Chat here is you are connecting ESP-xx type modules to existing AVR based Arduino

Moderator: igrr

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By HowardHo
#81337 From #include "ESP8266WiFi.h". The macro is in the header "eagle_soc.h"
As far as I can tell ....
ESP8266WiFi.h <-- WiFiClient.h <-- Stdint.h <-- Interrupts.h <-- ets_sys.h <-- eagle_soc.h

I hooked up CTS and RTS the standard way ...

Microprocessor RTS (Output/Driver) -----> 8266 CTS (input) (GPIO13)
Microprocessor CTS (Input) <----- 8266 RTS (Output) (GPIO15)

The signals are active low, i.e. when they are low the 8266 UART sends/receives normally. But this can be inverted in the 8266. BTW, I'm using a ESP8266Mod, but that shouldn't make a different.

Hope this helps.