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Simtec Hydra backplane (top)
Simtec Hydra backplane (bottom)
The Hydra was designed to take up to 4 additional CPU cards and to schedule a PiscPCs resources between them. Because RISC OS is not designed to support multiple processors, Simtec produced an interface module which could be used by multi-processor aware applications and allowed them to use the additional CPUs. The Hydra was killed off by the arrival of the StrongARM CPU because a single StrongARM CPU was faster than a fully populated Hydra. In 2003 Simtec sold off their remaining stock of Hydras in a clearance sale., at the time Simtec were quoted as saying that a Hydra with 5 ARM710 CPUs was slightly slower than a strongARM CPU.
The top picture shows the top of the Hydra backplane, at the back are the slots for 4 CPU cards (labelled ARM1 - ARM4). To the right is a vertical slot, labelled LOGIC, which, I think, contained the logic card to schedule access to the RiscPC for the CPUs (I don't have this card, but you can see it in pictures on the simtec website, At the bottom are the slots for the nornal RiscPC CPU and PC CPU cards (labelled ARM and 486 respectively).
The bottom picture shows the connector that fits in CPU socket on the RiscPC motherboard.
Further details can be found on the Simtec website under Legacy Products
The Simtec Hydra Overview is available HERE , note this is a text document.
There is a review of the Hydra in Risc User Volume 9 Issue 7.