DIY PC-Interface for Suunto Vyper

The Suunto Vyper is a great computer, but the PC-Interface seems expensive at over £70 (in the UK). With some electronics background I searched the web to find a more cost effective alternative. The result was this DIY interface.

This document describes a DIY interface for the Suunto Vyper wrist-mounted decompression computer. The same electrical circuit should work for the Cobra, but I don’t (yet!) have access to one to test this and the mechanical design will almost certainly need to be amended to fit the Cobra.

To see some user-built interfaces from this design check out the Interface Gallery! This includes a version built on veroboard.

DISCLAIMER: If you damage your Vyper with this Interface don't blame me!

Seriously though, there is no reason why anything should go wrong!

History

There are a number of pre-existing sites which describe DIY interfaces for Suunto diving computers. Probably the best known (at least here in the UK) is the excellent info for the Suunto Eon/Alpha and Solution from Dave Cordes. 

Further digging around on the 'net located Roland Wasmer's site where he describes an interface for the newer Stinger/Spyder models. This design appears to have been used as the basis for a (slightly different) design for the Vyper by Christian Erles at CERN. Some additional information and comments on building and testing Christian's design can be found at Reinhard Kapka's SCUBA DIY site (also a good source of smart electronics for dive lights and even pointers to DIY rebreathers !!!!)

All of these sites contain valuable information and I would encourage you to check them out.  Having absorbed the invaluable experience of these pioneers I developed the circuit below, containing ideas from the above sites. 

The Circuit

The circuit below is derived from Christian Erle’s circuit, with modifications inspired by Roland Wasmer’s and the comments from Reinhard Kapka.

It was originally built and tested from a Toshiba CDT660 serial port and appeared to work fine.

How it works (for those that care!)

Power is derived from the logic signals on the serial port, which are rectified by D1 thru D6 to charge C1 and C2. Schottky diodes would be more efficient, but are more expensive and the IN4001s seem to work just fine! R1 thru R3 provide some protection for the serial port. This combination provides a power supply of +9v and –9v (nominal). Mine actually delivers about +7.4v and –7.7v but this is probably influenced by the internal resistance of the test meter as well.

Diodes D7-D9 provide a (low-cost) voltage reference of about –1.5 volts which provides a reference for the comparator U1a and is shifted up by U1b to provide a logical HIGH signal to the Vyper in response to a HIGH from the com port. Adjust R6 to get about –3.2v on pin 7 of U1. The SUUNTO runs on negative logic (-3v and 0v).

PNP transistor Q1 acts as an inverting level shifter for the TD line, supplying a LOW logic level of about 0.2v in response to a LOW from the com port. In the reverse direction U1a provides a –9v (LOW) signal and +9v (HIGH or –3v from the Vyper) for the com port.

Improvements

Roland Wasmer uses a more sophisticated voltage reference in place of the 3 diodes and uses Schottky diodes for a more efficient power supply. This may be preferable and less affected by variations in serial ports – I may try this in a future version of the interface.

Testing

There isn’t much to go wrong here. Test the power supply to see you get a reasonable +/- supply, but don’t expect to get exactly +/-9v. Remember the com port needs to be active to drive the circuit. Check for –1.5v at pins 3/5 on U1 as the reference and adjust R6 to get –3.2v output on pin 7. Otherwise connect to your PC and run the “TEST” option in the Suunto software to detect whether the interface is recognised. I have had some problems here – you may need to increase the COM delay to 50 and try several times to get the interface recognised.

Circuit Construction

The circuit was constructed on a single-sided PCB, cut to fit inside a small case from Maplins. For this prototype I drew the PCB tracks by hand (!) but the layout is shown below for those with the capabilities to do this from the computer.  

If you don't want to bother with a PCB, then we now have a veroboard layout. This was developed and contributed by Martin Adams. Martin - you're a star!!

The PCB layout below is viewed from the copper side (i.e. with the components on the other side). Note that the KK connector is mounted on the copper side so that it projects up through the case to make contact with the Vyper sitting on the top.

The Mechanics

Frankly, the electronics is easy, the real problems start when you come to build the physical cradle to hold the Vyper (or whatever) firmly in place and make a reliable contact with the two contacts at the rear. This is the area where volume manufacturing has a big advantage and can design and build a customised moulded plug to fit the contact receptacle. Martin managed to find a plug that works - I'm checking it out and will add details to the site in the near future!

I made a few measurements and generally messed around and found that a 5 pin Molex connector was just the right width to fit snugly inside the contact receptacle on the Vyper, and that pins 2 and 4 would then be (almost) perfectly positioned to make a good contact. By filing a small slot in the center to accommodate the central rib of the receptacle and removing the uneeded pins I ended up with the current design. It may not look elegant, but it is simple and it works!