The hot spot

The hot spotThe PCI Industrial Computers Manufacturer’s group has sorted out its approach to hot-swapping CompactPCI modules. Eran Strod takes a look at what they decided. Opinion has been divided into two camps on the fundamental approach to providing hot swap capability in PCI-based industrial computers. The subject has been heavily debated by the PCI Industrial Computers Manufacturer’s group (PICMG) Hot Swap Subcommittee and a clear way forward has been established. One camp was in favour of using a passive backplane and delegating hot swap functionality to the board vendors. The other advocated an approach similar to that of PICMG Hot-Plug, using existing cards and adding functionality to the backplane to support Hot Swap. The advantages of the latter approach would be numerous: PICMG would not deviate from PCI Special Interest Group specification, no custom silicon would be required. Backward compatibility with existing CompactPCI cards would minimise cost of migration to the new standard. Interoperability problems would be reduced. A greater degree of control would make it easier to implement High Availability functionality.
The disadvantages of this approach centre on the fact that an active backplane would be required. This would: Lower the MTBF (mean time between failures) of the system. Increase the MTTR (mean-time-to-repair). Initial calculations estimated the active backplane MTBF to be less than 100,000 hours. Normally, when backplanes fail, the entire system is swapped. Higher failure rates are expensive in terms of repair costs and business disruption so a high backplane MTBF is essential. The MTBF of a passive backplane is many times that of an active backplane. A drastic reduction in reliability of the backplane may be acceptable for the commercial server market. However, the authors of the CompactPCI Hot Swap Specification are attuned to the needs of the industrial and embedded computing markets, which demand the highest MTBF and lowest MTTR possible. In the end, the arguments in favour of using a passive backplane were so compelling that they overwhelmed the advantages of using the PCI SIG Hot-Plug (active backplane) approach. The CompactPCI Hot Swap subcommittee chose a passive backplane approach and focused their energy on defining new board and platform features, which would support live insertion and extraction. CompactPCI?
CompactPCI is an industrial bus based on the standard PCI electrical specification, on a rugged 3U or 6U Eurocard packaging. Unlike its desktop cousin, the CompactPCI board uses a high quality 2mm metric pin and socket connector and can be front loaded into a rack mount system. CompactPCI effort was initiated in late 1994 under the auspices of the PCI Industrial Computers Manufacturer’s group (PICMG). PICMG’s mission is to extend the PCI standard, as approved by the PCI Special Interest Group (PCI SIG). As a compromise, functionality was added that would address some of the disadvantages of the passive backplane approach. The CompactPCI Hot Swap Specification provides interoperability with off-the-shelf silicon and legacy CompactPCI cards as well as compatibility with the spirit and letter of Hot-Plug.
PEP talk… CompactPCI brings economical Intel architecture chips to industrial control applications. PEP Modular Computers has introduced a Pentium-based CPU board for use on the CompactPCI bus. The CP310 board requires just a single slot in a standard 3U bus and is available with processors running up to 166MHz. The chip set includes level two write-back cache support and burst DRAM controller along with PCI and ISA-bus interfaces. An enhanced IDE controller supports two hard disks and one ATA flash hard disk. The main user I/Os – two comm ports, PS/2 keyboard and system reset – are routed through the front panel. This gives ease of use and better control of electromagnetic emissions. Some vendors may choose independently to implement live insertion and extraction on a CompactPCI system using the Hot-Plug approach – using existing CompactPCI boards with an active backplane. This is a workable model, but it does not conform to the CompactPCI Hot Swap Specification.
Eran Strod is a systems engineer with Motorola’s Computer Group


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