Machine Vision Digital Connection Standards - Part 2

This is the second part in a two part series that describes the current connection types used in machine vision and imaging. The first part can be found here.

IEEE 1394a & 1394b (Firewire)

IEEE 1394 (Firewire 'Apple Trademarked’) was developed by Apple Computers around 1986 for fast (at the time) and relatively low cost serial interface for delivering bulk data between personal computers. Other manufacturers such as Sony began using Firewire under license. Sony dubbed their Firewire connection 'iLink’ and this is Sony Trademarked’.

There is more than one software protocol used with the Firewire connection. Many digital still and consumer camcorders use Firewire but with the DV protocol. This is a compressed protocol and is not compatible with the DCAM protocol which is used in the industrial machine vision market.

DCAM which has more recently developed into the IIDC 1.03 standard is an architecture which defines Firewire camera functionality and how to control it. The idea being that any IIDC 1.03 camera can be plugged into a computer and work with any software that supports this standard. This capability is currently unique to Firewire and GigE Vision (w Genicam).

The IIDC standard defines such things as:

• Video formats e.g. 640 x 480 x 30 frames/s x 8 bit mono of 1280x960 @ 15fps x 16bit YUV422 colour etc.
• IIDC also scopes some custom formats which allows Firewire camera manufacturers to expand their functionality beyond the standard.
• How the camera is identified when plugged in.
• Assignment camera control registers.

Firewire has two formats IEEE1394a and IEEE1394b. IEEE 1394a:

• 400-Mbits/s capacity.
• Both isochronous and asynchronous.
• Isochronous is used for data transfer, guarantees 75% of bandwidth.
• Asynchronous is used for handshaking and control; if not used for this it can be used for data transfer as well.

With isochronous transfers, 1394a guarantees a 32 Mbytes/s of data transfer per Firewire port. Many 1394 cards have three connectors but these all connect to a single Firewire port (chipset) and so share the 32 Mbytes/s. Some other cards have 2 or perhaps 4 separate ports.

IEEE1394a carries power to devices but the standard defines the maximum cable length to be 4.5m. In reality high quality cables can typically successfully carry a IEEE1394a connection 10 metres. For longer distances IEEE1394a hubs can be used to retransmit the signal 10 metres further. For very long distances IEEE1394a Fibre extenders are available.

Cameras are now being released which comply with the newer IEEE1394b format. This can handle higher data rates of up to 800 Mbits/s (65Mbytes/sec sustained). IEEE1394b has all of the features associated with IEEE1394a and is backwardly compatible.

Firewire requires a Firewire hardware connection at the computer and while some computers are fitted with this connection it is by no means as prevalent as the USB2.0 connection. In fact there appears to be a small shift away from Firewire with some computer manufacturers such as Dell. If the computer does not have a Firewire connection built-in a Firewire adapter (card) is required, and these are available for desktops (PCI) and laptops (PCMCIA). While they can be thought of as a frame grabber they tend to be fairly low cost at < $100.

Firewire chipsets typically incorporate DMA engines and so while Firewire requires more CPU involvement than CameraLink it requires significantly less than USB2.0 and so is better suited to computationally intensive or multi-camera applications than USB2.0.

Firewire is best suited to applications that will benefit from or not be hindered by:

• Interchangeability of devices without losing software investment.
• Medium speed data-transfer rates.
• Relatively short cable lengths
• Relatively low CPU involvement with data transfer
• Port sharing for multiple cameras
• Non-deterministic transfers

Gigabit Ethernet (GigE)

GigE Vision (Gigabit Ethernet) is a new standard in the machine vision world. The AIA (Automated Imaging Association) sponsored the GigE Vision standard and it was developed over several years by a group of the world’s leading machine vision manufacturers. They saw the need to develop a connection that took advantage of the positive aspects of USB2.0 and Firewire but with features aimed specifically for machine vision. As a result GigE Vision is gaining rapid adoption by machine vision camera manufacturers with many now announcing a full range of GigE cameras.

