With the start of sales of RC-20, the programmer's manual for RC-20 was published in Japan, which describes in detail the options of connecting the watch to a PC and developing applications for them.

Original scan
Passed through OCR

The book was published with the kind permission of the author of the Youtube channel Vintage Digital Watches

To transfer data to the watch, you can use any USB to UART TTL module (which can be easily found on ebay or aliexpress). I used a module built on a CP2102 chip. But, without further development, you can either receive data, or send them (Because the RC-20 uses rs232c signal levels that are inverse to TTL levels). For transmit data, UART module with 2.5 Jack must be connected as follows:

|=TX=|====|=GND=>

For receive data from the RC-20:

|=RX=|=GND=|====>

FTDI
A fully functional data cable can be made using the FTDI chip (it is also used in many cheap USB UART modules). Connection diagram:

However, before use it is necessary to configure the chip with the FT_Prog utility. With its help, it is necessary to set the inverting of the RXD and TXD levels.

Note
It should be kept in mind that some 2.5 jacks do not completely enter to the input on the watch, in this case I slightly cut the plastic case, as in the photo:

The watch is put into receve mode as follows:

To transfer binary file to the watch, you can use the rc20dt application, which adds the necessary headers and sends the finished data to the watch.

Usage:

rc20dt [-p <port name>] <file name>

After the transfer is finished, the watch will show "PROGRAM RUN?"

To build the program any Z80 assembler will do (although some examples from the book use the i8080 notation). I used zmac.

A list of system subrouthine adresses can be found here

To demonstrate the potential of the watch, I wrote a clone of Google T-Rex game. The game sources and a binary ready to be sent to the watch are here.