grabber and relatively low cost in cameras makes it very attractive for many machine vision applications. Theoretical Data Rate (MB/sec) Camera Availability Hardware Interconnect Communication & Control IEEE 1394a 50 IEEE 1394b Analog Standard Double Speed Analog LVDS RS644 USB 2 ~10 ~ Camera Link 297 (Base) 594 (Medium) 891 (Full) High Low Highest Medium Low High Medium Yes Yes Yes Standard Standard Standard Ease of Use Easy Easy Easy Resolution Up to 3200x2400 and growing Up to 3200x2400 and growing 760x574 max. Yes for most Non- Standard Easy Medium 1600x1200 No Yes Yes Non- Standar d Non- Standard Non-Standard Difficult Easy Medium Very High Medium Very High Requires Frame Grabber No No Yes Yes Yes No Yes Functionality Low Low Medium High Medium Low High Cost Low - Medium Low Low High Low - Medium Medium Medium High Table 2. IEEE 1394 versus Other Standards 2.11 IEEE 1394 Cable The IEEE 1394a connection is made using a serial cable. This cable can provide a pointto-point connection between an IEEE 1394 host adapter and a device. An adapter may support multiple devices each independently connected but sharing the same bus. It is extensible in that devices supporting various IEEE 1394 data rates may occupy and share the same bus, but it will operate at a speed of the slowest device on the bus. The IEEE Cognex Corporation Page 9 of 24

10 1394a standard supports up to 63 devices sharing a total theoretical rate of up to 50 MB/sec (i.e. 400 Mbits/sec), the practical limit is 32 MB/sec. The IEEE 1394a cable is made with a 6-conductor cable that contains two separately shielded "twisted" pairs for transmitting data, plus two power conductors and an overall external shield. The two twisted pairs create a transmit/receive connection but are not bidirectional. One pair supplies the data while the other pair supplies the clock. At any one point in the time the data can transmit in only one direction. The power conductors (8 to A) supply power to some devices Most IEEE 1394 host adapters and devices use a 6 pin connector. Some devices, such as most digital camcorders, use a smaller 4 pin connector to save space. The 4 pin IEEE 1394 connector is a part of the IEEE 1394 standard intended primarily for batterypowered devices. Many PC laptops in the marketplace today have the 4 pin connection built in. You will need a separate power supply to power an industrial IEEE 1394 machine vision camera connected to a 4-pin connector IEEE 1394 Speed Table 3 shows the theoretical maximum data transfer rates for various technologies that are in the marketplace today. In practice, the actual data transfer rates of IEEE 1394 is lower than its theoretical maximum. The same is true for PCI, where the practical data transfer rate of PCI is conservatively ~60-70% of its theoretical peak. The IEEE 1394 bus technology is extensible with increasing data transfer rates. FireWire S100 operates at 12.5 MB/sec (i.e. 100 Mbits/sec) while FireWire S800 operates at 100 MB/sec (i.e. 800 Mbits/sec). Cognex Corporation Page 10 of 24

11 Peak Data or Bus Transfer Rates 2500 Theoretical Data Rate (MB/sec) USB 1.1 USB 2.0 Fire Wire S100 Cognex Supports FireWire S400 FireWire S200 FireWire S FireWire S800 FireWire S FireWire S PCI 32/33 PCI 64/ PCI 64/ PCIX 100 MHz 64 bit 1000 PCIX 133 MHz 64-bit PCI Express 1X PCI Express 4X PCI Express 8X Cam Link Base 594 Cam Link Medium 891 Cam Link Full * Note: PCI Express rates shown are one-half of theoretically possible since in machine vision applications frame grabber usage is generally one direction (writing to the bus) and can t take advantage of bi-directional transmission.. Table 3. IEEE 1394 versus Other Data/Bus Rates A typical industrial machine vision camera that generates a 640x480 sized 8 bit/pixel image running at 30 frames/sec will generate a data rate of 8.8 MB/sec. Image sensors continue to improve and can now generate much larger area scan image sizes, 1600x1200 is not uncommon in the marketplace today. This increased image size will require additional bus bandwidth. The IEEE 1394a bus technology has been widely adopted and there is a ready supply of industrial machine vision cameras that support it. However, the IEEE 1394b standard that supports 100 MB/sec to 400 MB/sec of data transfer rate is not yet fully supported. There is a very limited supply of host adapters and industrial cameras that can support it. 3 DCAM Overview The camera design guide for implementing a digital camera based on the IEEE 1394 standard is the IIDC 1394-based Digital Camera Specification. This specification outlines the hardware command set registers, data transfer modes, video format modes, Cognex Corporation Page 11 of 24

