Getting Camera Link specs right
Steve Scheiber, Contributing Technical Editor -- Test & Measurement World, 5/22/2006 10:15:00 AM
Comparing different aspects of electronics technologies requires that you compare them on the same basis--apples to apples, so to speak. In the February Machine Vision & Inspection Test Report, I discussed standard methods for transferring image data from a video camera to an analysis system. In the process, I looked at the specifications for Camera Link, GigE Vision, Gigabit Ethernet, FireWire, and USB. Since the article appeared, several questions have arisen, in particular questioning the raw speed of Camera Link.
All of the sources I spoke to--whichever standard they preferred--classified Camera Link as the fastest of the alternatives. Thier calculations of the actual speeds, however, did not always match. Some sources quoted the maximum Camera Link bandwidth at 7.14 Gbps. Others used 6.12 Gbps or 5.5 Gbps. Why the confusion? I recently went back to Steve Kinney, product manager at JAI Pulnix and a former chair of the Automated Imaging Association'sCamera Link Committee, for clarification.
The Camera Link standard employs chipsets that run at 40, 66, or 85 MHz. A base configuration uses a single Camera Link connector carrying one 28-bit video channel. Including a second connector adds two additional 28-bit channels. A medium configuration uses two of the channels, and a maximum configuration employs all three. Therefore, with the 85-MHz chipset, some sources report the maximum base-configuration speed at 2.38 Gbps, medium configuration at 4.76 Gbps, and maximum configuration at 7.14 Gbps. According to Kinney, however, those numbers overstate the case.
Those specifications assume that all Camera Link configurations use all 28 bits per channel. Kinney contends that only 24 bits per channel actually transfer data. The first channel’s remaining four bits provide camera timing. The extra bits on the second connector provide four lines for camera control and four lines for bidirectional serial communication. At 85 MHz, then, the maximum speeds reduce to 2.04 Gbps for 24 bits, 4.08 Gbps for 48 bits, and 6.12 Gbps for 72 bits. Ironically, a proponent of GigE Vision, not Camera Link, provided the higher numbers.
Other sources reported the Camera Link bandwidth at 5.5 Gbps. Kinney suggested that specification is also based on 28-bit channels, but uses a maximum chipset speed of 66 instead of 85 MHz.
When comparing specifications for making engineering decisions, be sure that the numbers translate accurately to the real world and directly to your specific application. The criteria around which they are established must be the same for all the alternatives. Otherwise, the decisions you make may not achieve the performance you are looking for.
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