The MicroTCA open computing standard can be adapted to provide a compact and cost-effective platform for high-performance systems in market sectors other than telecoms. Christian Ganninger and Keith Reynolds explain how.

The MicroTCA specification agreed in July 2006 was initially orientated to the requirements of network edge and access areas in telecommunications networks. In such applications, features such as redundancy and hot-swap capability are required in order to achieve the necessary high availability. With suitable modifications, however, MicroTCA forms an ideal basis for high-performance systems in other market sectors as well.

The MicroTCA specification (MicroTCA.0 R.1.0) extends the area of application of AdvancedMC modules, which were first defined for use as mezzanine boards in AdvancedTCA systems. For this purpose, there are special AdvancedMC carriers that are used in AdvancedTCA systems in place of AdvancedTCA blades. Now MicroTCA has taken these mezzanine boards and defined them as function modules that are inserted directly into the system.

Key benefits

AdvancedMC modules no longer adhere to the traditional 3U and 6U euroboard format for plug-in boards, but are now somewhat smaller with heights of about 75 and 150mm respectively for single and double modules. Increasing miniaturisation of components and parts means, however, that complete single-board computers can be realised on these small boards. It is thus possible with MicroTCA to build extremely small systems with high processing power (Fig. 1).

MicroTCA also uses serial data transfer via PCIe, Gigabit Ethernet, 10 Gigabit Ethernet, S-ATA or SAS and Rapid I/O and allows data transfer rates of up to 12.5Gb/s per port. With four fat-pipe ports available per module, a data transfer rate of 50Gb/s can be achieved between two modules.

A further advantage of MicroTCA is its integrated carrier and shelf management. This makes MicroTCA an interesting alternative for applications in those sectors requiring remote management or high data rates or where space is constrained.

Different requirements

Because MicroTCA was originally conceived for telecoms applications, certain features have to be adapted to suit the requirements of other market sectors. The maximum installation depth of a MicroTCA system is set at 197mm, so that the system with front cabling will fit into the 300mm-deep ETSI racks that are commonly used in telecoms infrastructure. However, there is no such restriction on the installation depth of systems for applications in say defence, traffic and transportation, medical equipment, industrial automation, instrumentation and control.

To achieve the 99.999% availability demanded by the telecoms market, MicroTCA defines redundancy in components such as ventilation, power supply, switches and all function modules. However, outside of the telecoms environment, such high levels of availability are rarely required, and there are also different requirements with regard to shock and vibration resistance and thermal management.

Possible modifications

In order to develop MicroTCA for markets other than telecoms, the standards and specifications of the respective markets must be implemented, and any telecoms-specific features that are not required in the new application should be trimmed away in order to optimise system costs (Fig. 2).

The simplest modification is to eliminate the redundancy, and MicroTCA systems of this type are already available. Such systems will use, for example, only one cooling unit and only one power module. And, with only one MicroTCA Carrier Hub (MCH) required, the size and number of layers in the backplane is reduced and the number of connectors halved.

This approach offers considerable savings potential, particularly in the area of the power supply. Rather than using a 600W plug-in PSU in the smallest space, a conventional open-frame PSU can be used in conjunction with a power management board. Since the enclosure depth is no longer restricted to 200mm, the PSU may be accommodated behind the backplane, thereby freeing up extra useable space in the board cage.

The MCHs are designed in accordance with the MicroTCA specification. The carrier manager and basic communications are defined via GbE on the main board. The second MCH tongue defines the clock signals and storage interface, and the third and fourth tongues are defined for the fat-pipe switch. This configuration in turn affords several possibilities for adapting the system to different requirements. PCIe is often used as the bus between the processor AdvancedMCs and I/O boards, and here one 2.5Gb/s lane is often sufficient. It is then possible to assign these PCIe lanes from the processor AdvancedMC board directly to up to 8 slots and thus save use of the fat-pipe switch on the MCH. The PCIe clock can be generated on an AdvancedMC module or on the backplane. Processor-to-processor communication is normally via Ethernet, and the 1GbE of tongue 1 of the MCH is normally sufficient for this. The user can therefore decide from the various options which functionalities are required and where costs can be saved.

Ruggedised MicroTCA

Particularly in transport and military applications, but also in industrial automation, a high level of shock and vibration resistance is often necessary. PICMG has therefore set up three MicroTCA sub-specifications to address these requirements. The first of these - MicroTCA.1: Air Cooled Rugged MicroTCA – was released in March 2009. Currently under development are MicroTCA.2: Hardened Air Cooled MicroTCA and MicroTCA.3: Hardened Conduction Cooled MicroTCA.

MicroTCA.1 opens the way to MicroTCA system solutions that meet the DL3 requirement class of IEC 61587-1. This includes peak accelerations of 25 g in shock tests and of 3 g in vibration tests and represents a requirement higher than that for MicroTCA.0 by a factor of between almost 4 and 6. Such systems are used, for example, in situations with high vibration levels, e.g. on rotating machines (offset printing) and in railway and marine equipment.

The most important difference to the MicroTCA base specification is the need to bolt the AdvancedMC modules on to the board cage. Only with this additional locking mechanism can the stringent mechanical requirements of MicroTCA.1 be fulfilled. For the front panels of the AdvancedMC module this means that the locking mechanism must be extended in both vertical directions and fixed to the MicroTCA system. It is particularly important that the locking is effected without any force being applied to the MicroTCA backplane connector. An ‘ordinary’ bolt would, when turned, push the AdvancedMC subassembly board generally in the direction of the backplane and so apply a force onto the connector. To address this situation, Schroff has developed a functioning and both mechanically and electrically tested AdvancedMC module fixing that fully satisfies the requirements of the MicroTCA.1 specification and allows locking without pressure on the connector. This solution (Fig. 3), for which Schroff has registered a patent, is a component of the MicroTCA.1 specification.

A bushing (blue) fixed to the front panel serves to accept an expansion plug (green) with an inner funnel. The bolt (grey) has a geometry that corresponds to that of the funnel of the plug. When the bolt is screwed in, the plug expands and becomes jammed positively against the bushing fixed to the front panel. No resultant force (pressure) occurs in the direction of the backplane connector. This solution - being the only one to date - is visually documented and given a Schroff order number in the appendix to the specification.

For more information about MicroTCA specifications and related products and services, visit the following websites: www.picmg.org, www.a-tca.com and www.schroff.co.uk.