A couple years ago during a Black Friday sale I purchased one Toshiba WT7-C Windows tablet. Featuring an Atom Z3735G processor and 1Gb DDR3 RAM, it's basically a full featured PC in a 7" tablet form. It had one MiniUSB port for charging and that's about it. No 3G, no GPS. I did say it was cheap, didn't I?
One year passed and I barely used it, mainly because the battery wouldn’t last more than a 3-4 hours on intense use. Then came the Windows 10 upgrade and I had just enough space for it. Barely. Windows 10 made it more responsive and I could finally use the native 1280x600 resolution. (owners surely know what I mean)
It was still borderline unusable because the small battery wouldn’t survive a full day. So I started using it in a self-made cradle, as a TuneIn music player, connected to the charger. I knew It had OTG support, I tried it with one OTG flash drive and it worked immediately. But it was either OTG or charging, not both. Searching for that matter online, I found some USB Y split cables claiming to support both data and charging at the same time but I knew I can make myself one. Or at least I can try.
Step 1: The Illusive "charging Downstream Port"
My first try involved splicing one OTG cable and injecting power from a third connector (simple Y with OTG). When I first connected my cable, I had data connection (the flash drive became visible in Windows Explorer) and the power indicator said charging but I quickly noticed the power level going down. So no joy. BatMon software made it obvious, there was still a charging status but nothing actually happened.
A few weeks later, while searching the matter online I came across this:
Two types of charging port exist: the charging downstream port (CDP), supporting data transfers as well, and the dedicated charging port (DCP), without data support. A portable device can recognize the type of USB port; on a dedicated charging port, the D+ and D− pins are shorted with a resistance not exceeding 200 ohms, while charging downstream ports provide additional detection logic so their presence can be determined by attached devices. (Wikipedia)
It became clear to me that I would need some kind of detection logic and that there's no DIY solution for this. That's what every forum I came across said. But I only became even more stubborn. I found some dismantling instructions on FixIt and I was determined go all the way and seep into the battery connector with some external li-ion charger, bypassing the onboard solution completely while ripping the whole thing open. Then I saw I still have about two years of warranty and common sense kicked in.
Up to that point I purchased one HAMA OTG USB hub. I figured I will need it . It had a micro-usb power connector on a side, for supplying additional power to the devices connected to its ports, two regular USB ports and one OTG cable to connect to the tablet. Opening it, I saw one SMD diode preventing power to flow from that port back to the tablet. I shorted it and tried it on that tablet. OTG data connection worked but still no charging. I figured the OTG input cable is to blame so I unsoldered it and replaced it with a standard micro USB. This move provided me with data transfer but fake charging. Then I started playing with resistors across the data lines. I figured the worst case scenario would have been to end data connection for that session and starting a high current charge process until reinsertion.
Step 2: It Works!
Then I found it: one 200 ohms resistor across the data lines. That tricked the tablet into charging at high current while still maintaining the data connection. I don’t fully understand why, but it does work. I now have on my desk a tablet that's charging while connected to a wireless mouse and keyboard and a USB hard drive. BatMon is showing more that 1000 mAh charging rate, depending on the battery's status. I finally achieved my goal! It's the lowest power all-in-one PC ever!
The home edition of windows 10 doesn’t allow inbound RDP sessions but Teamviewer makes up for that. I'd like to use it for something more exciting that a seldom used internet radio player.
Now I'm opened to suggestions: What would anyone do with a 24/7 connected Windows 10 tablet that's able to connect via USB to any USB device? What are your suggestions? Please, let me know. :-)