12 and bus serial bus management requirements. All digital cameras that conform to this standard are then suitable to be recognized on an IEEE 1394 bus. The specification itself is explicit and defines a clear protocol between the camera and the host bus adapter and as well as a camera control register set that enforces common functionality among all compliant cameras. 3.1 DCAM Architecture The DCAM specification is a hardware platform independent specification, where implementations are available for a wide variety of hardware platforms and operating systems. The diagram below shows the architecture used in the PC hardware platform supported by Cognex Corporation on the Microsoft Windows 2000 and XP operating systems. Client applications can use the CVL or VisionPro application programming interfaces to communicate with an IEEE 1394 digital camera as shown in Figure 1. Cognex has developed a generic driver adhering to the DCAM specification. This Cognex DCAM driver works with any IEEE 1394a compliant digital camera. The number of cameras is only limited by the available bandwidth on the network bus or by the adapter. Customer Application Cognex CVL/VisionPro SDK Cognex DCAM Driver OS Bus Driver OS OHCI Driver IEEE 1394 Host Adapter Camera Camera Camera Figure 1. Cognex DCAM Architecture Cognex Corporation Page 12 of 24

13 3.2 DCAM Specification The DCAM specification has gone through a number of revisions over the past several years. The most recently adopted version is The Cognex generic DCAM driver complies with digital cameras that support version 1.30 or greater.?? Version 1.04 is dated August 9, 1996?? Version 1.20 is dated July 23, 1998?? Version 1.30 is dated July 25, 2000?? Version 1.31 is dated February 12, DCAM Standard Video Formats There are two of types of video formats supported by DCAM: standard and custom format 7, see Figure 2. DCAM Video Formats DCAM Standard Video Formats DCAM Custom Format 7 Video Format Figure 2. DCAM Video Formats The DCAM specification explicitly lists many standard video formats. For each standard video format there are a wide variety of image sizes, frame rates, pixel sizes, and color types. The standard video format specifies the following types of data: monochrome 8 bits/pixel, monochrome 16 bits/pixel, YUV 4:1:1 12 bits/pixel (average), YUV 4:2:2: 16 bits/pixel (average), YUV 4:4:4 24 bits/pixel, RGB8 24 bits/pixel. This results in hundreds of possible standard video formats. Table 4 illustrates a subset of the video formats to give an indication on how many standard video formats are possible with the specification. Cognex Corporation Page 13 of 24

14 Image Size 640x x x1200 Format Bits/pixel Frame Rates (f/sec) Mono8 (grey) RGB (color) Mono8 (grey) YUV 4:2:2 Mono8 (grey) YUV 4:2: ,3.75,7.5,15,30,60,120, ,3.75,7.5,15,30,60,120, ,15,30,60,120, ,7.5,15,30,60,120, ,3.75,7.5,15,30,60, ,3.75,7.5,15,30,60 Table 4. DCAM Standard Video Formats 3.4 DCAM Format 7 In addition to the standard video formats there is a custom image format called Format 7. This special format allows full flexibility in a video format where the image size, pixel type, and frame rate can be customized. This type of specification allows camera vendors the flexibility to design and build digital cameras to specific requirements, and target cameras to operate within a defined range. Many cameras in the marketplace today support some of the standard formats and may also support some custom Format 7 video formats designed for the camera. Format 7 supports the same types of data as the standard video formats in addition to the following: bits/pixel, Signed bits/pixel, Signed bits/pixel, bits/pixel (Bayer pattern), bits/pixel (Bayer pattern). One of the special features of a Format 7 video format is the ability to specify a reduced image size corresponding to any region of interest (ROI) on the sensor. On some cameras this may increase frame rate due to the reduced ROI. FireWire cameras can be queried by software to determine the standard video formats and custom Format 7 video formats that are supported. 3.5 DCAM Features Features in DCAM correspond to acquisition properties in Cognex software. Values of contrast, brightness, and others, can be easily set at application run-time. Because the specification is explicit in defining the DCAM register space for DCAM features, all features can be easily changed at run-time. The features available in the DCAM specification include: Brightness, Auto Exposure, Sharpness, White Balance, Hue, Saturation, Gamma 1, Shutter, Gain, Iris, Focus, Temperature, Trigger, Trigger Delay, White Shading, Frame Rate, Zoom, Pan, Tilt, Optical Filter, Capture Size, Capture Quality, PIO, SIO, and Strobe Output. All of these features are supported with Cognex software either through an acquisition property interface or by direct control through Cognex Corporation Page 14 of 24

