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I've got my sim900 module working with arduino by using their software serial library, however, I want to eliminate arduino from the equation and have serial communication directly to sim900 module.

I'm using putty as my terminal emulator. It's serial is configured to COM1 19200 8 N 1 the same as device manager configuration for this port.

I connect straight from hardware serial on my PCs motherboard into serial-to-ttl interface board which connects to sim900 module. The board has 4 pins - VCC GND TX RX. They're all connected to my sim900 hardware serial as follows: VCC=5V GND=GND TX=TX RX=RX (Yes I know that it's always actually TX=RX and RX=TX, but when I connect it that way my interface board doesn't blink any led to indicate a transfer whereas it does when I connect TX=TX and RX=RX). The switch on the module is set to hardware serial pins as well.

So the only thing that happens when I send AT commands such as AT or ATI and press enter is that puttys cursor comes back to the beginning of command that I typed. No response.

I'm thinking that I'm not doing something that the arduinos software serial port is doing when it sends commands to sim900.

Can anyone help please ? It's literally been days of trying different configurations with no results.

In that time besides getting sim900 working with arduino software serial I verified that the hardware serial port on my motherboard is working correctly and the interface board is working correctly as well.

Shady Programmer
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  • Arduino hardware and shields do not generate a signal that is compatible with what your PC requires. Tutorial [is here](https://www.sparkfun.com/tutorials/215). Ask questions about it at arduino.stackexchange.com – Hans Passant Sep 02 '16 at 11:22
  • But with the use of my interface board I was able to read arduinos "hello world" output on putty therefore I've concluded that interfacing to arduino works fine but doesn't work with sim900. Also I've already asked that question on arduino.stackexchange and got directed here because it's not related to arduino but only sim900 shield and serial port !!! – Shady Programmer Sep 02 '16 at 13:17
  • If you suspect the serial data sent over the wire is different, then the next step is to hook up an oscilloscope and capture the signal in both cases and compare. Of course just capturing and examining the signal in the failing case on its own is also useful, but when you have one working situation and one non-working situation it is always easiest to start comparing those two. – hlovdal Sep 03 '16 at 06:49

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