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Devices:

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"

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    Could you share the complete command with us? – Seamus Feb 24 '20 at 22:20
  • Ah, you have some rubbish not flushed first. If you are using python, flush first. – tlfong01 Feb 25 '20 at 1:34

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