15 generic read/write methods to access the camera registers. See section 5.2 for more information. 4 IEEE 1394 Open Host Controller Interface (OHCI) The IEEE 1394 OHCI is an implementation of the link layer protocol of the IEEE 1394 serial bus, with some support for the transaction and bus management layers. It is based on the IEEE 1394 Open Host Controller Interface Specification release version 1.1, dated January 6, The IEEE 1394 OHCI also includes DMA engines for highperformance data transfer and a host bus interface. IEEE 1394 and the 1394 OHCI supports two types of data transfers: asynchronous and isochronous. Asynchronous data transfer puts the emphasis on guaranteed delivery of data, with less emphasis on guaranteed timing. Isochronous data transfer is the opposite, with the emphasis on the guaranteed timing of the data, and less emphasis on data integrity. Microsoft Windows XP and Win2K provide a built in bus driver and OHCI driver compatible with OHCI-compliant host adapters. The bus driver communicates with the DCAM driver and the OHCI driver communicates with the host adapter. Some adapter cards can also provide their own drivers. However, non-microsoft 1394 bus drivers are not supported by CVL or VisionPro Bus Driver OHCI Driver FireWire Adapter Figure 3. OHCI architecture 5 Cognex DCAM Implementation Cognex has implemented complete support for acquiring images from any DCAM version 1.30-compliant or greater camera using both CVL and VisionPro interfaces, as shown in Figure 4. The acquisition API is same as that of other Cognex frame grabbers. The key benefit using Cognex s interface rather than a camera vendor s SDK is that you can acquire images easily and then run Cognex s Vision Tools on the acquired image. This can be done without any need to learn and integrate a third party camera vendor Cognex Corporation Page 15 of 24

16 API. It also allows easy transition to (or from) a Cognex frame grabber if it turns out that DCAM is not suitable for the application. Customer Application (C++, C#, VB) Cognex CVL/VisionPro Cognex Common Acquisition Library Cognex 8100L Frame Grabber IEEE 1394 Host Adapter IEEE 1394 IEEE 1394 Cognex 8100M Frame Grabber Cognex 8100C Frame Grabber Cognex 8504 Frame Grabber Cognex 8501 Frame Grabber IEEE Cognex 8600 Frame Grabber Cognex 8120 Frame Grabber Figure 4. Cognex IEEE 1394 DCAM Support The Cognex DCAM implementation also allows users to acquire from cameras from multiple vendors. This allows mixing and matching cameras from different vendors in the same application. For example, in a two camera application you could have one high resolution camera from one vendor and a low resolution camera from a different vendor. The DCAM driver architecture is similar to all other Cognex frame grabber solutions. Users can recognize FireWire cameras in much the same way as recognizing Cognex frame grabbers. 5.1 Cognex DCAM Supported Video Formats Cognex supports both Standard generic DCAM formats and Format 7 video formats. Image pixel types that are officially supported are Mono8 and Raw8 (color Bayer). Other types may be supported in the future. Consult with your software documentation to determine what formats are supported. All video formats are supported by camera configuration files (ccf's), so it is easy to add support for a custom format 7 video format if required without requiring a new release of software. Cognex Corporation Page 16 of 24

17 5.2 Cognex DCAM Supported Camera Properties Cognex supports setting camera properties like contrast, brightness, and exposure through the advanced property interface classes available in our software. Thus, it is possible to change these properties at acquisition run-time and still achieve the best possible acquisition rate. All other DCAM properties are supported via our API software using generic read and write DCAM access register functions, as listed in Table 5. Note that most DCAM cameras do not support the entire list of features shown in Table 5. DCAM Features Run-Time Control Through Advanced Property Interfaces Run-Time Control Through Read/Write Register Access Functions Version 1.30 Gain (Contrast) Brightness Shutter (Exposure) Trigger Mode Auto Exposure Sharpness White Balance Hue Saturation Gamma Iris Focus Temperature Zoom Pan Tilt Optical Filter Version 1.31 Trigger Delay Strobe Output White Shading Frame Rate Prioritize PIO SIO Table 5. Cognex DCAM Feature Support Cognex Corporation Page 17 of 24

