By Ed Cady
It seems like every new board-to-board, including mezzanine, application could use a differential connector.
Whether parallel or serial, most new input/output (I/O) interfaces are based on rapidly increasing, higher data rates using balanced differential signaling. In some cases, certain newer connector systems that were designed and optimized for differential signaling have been selected by industry standards for their latest specifications. In other cases, older I/O connectors are being optimized with different internal ground and shielding structures while maintaining mechanical backward compatibility.
InfiniBand and Fibre Channel
Last summer, the InfiniBand Consortium selected a copper, four-lane (eight twinaxial conductors) serial differential cable connector for its 10 Gbps intercabinet, short-haul links. Three InfiniBand 10 Gbps connections fit on one peripheral component interface (PCI) or InfiniBand I/O bracket. This is a relatively new, pioneering consortium that selected a very new connector for 100 W differential cable links. Check out the InfiniBand Trade Association 1.0 Specification at www.infinibandta.org.
Back in December 2000, the 10-year-old Fibre Channel Standards Committee voted to select the four-lane InfiniBand copper cable connector for the 10 Gigabit Fibre Channel aggregated serial differential 100 W specification. This is also based on Fujitsu Takamisawa America Inc.'s (Sunnyvale, Calif.) MicroGigaCN connector product. Committee members will later decide whether keying is necessary or not to prevent connecting to an InfiniBand port inadvertently. It is likely that this connector and cabling system will see use in 10 Gbps asynchronous transfer mode (ATM) and synchronous optical network (SONET) electrical fail-over backbone applications. This same cable assembly could be used for bundling four separate 2.5 or 3.2 Gbps links together or as a switch fan-out with four separate cable ends using HSSDC-1 or HSSDC-2 connectors.
The data rate for each InfiniBand lane is 2.5 Gbps, while Fibre Channel will likely be 3.187 Gbps per lane. However, there is a proposal being made by Nortel Networks to consider using 3.125 Gbps per lane, as is being utilized for the 10 Gigabit Ethernet specification. But, these could be different four-lane cable assemblies because of data rate, electromagnetic interference containment and signal equalization optimizations. Keep up with Fibre Channel specifications at www.fibrechannel.org or www.t11.org.
This four-lane copper cable solution could potentially see use in some 10 Gigabit Ethernet applications requiring a low-cost solution for very short jumpers. It appears that the current 10 Gigabit Ethernet team is working cooperatively with the 10 Gigabit Fibre Channel team. They are developing different physical variant specifications, with the Fibre Channel team focusing on short haul and the Ethernet team doing long haul. If it all works out, they may achieve better time-to-market by borrowing from each other's work, as needed.
The Fibre Channel Optical Subcommittee will be voting this month to select a parallel optical connector for the same 10 Gigabit Fibre Channel specification. This will be a multimode 300 m maximum distance variant. The 12-position MPO optical connector that was chosen for InfiniBand is one of the leading candidates. Four fibers are used on each far side with four empty positions in the middle; that is eight fibers total. Other candidates include the 12-position SMC connector being promoted by Infineon Technologies and possibly a new eight-position connector from Stratos Lightwave LLC. There could be several more proposals offered at this meeting. Will the subcommittee choose just one or several connectors for the new standard?
Disk Drive Connectors
The newly developing Fibre Channel 4.25 Gbps copper physical specification for disk drives may warrant either an electrical improvement to the six-year-old, 40-position SCA device-to-backplane connection system or a new differentially designed connector. It seems that most current users would like to maintain mechanical backward compatibility while utilizing some possible internal SCA shielding improvements. Technical presentations will be made at the next Small Form Factor (SFF) Standards Committee meeting in early 2001. Check out the Web site at www.sffcommittee.org.
The new Serial ATA (SATA) Consortium is considering a new differentially optimized connector for its disk drive connections. But will this new connector and cabling work for both the first 1.5 Gbps data rate drives and the follow-on 3.0 Gbps version? If a dual-port SATA drive materialized, would it need the same connector as the new dual-port Fibre Channel drive or vice-versa? If an external link SATA specification materialized, would it need the same 100 W HSSDC-2 connector and cabling solution as InfiniBand?
The 100 W HSSDC-2 InfiniBand and the 150 W HSSDC-2 Fibre Channel connectors need further specification concerning keying and paddleboard chamfer dimensioning. It is likely that bulkhead cutout window dimensioning and connector electromagnetic-compatible shielding features, including a possible bezel of some sort, may be tweaked in later specification revisions. Paddleboard printed circuit board (PCB) thickness and gold finger pads may need to be specified as well for intermateability.
The new Ultra-4 Small Computer System Interface (SCSI) has a 320 MBps data rate that seems to work better over the five-year-old, 68-position VHDCI connectors. Ultra-4 SCSI still uses wide parallel differential technology. Newer VHDCI connectors that are backward-compatible have various internal shielding schemes that optimize signaling performance. Even the new PCI form factor High-performance Parallel Interface (HIPPI)-6400 uses double-stacked VHDCI receptacles and double-stacked cable plugs. HIPPI uses very wide parallel differential cabling. These two standards, as well as the IEEE 1596 Scalable Coherent Interface (SCI; a narrow parallel standard), are still benefiting the tweaked VHDCI connectors. Check out the SCSI Trade Association site at www.scsita.org for SCSI information and the HIPPI Standards Committee site at www.hippi.org or at www.t11.org for HIPPI information.
Backplane Connectors
Not only are cable I/O connectors becoming differentially designed more often, but so are most backplane connector systems. For example, the InfiniBand backplane uses Tyco Electronics' Speedpac connector, which has been qualified to handle the first generation 2.5 Gbps serial differential signaling. Another example is FCI Electronic's Metral 3000 backplane connector system, which has been qualified to handle 5.0 Gbps using certain PCB dielectrics. The market will see more during 2001. It seems like every new board-to-board, including mezzanine, application could use a differential connector.
Ironically, back in December 2000, a customer asked me for some help in designing a single-ended parallel copper cable solution. "Wow," I said, "this could be the last single-ended design of the year or even the decade. Just think, you got to use one hundred 50 W micro-coaxes in one short cable!" He said, "Maybe this will be a sort of final analysis ceremony for your signal integrity engineer!"
ED CADY, a Connector Specifier Advisory Board Member, is Advanced Market Engineer, FCI Electronics, 15005 N.W. Timmerman Rd., Forest Grove, OR 97116; (503) 359-4556; Fax: (503) 359-9273; E-mail: edcady@aol.com.
