PCB edited with KiCad nightly build of Jan 17th.

Sorry I don't have a lot of experience with ESP modules:
If you assume the ~CS and BOOT are floating at reset, does it need pull up/down resistors on GPIO0 and GPIO15 to boot in UART mode (I assume that is the desired state)?

Does the RST need R/C to reset at power up or the module already has these internally?

There is quite a collection of resistor values on the EXP1 and EXP2 pin are these calculated with something specific in mind? If so it would be good to add a comment in the schematics, why the values are what they are.

I see the resistor values are coming from https://github.com/gloomyandy/RepRapFirmware/wiki/SKR-Wifi most likely.

I noticed you are shorting the Reset and Wakeup. We should probably put that as a solder jumper, so the short can be removed if need be. Also add a jumper (on the bottom so it faces up) between the EXP1 pin 10 and the 5V power, you don't want to try and power the whole board from the USB connector on the add-on. It could be solder jumper as well.

I think the EXP1 and EXP2 connectors are flipped. You need to flip the board in order to connect to the PrntrBoard and then the EXP1 would be on the left and EXP2 on the right, which would not be correct (the ESP antenna facing right)

Distance between the connectors should be 21.3868mm (I know it's an odd number, I didn't pan it).

The Add-on looks good, thank you for your contributions.
However I think the previous PR has some issues with the main board, that we need to fix:

The sd-card has no CS pin connected, I don't believe it would work without one. Also there is nothing connected to pin 4 on the EXT2 connector. We have PD2 as spare (SDIO_CMD) - we can use it to drive CS on the sd-card. We don't have to use dedicated SPI for the sd-card. I think it can share the lines with SPI1, so we have 3 extra pins.

It would probably be best to connect one of the extra pins to pin 7 of EXT2. Currently it is shared with the sd_detect pin on the sd-card socket.

Added some more info to make schematics easier to understand.
Pins on EXT1/2 can be used freely as those must be defined in RRF config.

In the essence, to comply with RRF, this adapter must expose:

• SPI with CS signal,
• Data ready output (from ESP),
• Transfer ready input (to ESP),
• Reset line.

excerpt from board.txt

//ESP Settings
8266wifi.espDataReadyPin = A.15;
8266wifi.lpcTfrReadyPin = C.11;
8266wifi.espResetPin = B.10;
//ESP RX/TX Settings
8266wifi.serialRxTxPins = { PC7, PC6 } ;
serial.aux.rxTxPins = {NoPin, NoPin};

BTW, if SD card will not use hardware SPI, then I guess it would be problematic to update firmware. I.e. IAP bootloader uses SPI/SDIO to access SD card content.

I think I understand the ESP interface.

The issue is that on the PrntrBoard EXP2.4 (RST) is not connected - there is no trace to that pin in my original design. In the "reprap" screens that pin is usually used to detect if SD-card is inserted in the LCD sd-card slot. This is optional feature that I skipped because I was running out of pins.

Does this make sense?