Problem with 2 SIM800L

Hi everyone. I’ve just bought 2 SIM800L because the first one couldn’t get working. The second one, also didn’t work.

It’s very strange that 2 of this modules came faulty. Maybe I’m doing something wrong.

I attach an image to show the module.

I’ve tried with every baud rate and following the manual, no connection at all with USB2TTL. I dend the AT command to synchronize the UART, but module never respond.

This is how I wire it up:

5V → 5V of Power Source
GND → GND of Power Source
VDD → 5V and 3,3 V *** Read ahead explanation ***
SIM_TXD → USB2TTL’s RX
SIM_RXD → USB2TTL’s TX
GND → USB2TTL’s GND
RST → Not connected.

*** VDD ***: According to manual, this pin is to match the TTL voltage. So if I use 5V, this pin must be connected to 5V, the same for 3,3V.
I’ve tried with both voltage and not connected, nothing happened.

If I call from my mobile to the module, the power led turn off until I hung up. So it’s receiving calls.

¿ Any help with this ?

SIM_TXD --> USB2TTL's RX
SIM_RXD --> USB2TTL's TX

Why? The chip outputs TTL voltages already.

PaulS:
Why? The chip outputs TTL voltages already.

According to the manual, yes. One can decide which voltage to use: 5V or 3,3V. There is a pin for that in the module.