The backplane and crate infrastructure for the ATLAS Level-1 calorimeter processor (L1Calo) are presented, The L1Calo system is a relatively compact, high-performance real-time system with high channel density and interconnectivity between system modules, The design is based on a custom, monolithic 9U backplane populated completely with 2mm Hard Metric connectors,

The high pin count and monolithic design present new challenges, including high insertion/extraction forces and long-term maintenance and repair of the thousands of exposed male backplane pins. Here we present the solutions we have found for dealing with these issues, as well as accessibility, infrastructure for the hundreds of connected cables per crate, and maintenance and repair over the lifetime of LHC.

AbstractAbstract

ServicesServices

Mating forcesMating forces

IntroductionIntroductionMany high-performance real-time systems developed for LHC feature high channel density and large amounts of interconnectivity between modules. To achieve these high densities, system designers are turning to 2mm Hard Metric (HM) connector systems. 2mm HM connectors are widely used in industry and bus standards such as CompactPCI, and therefore provide a cost-effective solution.

However, 2mm HM connectors introduce new problems for the system designer. With insertion forces of up to 0.75N per pin, a 9U module requires hundreds of Newtons to insert or remove. Unlike earlier backplanes using female DIN 41612 euro--connectors, the 2mm HM backplane connector consists of exposed male pins that are easliy bent and damaged, presenting serious long-term maintenance issues over the lifetime of LHC. High channel density and processing speed also require higher power consumption, making power distribution an important design issue.

The ATLAS Level-1 Calorimeter trigger (L1Calo) group has adopted a common system crate architecture for the EM/Hadron cluster and Jet/Energy-sum processor subsystems that presents a near worst-case example in terms of these problems. We have developed integrated solutions for force reduction, integrated cable strain relief and power distribution that provide for accessibility and long term maintenance and repair. Elements of these solutions may be useful for other systems with similar issues.

Maintenance and RepairMaintenance and Repair

High-Density Backplanes: Problems and SolutionsHigh-Density Backplanes: Problems and SolutionsS. B. Silverstein, Stockholm University, Stockholm, SwedenS. B. Silverstein, Stockholm University, Stockholm, Sweden

For the ATLAS Level-1 Calorimeter Trigger Collaboration

Backplane and CrateBackplane and Crate

ReferencesReferences

The processor backplane is a monolithic construction with 9U height, and 21 single-width module positions. It is populated almost entirely with 2mm HM connectors, with 1148 signal and ground pins to each processor module. The PCB is 4.9mm thick, with 8 signal layers and 10 ground planes, Up to 384 serial LVDS cable assemblies and two rear tranisiton modules system results merging connected to the rear side of the backplane. Three high-current DIN connector pins at the bottom of each module position deliver up to 20A of low-voltage (3.3V and 5V) current.

The system is installed in CERN-standard Series 6000 LHC crates, which have been modified to accommodate our custom solutions for power distribution, strain relief, and insertion/extraction forces.

With 1148 signal pins per module position, the nominal insertion force for a typical module is nearly 500N, and the extraction force over 400N. Because the PB is attached to extrusions at the top and bottom of the crate, module insertion and extraction will cause the backpane PCB to bow horizontally. The resulting pin misaligment adds significantly to the mating forces.

To reduce backplane bowing, we use six vertical reinforcement ribs. They are manufactured from 5mm-thick brass (which matches the spacing between two adacent rows of 2mm HM connectors), and are fastened through the backplane to the top and bottom crate extrusions. Two retention bolts secure the ribs to the backplane at 1/3 and 2/3 of the backplane's height. This configuration dramatically reduces concave/convex bowing during module insertion/extraction with a maximum displacement of less than 1mm. Threaded holes on the back side of the ribs provide mounting points for the power bus bars and cable strain-relief system. IEEE-standard handles use leverage against the to the front crate extrusions to provide the necessary insertion/extraction force. For our 9U modules we have observed vertical flexing of the crate extrusions that cause the handles to slip, resulting in poor performance and accelerated wear. We have addressed the issue of wear by adopting solid aluminium handles. Furthermore, extrusion flexing can be reduced by securing the front-panels of inserted modules to the crate extrusions to provide vertical reinforcement.

Finally, each module includes a guide pin to ensure correct alignment during mating. This reduces the force of initial contact, and protects the pins from damage.

The backplane reinforcement system also serves as a mounting point for the cable strain relief system and the power distribution bus bars.

The cable strain relief system provides secure retention for up to 384 4-pair LVDS serial cable assemblies that provide the processor modules with input data. Vertical "forks" straddle the cables for one module position, and secure the connectors within the connector shrouds.

A large, robust power bus bar assembly is located at the bottom of the backplane, and distributes 5V and 3.3V power to each of the 21 modules through three high current pins rated at 20A each. While the trigger system is not designed for hot swapping, care has been taken to make the ground pins make-first break-last to minimize the risk from an accidental live extraction.

When signal pins become damaged, a repair and replacement procedure is critical. Tyco/AMP produces a toolkit (Part number 354687-1) that we have successfully used for field replacement of single connector pins, without removing the backplane from the crate. This kit works best for AMP brand connectors, whose pins are loaded into the front of the shroud and held in place by friction. Other connector brands such as ERNI are less suitable for field repair, since the pins are loaded from the back of the shroud, and are therefore less easily extracted. Nevertheless we have also succeeded in replacing several ERNI pins with the AMP toolkit with satisfactory results.

For more extensive damage one needs remove the backplane and repiace entire connectors, most likely by an external company. The backplane and its hardware form a single unit that can be relatively easily removed and replaced for this purpose.

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