RPi3B+, USB to TTL CH340, SIM800C GSM module
Here are the connection between the USB to TTL and GSM module respectively
5V-VCC, TxD-RxD, RxD-TxD, GND-GND
Then I attach this USB to TTL to one of the USB ports of the RPi3B+
Whenever I connect to this serial using putty, every first time I enter "AT" I get an "ERROR" then all subsequent "AT" will return "OK"