This Aquatica Switchboard Circuit (Replacement) can be used on the D90 and other recent Aquatica housings. The board has four 2 position switches. When all the switches are in the upper position, all contacts are active, allowing you to use a TTL converter, or housed TTL compatible strobe that is attached to the main bulkhead. When the switches are in the lower position (default), only 2 contacts are active. So the main connector will work in manual flash mode only. The secondary bulkhead is wired with only 2 contacts for manual strobe operation in order to avoid possible conflict with the camera's electronic circuit.