18 5.3 Cognex Multi-Camera/Multi-Vendor Support Cognex s DCAM implementation supports simultaneous acquisition from multiple vendor s DCAM cameras all connected to a single IEEE 1394 bus. There is no need to use the camera vendor s software to communicate to the camera, all this is automatically provided through a single common acquisition interface and a single driver. If needed, mixing a FireWire camera with a Cognex frame grabber is also supported in the same application. Cognex Drivers (DCAM and Frame Grabbers) Basler PCI Bus 0 IEEE 1394 Host Adapter PGR Cognex 8504 Frame Grabber PCI Bus 3 PCI Bus 1 IEEE 1394 Host Adapter Sony Teli Sony Teli Sony IEEE 1394 Hub AVT AVT Figure 5. Mixing FireWire Cameras 5.4 Cognex Dynamic Bandwidth Adjustment When simultaneous acquisition from multiple FireWire cameras is required, balancing the total amount of available bus bandwidth among all the cameras is a challenge. There needs to be a way for each camera to claim a certain percentage of the bus depending on how fast it needs to run. If each camera claimed 100% of bandwidth it required, then depending on the total bandwidth requirement, one or more cameras might be prevented from acquiring at the same time. Cognex introduced a new property called camera bandwidth that can be used to set the amount of bandwidth used by each camera on the bus. The value of this property ranges from 0.0 to 1.0. The default value for this property is 1.0 which means that each camera will use the maximum bandwidth it requires. Setting this property to 0.5, for instance, Cognex Corporation Page 18 of 24

19 will reduce the bandwidth used by the camera to half of its maximum bandwidth. This approach lets cameras share the available bandwidth among multiple cameras. This bandwidth property (BP) can be changed dynamically for every acquired image, if needed. This property can be used solve difficult applications that require dynamic rearrangement of bandwidth amongst cameras. The camera bandwidth property needs to be adjusted only when one needs to use multiple cameras simultaneously and when the combined bandwidth of all these cameras exceeds the available bandwidth. A single IEEE 1394a camera by itself will not exceed the maximum bandwidth available on the IEEE 1394a bus. But it is possible to set the camera bandwidth property even for a single camera. This makes the camera use a smaller amount of bus bandwidth than normal. Note that the BP setting only applies when operating the camera using Format 7 video formats. There is no dynamic bandwidth allocation possible when operating cameras using generic video formats. Also note that reducing the bandwidth that a camera uses by setting the camera BP will increase the time to transfer the image from the camera to the PC. This means image acquisition will take longer to complete and the camera s acquisition frame rate (FPS) will decrease. Figure 6 shows an example of balancing 3 cameras for simultaneous acquisition and the effect of setting the bandwidth property to prevent overload of the IEEE 1394 bus. Each camera can operate at a maximum of 640x480 at 60 frames/sec. Attempting to run all three cameras on the same bus at the same time would create a bus overload condition. By setting the bandwidth property to 60% for each camera, each would operate at 35 frames/sec and not overload the bus. Alternatively, each camera can support any value of bandwidth property value allowing the user to select the bandwidth allocation for each camera. Cognex Corporation Page 19 of 24

20 Total Isochronous 1394a Bus Bandwidth = 32 MB/sec Bus Overloaded BP = 100% Camera #1 60 FPS 18 MB/sec BP = 100% Camera #2 60 FPS 18 MB/sec BP = 100% Camera #3 60 FPS 18 MB/sec BP = 100% Bus Balanced BP = 60% Camera #1 640x FPS 10.8 MB/sec BP = 60% Camera #2 640x FPS 10.8 MB/sec BP = 60% Camera #3 640x FPS 10.8 MB/sec BP = 60% Bus Balanced BP = Mixed Camera #1 60 FPS 18 MB/sec BP = 100% Camera #2 640x FPS 5 MB/sec BP = 30% Camera #3 640x FPS 9.0 MB/sec BP = 50% 16 Bandwidth (MB/sec) Figure 6. Using Bandwidth Property to Balance Available Bus Bandwidth for Synchronous Acquisition 32 Below are two practical examples of how to make adjustments to meet IEEE 1394a bandwidth requirements. The first example uses the bandwidth property to reduce the pre-camera bandwidth used for a single adapter configuration. The second example uses an additional adapter to increase the total the bandwidth available to the application. Cognex Corporation Page 20 of 24

