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January 30, 2020 at 2:08 pm #64173
You can find the pin assignment in the hardware design details:
OS 3 (based on ESP8266) does NOT use 74HC595, mainly because ESP8266 does not have enough number of GPIO pins. Instead, it uses PCA9555 IO expander, which is I2C-based so only uses 2 pins and are shared with OLED and RTC.