Six-pin sockets provide power, 4-pin do not. A node can either be a power provider, a power sink or neither. There is no way for a node to identify itself as one that provides power out one port but not another. In addition, when there are two or more sockets on a node, power is either passed from one socket to the other or not. There is no way to be able to identify which sockets pass power and which do not. For example, if you had a four port node and two ports had 6-pin sockets and the other two had 4-pin sockets there would be absolutely no way for the bus manager to know that power is not passed through all sockets.

So, why, one may ask, is this all so important? Well, when endeavoring to create a bus manager that has power distribution management (PDM) capability, there must be a mechanism by which the PDM can determine the source of power and where and how that power is delivered to other nodes.

The Cable Power Distribution (CPD) Specification has gone to great lengths to make certain rules exist that enable good power distribution. Why? Because it has been found in 1394-1995 implementations that nodes that attach to the bus and simply receive a link-on with no forethought to power management cause bus failures when a link-on causes too much power to be extracted from the power provider - causing voltage droop, power brown-outs, etc.

Now that all of this has been said, it is important that I inform you that there is work being done in the Power Distribution Management Specification that may, in fact, create an opportunity for an OEM to have a mixture of 4-pin and 6-pin sockets and eliminate the need for the constraint set forth in the CPD spec. For example, an OEM may implement 1394b and have a GOF socket and a 6-pin socket. In this implementation, the same issue exists - one has/passes power the other does not. So this problem had to be solved.

Here is what is being done to solve the problem. Any node that has a mixture of like-media sockets must declare their power class field in their self-ID packet to be power class four. This requires such nodes to have a link and to have CSR space. CSR space must contain registers that identify port capability. One of the port capabilities that shall be identified are where a port consumes power, sources power, and passes power or not.

BUT, since PDM has not yet been ratified, the CPD specification is the "standard" OEM's must abide by.