So, after getting serial working on a PC, I decided to see if it would work with and Android phone, and it does, at least with one, a Motorola X4; and should work on any device that supports USB-OTG
HW AND SW:
- Phone capable of USB-OTG (On-the-go, gives a USB host mode). There are apps on the play store to test whether your phone can do USB-OTG; some phones may have to be rooted for this to work, but not the X4.
- USB-OTG adapter from Micro or USB-C male to a USB A Female socket, as needed by phone.
- Serial USB Terminal software, such as https://play.google.com/store/apps/details?id=de.kai_morich.serial_usb_terminal
- USB-Serial converter, the app above lists supported chipsets, I used a generic CP2102 device from Amazon https://www.amazon.com/gp/product/B072K3Z3TL that can do 3.3v
After that, it's simple, install the app, then just wire the USB-serial converter to J7, the pin numbers below are J7 pins.
- Pin 1 = GND
- Pin 3 = VCC (3V3)
- Pin 4 = TXD (not strictly needed, since counter doesn't take input?)
- Pin 5 = RXD
Finally, plug the USB-serial converter into the OTG adapter, and then into the phone; you'll be asked whether to always launch that terminal app when the device is plugged in. Then, the settings are 9600 baud, all others are default. Last, click the "connect" button at the top of the terminal screen.
You should see something like the following. No, it's not GeigerBot, but it'll do for my simple needs of not counting CPS and CPM manually.