• Background
• Plans and Stuff
• Making the thing
• Rating

Background

Having recently decided to buy an interface to link my Suunto Cobra with my PC, I was freaked by the price of these things. Ninety quid or more for what is little more than a serial cable. Sod that! And the other manufacturers are no different, with prices ranging from £60-£100+. At least Suunto provide the software free.

So what's going on? The modern dive computer is a highly sophisticated piece of gear filled with expensive gubbins, and definitely worth the money considering what it does: lets you dive more safely and more often. The interface, however, is typically a plastic box with a few quid's worth of electronics rattling around inside (Fig. 1). So why do they cost up to half as much as the computer itself? Because they've got you over a barrel, that's why. Until now....

A nice shiny Suunto Cobra. Quality kit and worth every penny. Buy one now.	A piece of tat made from stuff they found in an old biscuit tin. Yours for just 25% of the cost of the computer. Includes a free floppy disk. 'Nuff said.

Figure 1. Spot the difference

Of course, these interfaces must be full of the latest hi-tech gadgetry to qualify for that price point, right? Wrong. The electronics are as basic as it gets - 11 diodes; 10 resistors; 2 capacitors; 1 transistor; 1 IC. Total cost to you guv, £2.33. Wholesale, including cables etc., you're looking at a couple of pounds. Now I know companies have overheads and shareholders to think of, but charging close to £100 quid retail for something that costs two quid to make is a disgrace. Robbing bastards, basically.

Plans and stuff

A quick search on the Internet confirmed my suspicion that these interfaces must be simple things - they just pass data to your PC where all the work is done - and are therefore fairly easy to make. In fact the hardest bit seems to be fashioning a plug for the proprietary (they would be wouldn't they?) contacts on the computer.

A few sites have plans for a DIY interface for the Suunto Vyper (the Cobra's non air-integrated stable mate). There don't seem to be any plans specifically for the Cobra, but they both use the same protocols and it was suggested that the Vyper one should work. (It does!). Steve Prentice's site is the most comprehensive and my interface is based on the material there. Many thanks to him and the others whose ideas I've borrowed.

Making the thing

I decided to have a bash at etching a PCB for a laugh. If you can't be arsed or don't have the gear, it is a lot easier to make the Veroboard version.

Working from Mr Prentice's schematic, I drew up and routed the PCB in P-CAD 2000 and transferred the design to the board using the 'acetate print-n-iron' method (there's a good beginners guide to making your own PCBs at Technick.net ). I didn't get great results and had to go over the tracks in pen. Electronics shops sell these pens for several quid, but they're usually just Staedler 358 permanent transparency pens - the type they have in most offices :). It was then etched using ferric chloride.

Parts were from Maplin and RS Electronics. RS has better pics and it is generally easier to find components, but everything is prepackaged and the cheaper components like resistors come in multiples. Maplin will sell you a single diode if you want.

This was my first attempt at PCB making. Here are some beginner's tips that I learned the hard way (the first board was crap):

• Make the tracks FAT. I mean at least 25 mil (c. 0.65mm). Make the gaps at least 20 mil. (This is especially important if you use the acetate transfer method, which is not very precise, and/or your soldering skills are as bad as mine.)
• If using an autorouter, don't be happy with the first routing pass. Fiddle with the layout until you get the fewest/shortest tracks possible. Tweak by hand to optimize. The less track you have, the fatter it can be in the same size board.
• Heat up your ferric chloride solution using a water bath, otherwise it'll take all day and the quality will suffer too (get advice on this - ferric chloride is nasty stuff. Wear goggles and gloves.)
• Triple check everything before soldering a component. Putting things in back to front can be bad.
• Buy a desoldering pump/braid :)
• If it doesn't work, don't do what I did with version 1.0 and smash the f*** out of it with a hammer to teach it a valuable lesson, only to find out later that it was probably a problem with the serial port. Test it at every stage - disconnected; connected without the IC (if using a DIL socket); complete. Compare the board to the schematic and your routed design.
• Check the PCB in sections. If the power section isn't putting out the right voltage, for example, it probably isn't worth troubleshooting the logic bits.
• Try another PC if nothing seems wrong but it's still not working. The interface simply wouldn't work on my main PC (ABit KT7-RAID motherboard). The power circuit was giving out +0.3v instead of +9V, even though the input from the serial lead seemed fine. After much hair pulling, I tried it on my other computer (ABit BM6 motherboard) and it worked immediately.
• If the dive manager software doesn't recognize the interface, set the 'COM delay' in the Transfer/Setup menu to 50 and try again. Restart the program if it seems to hang, and try again.

The contacts in the Cobra are two small pins recessed in a rectangular cutout - quite different from the Vyper, I think (never seen one). Looking at the back of the Cobra with the hose on your left, the left pin is 'Ground' and the right pin is 'I/O'. The connector at the moment is me - holding two bits of wire against the Cobra and pressing "Transfer" with my nose. I'm fiddling with 'proper' connectors and will post details once I've got something that works.

If anyone wants any advice regarding making the interface, feel free to email me.

Rating

• Difficulty: 6/10.
  Depends on how good you are at electronics, though problems may arise that are nothing to do with the board itself. PCB production was the most involved bit - the Veroboard version would be relatively simple.
• Geek factor: 9.5/10.
  Electronics is geeky. Etching your own PCB is a special geek subset in the geekiness of electronics.
• Satisfaction: 10/10.
  Mwahahaha! It works!! I saved myself 90 quid and can now play with pretty graphs of my dive profiles and have the computer fill in my logbook for me. Not to put too fine a point on it, they can stick their overpriced piece of cack up their arse.