Deploying legacy CompactPCI systems for today’s high-speed serial applications.
Charlie walks us through a set of options designers now have as two new PICMG draft specifications affecting high-speed I/O in CompactPCI-based systems near completion. This article focuses on the Ultra Hard Metric (UHM) socket connector from 3M. For more on this connector and the new draft specifications, see Pairing CompactPCI compatibility with high-speed serial data transfer, also in this issue.
The CompactPCI specification is a true workhorse, showing itself to be rugged and reliable for embedded systems used in a variety of harsh environments. Systems designers and users have found the CompactPCI specification to be flexible and upgradeable enough to sustain its usefulness over the years. For example, system designers enjoy a number of options when upgrading deployed, legacy CompactPCI systems for today’s high-speed serial applications.
One new option is defined by a new draft specification from PICMG. PICMG has made a commitment to create specifications that enable incremental, economical upgrading of legacy CompactPCI systems. The new draft PICMG 2.30 CompactPCI PlusIO specification is based on the PICMG 2.0 core specification and defines the migration path from parallel PCI to the serial PCI Express. The draft specification is largely complete, and final changes are in process prior to ratification.
The new draft specification defines the pin assignment and the function of the user pins on the J2 connector for 32-bit system slots. The J2 pinout consists of 22 rows of six pins set in a 2 mm x 2 mm Hard Metric (HM) grid, where the outer rows are specified as grounds. The current pinout does not provide for high-speed signals such as PCI Express, SAS/SATA, USB, and Gigabit Ethernet, whereas the draft 2.30 specification does. However, the new pin assignment defined by the PICMG 2.30 standard cannot be fully taken advantage of without upgrading the connector as well. Standard unshielded 2 mm HM connectors used in current CompactPCI embedded computers can only enable transmission to 1.5 Gbps and cannot support the high-density pinout of high-speed differential signals that many new applications demand (Figure 1).
The draft PICMG 2.30 specification specifies a new board connecter, the Ultra Hard Metric (UHM) Socket Connector from 3M (Figure 2). The UHM socket connector improves signal integrity performance, which is required to transmit multiple high-speed protocols. Also, the UHM socket can mate to a standard 2 mm HM backplane header. The UHM socket connector provides full shielding, low crosstalk, and high signal density, while its controlled impedance characteristics offer increased performance. The UHM socket connector’s patented and patent-pending “virtual coaxial box” shielding technology dramatically reduces the severe crosstalk commonly experienced by 2 mm HM connectors above 1 Gbps speeds, and it can enable speeds greater than 7 Gbps.
Based on the PICMG 2.0 CompactPCI core specification, the draft 2.30 specification enables designers to make modular upgrades to incumbent systems and protect their investment in legacy hardware. To preserve interoperability of modular embedded systems, the draft specification allows backward compatibility to support investments in legacy PCI bus cards and backplanes fitted with 2 mm HM connectors. The PICMG 2.30 draft specification allows designers to “future proof” CompactPCI systems by creating a way to use both legacy processing cards and new advanced serial cards on the same backplane.
New lease on life for legacy hardware
UHM socket connectors are inter-mateable with both legacy and new equipment built to meet IEC 61076-4-101. The UHM socket connector goes on the circuit card and can plug into the current backplane. (However, system performance is limited by the backplane construction and whether the traces can accommodate intended data rates.) The UHM socket connector bridges the older cards and the newer ones as part of a hybrid backplane, allowing for a cost-effective upgrade while extending the life of legacy hardware.
With the UHM socket connector, the designer can build a board with high and low capability on the same pinouts (Figure 3). Signal speed limitation would now be a function of the backplane, due to construction and design, and maximum speed is based on the capability of the backplane. Replacement of the backplane is not trivial, but it is an option that would provide enhanced capability while preserving the majority of the installation.
The PICMG 2.30 draft specification establishes an excellent path to upgrade to high-speed serial applications while protecting investments in hardware, but designers do have other choices.
The designer has the option of replacing the component board connector with the UHM socket connector and continuing to utilize currently defined J2 pinouts for that system. In other words, the UHM socket connector can be used even if the PICMG 2.30 draft specification is not utilized.
Not always Costco-size quantities
Some systems may not need the large quantity of high-speed signals prescribed by PICMG 2.30 and enabled by the UHM socket connector. These systems can look to more cost-effective upgrading options. One method is to dedicate pins to ground in the user-assigned section of the J2 connector (or J3, J4, or J5 in 6U systems). That will shield and increase the performance of a select few pins without changing the connector, though density is sacrificed.
Another way to squeeze speed from a system is to bypass the backplane entirely and add high-speed cabling off the opposite side of the backplane pin field. This option requires using extended length backplane pins. The system must also have the room behind for cabling. There is little disruption to the signal as it passes from the card straight into the high-performance cable.
These last two options do not require a new backplane. However, they are application-dependent, will work only in certain situations, and will not overcome board limitations.
Data rates exceeding 7 Gbps
Some applications need even higher speeds than what PICMG 2.30 and the UHM socket connector can provide. Going further up the upgrade path, designers have the option of doing a significant overhaul. Where data rates in excess of 7 Gbps are required, then one answer is likely to be found in CPLUS.0, a higher-speed upgrade to CompactPCI that is under development. The PICMG CPLUS.0 development effort is a separate draft specification for new systems. It is based on CompactPCI and remains compatible with existing IEC 1101 mechanicals, but it only supports modern point-to-point connections. PICMG CPLUS.0 defines a new system slot connected to up to eight peripheral slots via a star topology. CPLUS.0 specifies the AirMax connector from FCI, which is expected to support data transfer rates of up to 12 Gbps and beyond.
Designers and suppliers will soon have a range of options to support the large CompactPCI ecosystem. The PICMG standard will continue to provide flexible high-performance capability for rugged and demanding environments, while new connectors will enable higher speeds. These options allow legacy systems to remain in use and to integrate with the higher capabilities coming on line, thus protecting user investment.
Charlie Staley is a Market Development Manager for the Electronics Solutions Division of 3M in Austin, Texas. Charlie is a connector veteran with more than 20 years in the industry and more than 10 years at 3M. He has worked in sales, applications engineering, development engineering, and marketing, and continues to work on projects that meet the evolving needs of the customers.