Here is a Subsea BOP Multiplex Control pod that is used on one of the rigs I am over.

The bell on top is a one atmosphere housing that communicates to the surface via fiber optic umbilical and also receives 480vac from surface. Deployment depth is rated to 10,000ft and there are two mounted on the subsea BOP stack to allow for redundancy if one fails. There is a bank of GE Fanuc PLC's mounted in the bell and they output a 24vdc signal to a solenoid operated pilot valve which shifts a larger hydraulic valve that send hydraulics to the desired actuator on the BOP stack, to close the well bore off for example.

Model in MoI and render in Carrara.