Author: Manfred Schmitz, Chief Technical Officer, MEN Mikro Elektronik
The structure of PICMG's specifications always follows the same principle: A base specification is extended by follow-on standards. The base specification of the well-known, widely used CompactPCI standard is designated PICMG 2.0. The zero after the point indicates that this is a base specification. Standards building on it and extending it have higher numbers with the same prefix. PICMG 2.16, for instance, describes Ethernet over the backplane for 6U CompactPCI. PICMG 2.30 - CompactPCI PlusIO - defines the usage of the user I/O signals of J2 for 3U and 6U boards, in order to make modern serial buses usable together with the parallel PCI bus. In this way, PICMG 2.30 details PICMG 2.0 CompactPCI.
CPCI-S.0 is the base specification of the new CompactPCI Serial standard. On the one hand a base specification defines as little as possible to stay open for later technological change, but on the other hand this specification needs to be complete enough to guarantee smooth interoperability of the products of different manufacturers - which in the end is, or at least should be, the basic idea of a standard.
Being a base specification, CPCI-S.0 must not refer to other base specifications. The mechanics of parallel CompactPCI is basically identical with CompactPCI Serial. Still, CPCI-S.0 must again describe all the technical details. On the one hand CompactPCI Serial is widely redundant to CompactPCI, but on the other hand a standard like this is easy to read, because one document contains all the technically relevant details.
CPCI-S.0 is the first standard to assign very clear "requirement numbers". Every single detail, every "shall", has its own unique number. This may seem very bureaucratic at first, but it greatly facilitates the verification of standard conformity. It allows to capture the specification using a suitable tool like DOORS and to exactly check the degree of compliance of a specific implementation. Such requirement tracing is the prerequisite for setting up safety-critical systems and is demanded in aviation, for example.
The CPCI-S.0 base specification covers all areas, i.e. both mechanical and electrical. One chapter gets special prominence. It specifies the requirements for interoperability with parallel CompactPCI. This permits a simple migration compliant with the standards from CompactPCI (PICMG 2.0) to CompactPCI PlusIO (PICMG 2.30) up to CompactPCI Serial (PICMG CPCI-S.0).