PCs and cameras plug and play

Camera Link simplifies the camera-to-frame-grabber connection.

Jon Titus, Editorial Director -- Test & Measurement World, 3/1/2001

When it’s time to design or implement a machine-vision system, your biggest headache shouldn’t be how to connect a digital camera to a PC. You’d expect a standard cable to connect the two. But until recently, camera manufacturers and frame-grabber suppliers provided a variety of connectors that in turn required custom-made cables. The price of those special cables could quickly increase the cost of a machine-vision project. Now, a standard called Camera Link makes it easier and cheaper to make the camera-to-PC connections.

The machine-vision industry has long known it needed a simple way to connect digital cameras and PCs. But the industry comprised many small companies, none of which had enough clout to force a standard on the others, so standardization efforts made little progress.

Several years ago, the Automated Imaging Association (www.machinevisiononline.org) proposed a 68-pin interface that used the same connectors and cables as SCSI peripherals. As a plug-and-play standard, the AIA’s configuration worked well, and companies such as Kodak and Data Translation provided compatible products. But the large connector required extra panel space, and the thick mating cable proved difficult to work with. Few other suppliers adopted the standard.

After the aborted attempt to adopt a standard, camera manufacturers “discovered” low-voltage differential signaling (LVDS) chips that serialize data and send it up to 10 m at rates approaching 1.9 Gbps, and with few errors. Although primarily meant to communicate with LCDs, the chips seemed ideal as an interface for digital cameras. The LVDS technique itself was standardized as EIA-644 in 1996; the new Camera Link standard simply builds on this existing physical-layer standard as well as on standard ICs available from National Semiconductor ( www.natsemi.com). Other IC suppliers offer similar LVDS ICs, but they’re not guaranteed to be compatible.

The Camera Link standard (Ref. 1) prescribes a 26-pin Mini Delta Ribbon connector (MDR-26) from 3M (www.3m.com) and a standard cable that makes the interface a true plug-and-play affair. All signals are unidirectional and differential ( Figure 1).

Data transfers from a camera to a frame grabber take place in 28-bit bundles—24 bits of data and four status bits: frame valid, line valid, and data valid, and a spare bit for future use. The 24-bit data path—divided into 8-bit ports labeled A, B, and C—lets the interface easily handle monochrome and color digital cameras with a variety of resolutions and data formats.

An LVDS interface chip serializes the information and transmits it over four differential data channels. The chip also sends a differential clock signal to the receiver that reconstructs the serialized data into 28 parallel TTL/CMOS outputs. Data-transmission takes 10 of the Camera Link’s 26 signals.

The Camera Link interface also provides for serial communication of command and control information between a camera and a frame-grabber board. In most cases, cameras can receive transmissions from the frame grabber, but transmissions from the camera to the frame grabber are optional. The serial-communication circuits use four differential signals, and they use the same protocol of start, data, and stop bits as standard serial ports on a PC. But don’t confuse the Camera Link serial port with the usual COM ports built into PCs.

Eight Camera Link signals provide four direct control lines, CC1–CC4, reserved for general-purpose camera control. Camera manufacturers can define these signals as needed for their different camera models. The signals find use in applications that require faster response times than are possible using the serial port. The last four Camera Link pins connect to the cable’s inner shield. The cable provides no power to a camera.

The above description of Camera Link refers to the standard’s base configuration. The standard lets manufacturers add a clock signal, four control signals, and ports D, E, and F—for a medium configuration. A medium configuration expands to a full configuration with the addition of a clock signal, four control signals, and ports G and H. So, camera and frame-grabber manufacturers have plenty of additional data capacity should they need higher data-transfer rates and wider bus widths.

The medium and full configurations do not add additional camera-control or serial-port signals. A full or medium configuration requires only one additional 26-pin connector. The extra data lines let cameras transfer video information in parallel to several PCI-based frame grabbers, no one of which could keep up with the data rate from a high-speed, high-resolution camera.

Standard covers little software

Although the standard clearly specifies electrical connections and protocols, it provides little standardization of software. The only Camera Link software standard relates to four general API calls that control the frame-grabber’s serial communication. These APIs open and close the serial channel and transmit and receive serial data. When it comes to controlling the camera and moving the camera’s data from the frame grabber to the host PC’s memory or disk, you must use drivers that accompany the frame-grabber. Because the Camera Link standard specifies how data transfers take place, frame-grabber vendors can supply drivers for most Camera-Link-compatible cameras.

The standardization of camera and frame-grabber communications means you can now get an application up and running in days instead of weeks. And because the commands that control a camera come from the host PC, you won’t have to spend time configuring camera DIP switches or jumpers. If you must change a camera’s settings, all it takes is running the camera-setup software again.

The Camera Link standard lets you concentrate on features when you shop for cameras. You no longer have to choose from a limited number of cameras that work with a specific frame grabber, or vice versa. Switching cameras means simply loading a new driver and reconfiguring the system for the new camera type. You don’t have to spend several hundred dollars on a special cable for each new camera type. And you don’t have to fool with hardware settings in the PC or camera.

Depending on how software vendors write their driver code, you may find switching to Camera Link hardware easier than you think. If a vendor uses the same API calls in all its frame-grabber drivers, you can simply add a Camera Link frame grabber, run the configuration software, and use your existing software. Check with frame-grabber manufacturers to find out what an upgrade to Camera Link equipment involves in the way of software changes.

You may need more I/O lines

You may want to buy I/O boards from the same vendor that supplies your frame-grabber boards because the vendor probably has experience using both board types and their drivers in a vision system. If you’re building an extensive machine-vision system, you may want to consider boards that share timing signals and other information as a tightly integrated vision subsystem within a PC.

Unfortunately, standards don’t stand still for long. Dalsa ( www.dalsa.com), a supplier of high-end digital cameras, has already proposed extensions to the time-multiplexing and data-rate-division features of the standard. (Ref. 2) These efforts don’t threaten the Camera Link standard, but they may cause some confusion as customers wonder what will change next. In any event, today’s Camera Link standard will remain useful for a long time. We’ll have to see if various “extensions” splinter future efforts to maintain one standard.

Although the Camera Link standard eases hardware and software design, perhaps its biggest advantage is that it has fostered cooperation between camera manufacturers and frame-grabber suppliers. Their efforts to standardize what was previously a confusing and expensive connection between their products will pay off for buyers. T&MW

References

1. You can find the Camera Link specifications in an Adobe Acrobat file on the Automated Imaging Association Web site: www.machinevisiononline.org. To find the document, search the site using the term “Camera link.”

2. Information about the Dalsa extensions to Camera Link is available at the company’s Web site: www.dalsa.com . Look for the Dalsa Implementation Roadmap.

Jon Titus has written real-time software and designed embedded systems and computer/instrument interfaces. He worked in electronics for 10 years and spent nine years at EDN magazine prior to joining T&MW in 1993. He has a BS from WPI, an MS from RPI, and a PhD from VPI.