GigE Vision has created a lot of excitement in the industry because of its enhanced data capacity, long cable lengths and interchangeability features. Perhaps its strongest feature is that it works over the Gigabit Ethernet networking standard which virtually all computer manufacturers are now equipping their new motherboards with. Gigabit Ethernet is the extension of the 10/100 BaseT ethernet we have all used for networking for many years. Because ethernet is so widely used there are already a wide range of accessories available such as hubs, switchers, fibre extenders etc.

GigE offers 1000 Mbits/s = 125 Mbytes/s maximum transfer rate. In reality you can expect to sustain approximately 100MB/sec with the correct software drivers. This means plenty of capacity for high resolution and high frame rates. GigE can work happily with cable lengths of 100m (Cat5e, Cat6). Add some switches or fibre extenders and it becomes virtually unlimited.

As mentioned above GigE uses computer industry standard protocols. GigE Vision data is sent over the UDP layer. This offers much faster data rates than TCP/IP. However UDP does not offer QOS (Quality of service) i.e. data is not checked and re-requested if it becomes corrupt or lost, so with UDP alone you cannot guarantee that the data sent is actually received. TCP/IP is typically the office standard mechanism for moving data across a network. It offers QOS but is relatively inefficient because the TCP software must support a myriad of data protocols that are required of a connectivity medium that is designed as a generic office network. TCP packet decoding can use substantial CPU resource and so is not optimized for vision where large amounts of data need to be transferred quickly. For high speed transfers with data integrity and minimal CPU intervention GigE Vision has adopted TOE (TCP Offload Engine) which is built into the newer Intel Ethernet chipsets. This provides data reliability and frees the CPU from much of the work required for data transfers.

To achieve the fastest data rates Gigabit Ethernet should be used as a direct peer-to-peer configuration where the camera is connected directly and exclusively to the network interface card. In this way jumbo packets can be used and the maximum rates achieved. You may choose to share the Gigabit Ethernet connection with more than one camera. This will still allow jumbo packets and the maximum data rate but the data rate needs to be shared by the cameras so reducing the allowable data rate for each camera. GigE Vision cameras can also be used on a Gigabit Ethernet network with other data devices i.e. office network with computers and printers etc. However maximum data rates cannot be achieved in this way because a filter driver needs to be adopted to filter the video data from the normal network data and there is bandwidth sharing.

Similar to Firewire and to USB2.0 Gigabit ethernet is a packetised protocol. The video data is split into packets, transmitted and then collected at the computer where the packets are reassembled into an image. Similar to Firewire and USB2.0 Gigabit Ethernet has some time latency and so is not deterministic as is CameraLink. However being significantly faster than Firewire and USB2.0, Gigabit Ethernet has lower latencies.

Camera control is built into the GigE Vision connection and there has been a recent development of the Genicam standard (by the European Machine Vision Association) which is similar to the IIDC standard for Firewire. It defines the standard modes and formats a camera will run in (resolution, colour bit depth etc) and defines a standard protocol to communicate and control the camera. This will provide camera interchangeability to users, integrators and OEMS to protect software investment and make them less reliant on one camera or manufacturer.

Gigabit Ethernet does not carry power along the same cable and so cameras must be powered independently. We have heard however that this is being addressed and so we expect this will be a feature of GigE Vision in the not too distant future.

GigE Vision is best suited to applications that will benefit from or not be hindered by:

• Interchangeability of devices without losing software investment.
• Uses standard computer hardware
• High speed data-transfer rates.
• cable lengths up to 100m
• Relatively low CPU involvement with data transfer
• Port sharing for multiple cameras
• Non-deterministic transfers are OK.

To summarise all of the above discussion into a very simple snapshot:

• Lowest cost - USB 2.0
• Longest cable - GigE
• Highest Speed - Camera Link
• Standard software - Firewire and GigE

Be sure when planning your machine vision system that you understand what is available and how it will tie into the environment you are working with, also how it will work with the specific camera/frame grabbers you are intending to use. If you’re not sure then consult with AES and we’ll offer the right advice that will be able you to supply the solution that the customer requires first time, every time.

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