Designers of high-throughput, multi-camera machine vision systems have grown dissatisfied with aging standards and have found a new champion, CoaXPress (CXP), a high-speed, point-to-point, serial communications interface that runs data over off-the-shelf 75Ω coaxial cables.
The original CXP, introduced in 2008, supported a maximum data rate of 6.25 Gbps, approximately six times faster than GigE Vision and 40 percent faster than USB3. Version 2.0 of CXP has added two more speeds: 10 Gb/s (CXP-10) and 12.5 Gb/s (CXP-12). CXP 2.0 is ideal for supporting multiple high-resolution cameras and does not require the complexity or cost of multiple cables and connectors and is a platform that is easily adapted and scaled to meet changing requirements.
Also, CXP offers greater flexibility for system integrators who previously were handcuffed to a few metres of cable if using Camera Link or USB3. Below are CXP’s maximum cable lengths:
CoaXPress Transmission Distances
- Data Rate Maximum Distance
- 1.25 Gbps (CXP-1) 105 metres
- 3.75 Gbps (CXP-3) 85 metres
- 6.25 Gbps (CXP-6) 35 metres
- 12.5 Gbps (CXP-12) 25 metres
Uncompressed data, power and low-speed uplink are all simultaneously distributed over a single coaxial cable, reducing complexity and potential points of failure. Coaxial has inherently excellent protection against EMC/radio frequency interference, minimizing the risks of costly downtime or latencies. Finally, CXP supports GenICam. Widely adopted by industry partners, GenICam simplifies application development or upgrading components.
Is GigE vision the answer?
Only GigE Vision can compete with CXP on cable length in multi-camera systems. Going head-to-head with CXP 2.0 is the new 10 Gigabit Ethernet (10 GigE Vision) interface. It provides a tenfold increase in data transmission speeds over its predecessor, GigE, and was specifically targeted for high-speed testing environments.
Unfortunately, there are some serious drawbacks. For one, 10 GigE Vision is exceptionally power hungry. It requires up to seven watts for operation, not including the cameras’ power requirements. Power consumption is roughly twice that of other interfaces. Nor has 10 GigE shown itself to be efficient with handling heavy data loads. It leans on the PC’s CPU and internal memory bus for operation because processing and buffering cannot be offloaded to a frame grabber FPGA or to memory.
PnP discovery and operation is also required under all circumstances, making for a complex system subject to bottlenecks. And while power over cable and real-time triggering are said to be planned for its next version, 10 GigE does not offer them now. Ironically, high-data applications using 10 GigE actually need a frame grabber to offload CPU and memory, therefore eliminating what was the principle benefit of the standard over CXP. Cost benefits of GigE Vision are further diminished because expensive, high-end server components serve as its backbone. All things considered, 10 GigE appears to be a step backward, rather than forward.
Similarly, USB 3.1 Vision (SuperSpeed+) has fallen short of expectations. Limited to one to three meters with a passive cable, USB 3.1 Vision requires expensive active cables for each camera on a typical system.
CXP in multiple camera systems
Multi-camera systems have been a fixture in machine vision for decades. What is new is CXP. It allows multiple cameras to be linked by a single frame grabber over long, inexpensive and very robust coaxial cables with zero latency and exact synchronization. Various resolution cameras set at high or low frame rates can be linked to a single CXP frame grabber, each performing a different inspection task. Even CMOS and CCD sensors can be mixed in the configuration.
One of the arguments against CXP is that it requires a frame grabber, an expense that USB3 and GigE Vision dodge. Mistakenly, the impression is given that a multiple-camera system based on CXP is therefore more complex and expensive. Yet this ignores the fact that the load on the PC significantly increases with USB3 and GigE Vision. What savings are realised with USB3 are quickly negated by the cost of computing resources, while an expensive network card must be purchased for GigE Vision for proper operation.
Here is another point: The required precise synchronising of cameras in a multi-camera configuration is the byproduct of a deterministic interface. Both CXP and Camera Link are inherently deterministic. GigE Vision and USB3 are not. Workarounds are possible yet these steps invite unstable performance and latency when nodes are added or when bandwidth is shared due to packets being dropped. As Camera Link is not an option for high-speed multi-camera systems this leaves CXP as the only practical choice.