Author: Manfred Schmitz, Chief Technical Officer, MEN Mikro Elektronik
Most modular computers support physical addressing. For CompactPCI Serial – PICMG CPCI-S.0 – this addressing is particularly important.
Modern serial buses such as SATA, PCI Express or Ethernet support all mechanisms for automatic configuration to simplify installation of new hardware. For instance, if you connect a hard disk to a computer using SATA, this drive will be automatically recognized and can be used without any further configuration. This advantage can at the same time be a pitfall – just think about a hard disk RAID configuration. In the case of a failure, you have to find and replace the right drive. This problem is also known with USB devices. If you connect two USB Flash drives to a computer, you don't know which drive is accessed where.
This is why physical addressing is a necessity for modular computer periphery. CompactPCI Serial supports this type of addressing. The base specification of CPCI-S defines one system slot and up to eight peripheral slots. The system slot is recognized by a special signal (SYSEN), not needing any additional address information. Even if the base standard does not explicitly describe this, you can of course expand the eight peripheral slots using suitable bridges. Since a 19-inch card cage accommodates a maximum of 21 slots, 4 lines are provided for the distinction between these slots. Depending on the backplane position, these lines will be either left open on the backplane, permanently connected to ground, or connected to ground with a 1 kOhm resistor. This results in 24 different combinations.
To assure the compatibility of different manufacturers, the CompactPCI Serial specification prescribes the order in which interfaces like SATA must be implemented on the system slot, if not all 8 interfaces can be supported. If a CPU board is able to support only 2 SATA channels, for example, these need to be SATA 7 and 8 (SATA 1 to 6 remaining unsupported). The base specification defines slots 7 and 8 to be implemented at the very right of the system. The recognition mechanisms of RAID controllers expect the first drive at address 0. For this reason, the slot at the very right of the system has ID 0 (all 4 address lines are connected to GND). Accordingly, the neighboring slot to the left has ID 1, and so on.
The physical addressing of CompactPCI Serial is simple and functional. It is compatible with standards such as SFF-8485 (Specification for Serial GPIO (SGPIO) Bus) for hard drive RAIDs. This allows to implement inexpensive, optimized solutions.