21 An application requires 2 identical cameras each using a Format 7 video format: 640x480, 60FPS. Notes:?? The bandwidth property (BP) default is 1.0?? The maximum packet size on an IEEE 1394a bus is 4096 bytes?? The number of packets sent per second on an IEEE 1394a bus is 8000?? The amount of time to transfer one packet on an IEEE 1394 bus is 1/8000 or 125 microseconds Bandwidth used by a camera is represented as bytes per packet (BPP). This value is found inside the camera configuration file (CCF) for the video format. For this example, the BPP for each camera (from the CCF) is Bandwidth required by each camera = BP * BPP * 8000 / (1024 * 1024) = 1.0 * 2560 * 8000 / (1024 * 1024) = ~19.5 MB/sec Total bandwidth required by both cameras = ~19.5 MB/sec * 2 = ~39 MB/sec Note, Maximum The maximum Bandwidth video allowed (isochronous) on IEEE bandwidth 1394a bus supported on an IEEE 1394a bus = 4096 = 4096 * 8000 * 8000 / 10e-6 / (1024 * 1024) = 32 = MB/sec ~32 MB/sec Therefore, if the cameras were running simultaneously, the bandwidth required would exceed the maximum available bandwidth on an IEEE 1394a FireWire bus. To allow both cameras to run simultaneously, the bandwidth property value could be changed for example, from 1.0 to 0.5 (half the maximum). In this case: Bandwidth required by each camera = 0.5 * 2560 * 8000 / (1024 * 1024) = ~9.8 MB/sec Total bandwidth required by both cameras = ~9 MB/sec * 2 = ~19.5 MB/sec Thus, it is possible to adjust each camera s bandwidth using the bandwidth property. Now both cameras can share the available bandwidth on the bus and can acquire simultaneously. Note that the bandwidth property will affect the image acquisition frame rate. In the above example, Cognex Corporation Page 21 of 24

22 The following example uses an additional IEEE 1394 adapter to increase the total the bandwidth available to the application. An application requires performing an inspection at up to 30Hz using 3 FireWire cameras. Two of the cameras are the same and use a 640x480 video format. The third camera uses a 1024x768 video format. The cameras must be able to run simultaneously. Bandwidth used by a camera is represented as bytes per packet (BPP). This value is found inside the camera configuration file (CCF) for the video format. For the 640x480 cameras, the BPP from the CCF is For the 1024x768 camera, the BPP from the CCF is Total bandwidth available on an IEEE 1394a bus in BPP = 4096 Total bandwidth required by all three cameras in BPP = = 5632 Therefore, if all 3 cameras were running, the bandwidth required would exceed the maximum available bandwidth on an IEEE 1394a FireWire bus. To allow all cameras to run simultaneously, the cameras could be placed on two IEEE 1394a buses by using 2 IEEE 1394a adapter cards. For example: Bandwidth required on bus A using the 2 640x480 cameras = = 2560 Bandwidth required on bus B using the 1024x768 camera = 3072 Thus, it is possible to place cameras on multiple IEEE 1394a buses without having to decrease camera frame rate. Now all cameras can acquire simultaneously. 5.5 Cognex DCAM Topology The Cognex API provides functions to understand the topology of the cameras on each bus. Information such as node number, part number, vendor identification can be easily retrieved to create a topology mapping of all cameras. 5.6 Cognex Advanced and Non-Compliant Feature Access The Cognex API provides generic read register and write register functions that can be used to communicate with any register in the DCAM register space. This allows the user to set any custom or advanced feature supported by the camera, if needed. There are many cameras that have implemented advanced features not defined in the DCAM Cognex Corporation Page 22 of 24

23 specification. Some of them include time stamping, auto synchronization, trigger error detection, and broadcast acquisition. 6 Conclusion FireWire cameras can be one of the most cost effective choices for many machine vision applications. As long as the application takes the limitations into account and has mechanisms to deal with these, there is no reason to think that FireWire cameras cannot meet the same machine vision requirements as traditional frame grabbers. FireWire is a registered trademark from Apple Computer, Inc. Cognex Corporation Page 23 of 24

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