We’ve been on a multiyear quest to make a durable, affordable, usable probe cable for the Bus Pirate. The one currently sold by Seeed is inexpensive (good) but I’ve never been a fan of the probe hooks used. I typically use male:female, or female:female individual probe wires sold in bundles at Seeed, Adafruit, etc.

With the Bus Pirate education kit almost sorted, we MUST have a probe before launch. Ideally it will be something with male ends that stick into a breadboard and labels on each probe.

A few times over the past year I’ve tried to get a new cable put together with labeled probes, but various barriers and misunderstandings about heat shrink and cable labeling always got in the way. It’s beyond time to bite the bullet and make an example to shop around for quotes. This is a opportunity to buy a new toy: a shrink tube printer.

Follow below the jump for our experience shopping and using heat shrink tubing printers.

At first I looked at several DYMO RHINO models. They are cheap in the US, but extremely expensive in China. All use special cartridges of shrink tube that run about $30 for 5 feet. To compare – 200meters of shrink tube is about $5 in Huaqiangbei… There’s also the risk that none of the sizes of shrink tube cartridges available would be adequate or the right color. I mean, what if we want hot pink labels?

A Taobao dealer selling the DYMO RHINO referred us to a professional heat shrink printer that’s only about $100 more expensive. In the end the professional setup was less than $40 more expensive because we bought cheap heat shrink in a bunch of sizes and colors instead of the expensive cartridges used by the RHINO.

We chose the L-Mark LK-320P over other brands because it’s exported and very google-able. Some of the other options were more enticing, but seemed less known which could lead to ribbon supply issues later. The “P” version means it has a USB port and connects to a computer, where it’s much easier to deal with the Chinese interface (and hack?). We paid about $250 for the Chinese version of the machine. You can find the English export version on Alibaba for $400+. We ordered it from a company in northwestern Shenzhen (Bao’on) in the morning, it arrived in the evening by courier.

In the box we got the printer, one label cartridge, one black acetate transfer ribbon (called color tape in Chinese), software, manual, and some cables.

We tested it out with the label cartridge first. Not only is the interface (of this version) in Chinese, it’s about the clunkiest thing ever. After some fumbling we spit out a few labels with some confusion about what word too long errors and paragraph settings meant.

Next we removed the labels and stuck in some yellow heat shrink tube picked up yesterday in Huaqiangbei. Did you know most heat tube is already printed? I had never noticed before until I tried to find blank tube. I ended up with something with small printing, today I can pick up a reel we ordered from the factory with no printing.

We printed a few Vpu (voltage pullup probe) labels for the Bus Pirate cable. The tube seemed to wander around and printing was inconsistent. Then we realized the label cartridge has a plastic arm that works as a feeder for the tube, holding it down during printing. It’s not clear if this is intentional, and it isn’t mentioned in the manual. With the tube threaded through the cartridge things started to work great.

The interface, even if not in Chinese, is wretched. We decided to give the PC connection option a shot. Drivers for the USB to Serial chip (by Winchiphead) failed on Windows 7. Ok. Drag out the wonky Windows XP netbook and everything installs fine.

The printer enumerates as a USB to serial device so it’s probably very sniffable and hackable. Open it up, bring out the serial, and use a modern converter chip with updated drivers. That presents a minor issue as the software seems to look for it’s specialized drivers (no way to set a serial port manually), but sniffing would be very easy and a simple app to send commands from a CSV spreadsheet could be whipped up in no time… But I digress, none of this gets us closer to a sample Bus Pirate probe cable.

The software isn’t great, but it’s way better than the terrible menus and poorly debounced keyboard on the machine itself. Each heat shrink label tube diameter, length, font size, cut type (half, line, none), darkness, repetitions, etc are set in a spreadsheet like interface. It’s in Chinese but you only need to recognize a few characters to get the basics, mostly cut type. There’s an option to import a spreadsheet of labels, which seems like the easiest option, but as of yet we’ve found no example of the format needed (and the native file is binary which gives no hints). There’s an English version of the 320/330 so the software might be available in English, but the downloads on L-Mark’s English website fail.

We were up and printing meters of probe labels in a few minutes with the PC connect software.

And then the acetate transfer ribbon (ink, basically) broke. Just snapped. And we had no spares. Using a little screw driver and the smallest fingers in the room we pried open the ribbon cartridge with minimal damage. We tied the ribbon back together and did what we could to rethread it properly. Amazingly this worked!

One thing we spotted in the cartridge is possibly an RFID chip to either ID the type of ribbon, or worse, to prevent third party replacements. The ribbon isn’t “cheap” at ~$6 for 80meters. Other machines are much cheaper. However given that we could open the cassette and repair it with minimal damage, we’ll probably just respool it with cheap (and plentiful, and high quality) acetate ribbon that’s used all over Huaqiangbei to print fake cell phone battery labels with the Postek G-3106 or C-168 (our model).

Verdict: it works, and we’re closer to a probe cable I like. Worth the money? Sure, especially if I bought it 12 months ago and didn’t waste so much time with false starts on the cables this past year. I wouldn’t buy it from Alibaba for $400, that’s for sure, and I would stay away unless you’ve got some basic Chinese reading skills or a friend who can help translate.

Print quality seems to depend heavily on the shrink tube. Cheap, highly flexible tubes that compress flat in the printer look beautiful. Expensive, thick tubes range from fair to crap.

We picked up 20 meters yellow and 20 meters white high quality shrink tubing, 3.2mm diameter, 3:1 shrink ratio on Taobao for $0.20 per meter. Most tube has some kind of marking on it already, but this tube is totally blank. We thought it would be perfect for printing, but it’s too stiff and doesn’t hit the print head properly. The white tube printed ok, but the yellow tube looks awful.

Next we grabbed a reel (200 meters) of white and yellow shrink tube from Huaqiangbei for less than $7.00 per reel. This stuff is very thin, flexible, and the prints look great. It already has marking along one side, but if we hold it right it stays on the back of the tube and doesn’t interfere with the print. It’s a pain, but it’s just a prototype.

From left to right: expensive white, expensive yellow, cheap white, cheap yellow.

Sample cables

Here’s a sample cable in yellow, and one in white. The white one looks cleaner, but the yellow looks a bit cooler.

Additionally, there’s two styles of printing – one justified to the left edge of the pin housing, the other centered in the shrink tube. Both cables have half of each. I like the left justified better because the print is much larger and more readable over the crimp housing, however it’s also “upside down”. I’d prefer right justified but the label printer doesn’t have that as an option.

Getting a quote

Fancy printed cable in hand, I set out to Huaqiangbei to get a quote on the cable. Surely with an example cable in hand we can get some quotes from cable makers in the world’s largest electronics market.

What a shock… NOBODY does heat shrink labels, and even the heat shrink manufacturer’s representative said our printing was better than theirs (minimum order 5000 rolls for custom printing). Seeed’s cable girl gave a great tip though – dip the cable in boiling water to shrink it. It looks beautiful, but doubts about residual water remain.

The next step

There’s plenty of suppliers to make the cables, but they don’t do shrink tubes. We did find a supplier to do custom printed shrink tubes, but they’re 7 times more expensive than printing on the L-Mark.

Next we’ll order 200 probe cables, and a set of shrink tubes for 100 cables. The cable manufacturer is willing to put the professionally printed tubes on 100 cables for free. The other 100 cables will be fitted with our own labels printed and shrunk in-house. This should give an idea what the cost of our various options vs quality are. All 200 cables will probably go in the free PCB drawer, but might also end up at Seeed Studio.