Glucometer notes: GlucoRx Nexus

This is a bit of a strange post, because it would be a glucometer review, except that I bought this glucometer a year and a half ago, teased a review, but don’t actually remember if I ever wrote any notes for it. While I may be able to get a new feel for the device to write a review, I don’t even know if the meter is still being distributed, and a few of the things I’m going to write here suggest me that it might not be the case, but who knows.

I found the Nexus as an over-the-counter boxed meter at my local pharmacy, in London. To me it appears like the device was explicitly designed to be used by the elderly, not just because of the large screen and numbers, but also because it comes with a fairly big lever to drop out the test strip, something I had previously only seen in the Sannuo meter.

This is also the first meter I see with an always-on display — although it seems that the backlight turns on only when the device is woken up, and otherwise is pretty much unreadable. I guess they can afford this type of display given that the meter is powered by 2 AAA batteries, rather than CR2032 like others.

As you may have guessed by now from the top link about the teased review, this is the device that uses a Silicon Labs CP2110 HID-to-UART adapter, for which I ended up writing a pyserial driver, earlier this year. The software to download the data seems to be available from the GlucoRx website for Windows and Mac — confusingly, the website you actually download the file from is not GlucoRx’s but Taidoc’s. TaiDoc Technology Corporation being named on the label under the device, together with MedNet GmbH. A quick look around suggests TaiDoc is a Taiwanese company, and now I’m wondering if I’m missing a cultural significance around the test strips, or blood, and the push-out lever.

I want to spend a couple notes about the Windows software, which is the main reason why I don’t know if the device is still being distributed. The download I was provided today was for version 5.04.20181206 – which presumes the software was still being developed as of December last year – but it does not seem to be quite tested to work on Windows 10.

The first problem is that that the Windows Defender malware detection tool actually considers the installer itself as malware. I’m not sure why, and honestly I don’t care: I’m only using this on a 90-days expiring Windows 10 virtual machine that barely has access to the network. The other problem, is that when you try to run the setup script (yes, it’s a script, it even opens a command prompt), it tries to install the redistributable for .NET 3.5 and Crystal Reports, fail and error out. If you try to run the setup for the software itself explicitly, you’re told you need to install .NET 3.5, which is fair, but then it opens a link from Microsoft’s website that is now not found and giving you a 404. Oops.

Setting aside these two annoying, but not insurmountable problems, what remains is to figure out the protocol behind the scenes. I wrote a tool that reads a pcapng file and outputs the “chatter”, and you can find it in the usbmon-tools repository. It’s far from perfect and among other things it still does not dissect the actual CP2110 protocol — only the obvious packets that I know include data traffic to the device itself.

This is enough to figure out that the serial protocol is one of the “simplest” that I have seen. Not in the sense of being easy to reverse, but rather in term of complexity of the messages: it’s a ping-pong protocol with fixed-length 8-bytes messages, of which the last one is a simple checksum (sum-modulo-8-bit), a fixed start byte of 0x51, and a fixed end with a bit for host-to-device and device-to-host selection. Adding to the first nibble of the message to always have the same value (2), it brings down the amount of data to be passed for each message to 34-bit. Which is a pretty low amount of information even when looking at simple information as glucose readings.

At any rate, I think I already have a bit of the protocol figured out. I’ll probably finish it over the next few days and the weekend, and then I’ll post the protocol in the usual repository. Hopefully if there are other users of this device they can be well served by someone writing a tool to download the data that is not as painful to set up as the original software.