What’s New in CLHS Rev 1.1?
Written by Mike Miethig
The CLHS committee has published release 1.1 in order to catch up with released products and provide a development path for increased bit rates using the same connectors. These capabilities have been added as a result of recently released cost effective FPGAs and the availability of fiber modules able to achieve the included bit rates. Rev 1.1 change is limited to increasing the speed of the connector styles all ready in use to meet the demands of higher speed cameras which are being brought to market now and in the future.
The M protocol C2 connector has an increased speed from 3.125 to 5.0 Gbps per lane bringing that connector bandwidth from 2.1 GByte/sec to 3.3 GByte/sec. Products implementing 5.0 Gbps have been available for several years and the specification is just catching up. This connector features 7 down lanes and 1 up lane and the ability to support active optical cables. The copper cable, which is similar to the Camera Link cable, is able to reach 15 meters at 3.125 Gbps and carries about 3x the CL Deca Bandwidth with error detection and correction capability.
Rev 1.1 introduces the C3 connector, which is an increased speed version of the C2. The C3 is the same mechanically as the C2 just slight internal connector changes to raise the connector performance to 20 Gbps. The X protocol is defined for the C3 as it is about 25% more efficient than 8b/10b encoding and has a default bit rate of 10.3125 Gbps. The X protocol also features Forward Error Correction and is able to correct a burst of 11 error bits. The higher bit rate and higher encoding efficiency means the 7 down lanes transfer a maximum of 8.4 GBytes/sec. The plug on AOC cables developed for the C2 products were developed with the C3 requirements in mind and as a result it is possible to send that data 100 meter through a 3mm diameter cable. In fact the AOC cables use 14 Gbps capable optical engines, but Rev 1.1 only lists 12.5 Gbps as the next allowed speed to ensure margin. At 12.5 Gbps, the C3 cable sends 10.2 GByte/sec of data, the highest of any machine vision standard. That’s a lot of data and frame grabber manufacturers add a second C3 connector and turn on the unused transmitters and forward the data to slave frame grabbers/PCs so that processing can be accomplished on multiple PCs. This is a very cost effective method to achieve parallel processing. Some applications need more than 6 PCs to keep up with one camera. This simple high speed processing architecture is a feature made possible due to the asymmetric bandwidth cable and small connector size found only in CLHS.
Rev 1.1 improves the bit rates of the F2 (SFP+) connector as well. The original specification defined 10.3125 Gbps for the SFP+ connector which uses the 64/66B encoding with the Forward Error Correction (FEC) of the X protocol. The shared IP core available from the AIA is able to support higher speeds and the newer mid tier FPGAs are now able to support bit rates up to 16 Gbps and so CLHS decided it was time to increase the bit rate for the F2. Data rates now include the default 10.3125, and added 12.5, 13.75 and 15.93755 Gbps which can be generated by Altera and Xilinx FPGAs from a 156.25MHz reference clock. The original 10.3 Gbps matches the recently introduced CXP-12 in terms of data throughput at 1200Mbyte/sec. At 15.9 Gbps CLHS achieves a throughput of 1850 Mbyte/s on the single lane. Note worthy is that a dual LC fiber cable costs about $10 for 20 meter is about 2? mm in diameter and can be tied in a knot without loss of signal and is able to reach 300 meters. Bonus features of fiber include its immunity to ground bounce and radiated emissions found in factory environments, which may cause bit errors for copper interfaces. The LC connector has been used in military applications and is a robust connector.
Another reason machine builders should look for fiber cabling is that once installed, it will be able to support cameras with increased bit rates. Telecom installations are able to run 28 Gbps on fiber that was installed for 10 Gbps. We have seen that coax solutions need to change connectors/cables with increasing bit rates.
The CLHS committee has evaluated its shared X protocol IP core at the increased 28 Gbps and found that it works at these speeds without change. CLHS is patiently waiting for pricing to come down to make it reasonable to put the 28 Gbps speed into the standard. CLHS may not have to wait too long: 2 years ago the 28G SFP28 module was priced at $800, today its around $70. When pricing of the FPGAs and the optical modules become attractive, CLHS will again revise its standard to leverage off the shelf technology for the benefit of machine vision.