Reverse engineering Mantis / Meridian USB cradle

[David Baumeister]

Hello,

This might be slightly off topic; I'm looking for some info on the Scubapro Meridian / Mantis cradle.

I'm attempting to build a DIY USB cradle for the Scubapro Meridian / Mantis dive computer and currently I'm stuck on the cradle detection/wake mechanism.

So now I am looking for some info on how the real cradle makes itself apparent to the watch. If anyone with the cradle at hand might find this interesting, I would be grateful for answers to any of these questions:

In order of effort (easiest first):

- Does the watch beep when the cradle is first connected physically (before any software is run)?

- Where exactly does the cradle make electrical contact on the watch? (my current understanding is the data contact on the side and 2 grounds near the buttons)

- What resistance or impedance does the cradle present between the data contact and watch case/GND (without watch inserted but cradle powered)?

- What voltage does the cradle drive on data vs GND (again without watch inserted but cradle powered)?

- If no static voltage, what signal does the cradle drive on the data line (again without watch inserted but cradle powered)?

If anyone might find this interesting, here's my current progress:

- Successfully implemented half-duplex serial using the on-board serial chip of an arduino

- Handshake (13-byte greeting + 0x11 ack) works with loopback testing on second arduino

=> Serial communication seems successful

- The watch drives data line high every 1s and twice quickly every 5s after button press (roughly 3V)

- The watch's own pulses show interesting RC-like decay behavior (~1ms time constant with high-impedance loading), which suggests it may be sensing a specific impedance or waiting for a specific response pattern, but I can't determine what without more information about the cradle's actual behavior.

So I am confident that communication with the watch should be possible once its in transfer mode. The missing link is how to trigger transfer mode on the watch.

Any insights into the hardware-level cradle detection would be greatly appreciated. If others have reverse-engineered this, I'd love to know the details.

Thanks in advance,

David