Note this is not an offical Phase One Supported solution and is offered as a workaround only.
The Contaxt 645 has a limitation in the body which limits exposure time with the Phase One digital backs. The way around this is to make the back think it is a large format solution.
You will need:
A post-stick note or thin tape
A wake up cable and multi pin adapter (or a kapture group one shot cable)
1. Cut the post-stick note, along the sticky tab, so it is roughly the same size as the contacts on the body.
2. Place the paper over the contacts and press firmly so the paper holds in place.
3. Carefully attach the back, making sure the contacts from the body and the back are isolated by the paper strip. You may encounter focus and light leak issues if this is not seated properly.
4. Attach a (lockable is preferable) mechanical release to the B port.
5. Connect the wake up cable and multipin as shown.
To perform a capture follow this sequence.
Switch AF to manual
Press the wake up cable (once) to start the back
Press the plunger to start exposure. When done release or unlock.
Press the wake up cable again to stop the back.