Background - I'm building a uC unit to manage the solar system on my camper vehicle. It's Arduino based. It was intended to be integrated to a smart phone app over Blootooth. I also wanted the HC-05 or HC-06 Bluetooth modules to also do Over The Air (OTA) programing of the Arduino.
I've given up with OTA on the HC-05/HC-06 modules. I can't find a way for them to reset the Arduino immediately prior to programming and also facilitate the smartphone app channel.
Now I want to use a standard USB to Serial TTL bridge and two ESP8266-01 modules as a transparent serial to serial over wi-fi bridge just for OTA. The app channel will stay bluetooth hopefully.
For this I need TXD, RXD as you would expect and I also need DRT to pass through to reset the Arduino so the the bootloader starts so I can program it OTA.
PC --> USB port --> USB to TTL serial --> ESP8266-01 (wi-fi) ESP8266-01 (serial) --> Arduino.
I also have an ESP6266 nodeMCU (ESP-12 or ESP-12E) module.
I did think of abandoning blootooth and just using WiFi but it's too hard to do the app via Wifi and the ESP-12 doesn't have enough analog inputs.
Is there some standard firmware(s) that I can flash to make these ESP-01 do a transparent link with DTR?