No communication between GPRS module RaspberryPi Zero serial GPIO ports

Question

I am trying to use my RaspberryPi Zero to communicate with an A6 GPRS module (http://www.inkocean.in/gprs-a6-module-sms-board-gsm-gprs-wireless-data-transmission-over-sim900a)

Drawing of physical connections:

Steps taken:

• connect module's U_TDX pin to PiZero's TXD0 (GPIO14)
• connect module's U_RDX pin to PiZero's RXD0 (GPIO15)
• connect module's GND pin to PiZero's Ground
• power up pi using micro-usb
• power up module using it's own micro-usb port
• disabled serial port getty/logging using raspi-config
• reboot
• added linux user to group dialout
• command: ls -l /dev/ttyAMA0 output: crw-rw---- 1 root dialout 204, 64 Oct 19 10:17 /dev/ttyAMA0

Now I should be able to communicate with the module and monitor the serial port using a serial monitor, i have tried using screen for this:

screen /dev/ttyAMA0 115200

But I get a blank screen without errors.

According to this blog (https://learn.adafruit.com/fona-tethering-to-raspberry-pi-or-beaglebone-black/setup) which uses a similar module, i should be able to type "AT" + enter (without seeing the feedback of chars being typed) and I should receive "OK", which I do not.

Potential problems:

• the module is borked (doesn't seem so; I get a solid red led and a blue led blinks a few times after boot, I assume it's making connection to the sim's network)
• wiring is incorrect, perhaps use different pins on the module?
• I have not yet correctly configured the serial monitor screen or am using the wrong baud rate?

What I have tried:

• connecting the zero's serial ports to the module's R232_RXD and R232_TXD ports
• using minicom as a serial monitor
• using PySerial to send/read data to/from the module

What I havn't tried:

• connecting the zero's serial ports to the module's H_RXD and H_TXD ports as I assume these are not the ports I need

Any help solving or even just debugging this issue would really be appreciated, as I am really stuck. Thanks.

Answer 1

If you have already connected the Pi Zero TX/RX pins to the GPSs RS232 TX/RX, then you may have a 'problem'. Voltages for RS232 communication can be anything from -15v to +15v, and the Pi Zero is expecting +3.3v and 0v... you may have killed those pins.

It seems (from the page you linked to) that the U_TXD and U_RXD pins on the GPS use TTL levels (+5v and 0v), so those are not safe to use either unless you have a level shifter...

My first step would be to check that the Pi Zero TX/RX pins are still working - link them to another Pi (if you have one), open a terminal on each and make sure you can Tx/Rx both ways. You might also be able to simply link Tx to Rx on the Pi Zero, open a terminal and see the sent data reflected back to you...

Answer 2

I eventually found the problem, which was alot more basic than I thought.

The problem was the wiring of the module to the pi. The wiring of the pins should look like this:

So that:

• The module's trasmitter pin (U_TXD) connects to the Pi's receiver pin (RXD0)
• The module's receiver pin (U_RXD) connects to the Pi's transmitter pin (TXD0)

As shown by this image:

Which makes a whole lot of sense. I hope this helps other people that might be stuck on the same (basic) issue!