OK, Rand we can fix this.
Olijouve posted the series firmware on the GitHub.
1-Do you have a MicroSD card of 8gb or less? A 16gb might work, might not. If so format it Fat32, with default allocation size. Or use this software tool to format: SD Memory Card Formatter | SD Association
2- Put the “Firmware.bin” on the MicroSD card. Nothing else.
3-On the SKR Pro 1.2 Pro board, disconnect the power leads, for this part you will not need 12v power. Disconnect the TFT35 as well.
4-Orient the board with the stepper drivers along the top edge. On the board, left middle, is a jumper for using either USB or power, set the jumper for USB power. Plug in a usb cable. The stepper lights should light up, as well as the lights middle right that indicate logic power.
5-Unplug the USB cable.
6-Insert your freshly formatted MicroSD that has the Firmware.bin file on it.
7-Plug in the USB cable, the lights will come on, and near the middle of it, a green light should flash like mad for 5 or so seconds, this is the board updating the firmware.
8-After the light quits flashing let it set for a minute as is then unplug the USB Cable and remove the MicroSD Card.
9-Reset the USB Jumper, left middle, for main power.
10-Plug the TFT35 back in, the EXP cables and the black ribbon. Re-connect the 12v power
11-Power on. Check the firmware version, It should now be 515S.
If I remember correctly the Series firmware doesn’t use end stops, so they may or may not work, likewise it cannot auto-square, as that feature requires end stops, and individual stepper drivers for all the steppers, 2 micro switches each on X and Y as the steppers are driven individually. Series wiring actually is a bit more difficult to get the machine square, as mechanically you have to get the X and Y axis as square as possible by tightening or loosening the Truck bearing bolts. When I built my first Primo, it was series wired. It took some tinkering to get the frame, the X and the Y square. Take your time, I used a spacer block between the Corner Top, and the Truck to check that spacing with the Core in the lower left corner (the Home corner)