I have an Arduino GSM Shield mounted on an Arduino Uno. Can I stack this GPS shield above Arduino GSM shield? Will there be any pin configuration problem?

In GSM Shield hardware pin’s configuraton are as below:

0 - Not connected
1 - Not connected
2 - GSM Rx
3 - GSM Tx
7 - Modem reset

If there is any problem, how can it be solved? I would be happy if there are any alternatives for the GPS shield.


The schematic of the Arduino GSM shield: schematic of the Arduino GSM shield

The schematic of the SparkFun GPS shield: schematic of the SparkFun GPS shield

  • Start by finding the circuit diagrams of both shields and lining them in your question.
    – jippie
    Commented May 4, 2014 at 9:51
  • According to the GPS shield quick start guide (sparkfun.com/tutorials/173), it seems possible to select different pins for serial communication than the default ones (2 & 3) that conflict with yout GSM shield. So that should be OK.
    – jfpoilpret
    Commented May 4, 2014 at 11:15
  • @jippie - I've added the schematics Commented Feb 12, 2015 at 4:16

3 Answers 3


enter image description here
Photo by John Boxall

Which pins do they use?

To determine if two shields are compatible, you have to read the specifications of both shields.

The best source for this information is the data sheets supplied with the shields. However there is also this Shield List

Can you reassign pins?

Some shields provide a means to alter which pins are used for communications. This can take the form of a switch or a jumper - either pull-off or solder-bridge type. (the GPS shield in the question has jumpers).

Where the shields do not support pin-reassignment or provides insufficient flexibility, to work-around incompatibilities you can use an in-between shield designed for the purpose.

here is one example:

You hear it all the time in the Arduino community, “will this shield work with my other shield?.” I think we’ve all been disappointed to find that two of our favorite shields compete for control of a digital pin here or a serial pin there. While there are sometimes some clever workarounds from elegant code modifications to aggressive ‘greenwiring,’ Mayhew Labs may have come up with the simplest solution thus far: The Go-Between Shield.

Are any alternatives for the foo shield

I notice that Arduino.cc now point people to the list referenced above. They used to have their own list.

Product recommendations are off-topic here but, obviously, you can search

  • +1 for the link to Arduino shields references, and for the go-between shield as a generic solution to this kind of issue. However, in this specific case, a go-between shield seems useless as the GPS shield allows changing the TX/RX pins (2 & 3 are just the default pins) quite simply.
    – jfpoilpret
    Commented May 4, 2014 at 11:46
  • @jfpoilpret: Good point. I'll update answer. Commented May 4, 2014 at 15:19

From the shield description, the TX and RX of the GPS are mapped on pins 2 & 3, like your GSM, so you'll have to change that.

I'm using Adafruit GPS + flash card shield, which is a bit cheaper (and uses pins 7&8).

  • But my GSM shield uses pin 7 uses modem reset. Will It create any problems by placing GPS shield on the GSM shield ?
    – Manihatty
    Commented May 4, 2014 at 13:15
  • Yes, very same thing. Depending on what you want to do, why not use a breakout plugged on a breadboard ? You'll be able to choose pin assignment. Commented May 4, 2014 at 15:34

This answer does not directly address the pin conflict issue, but rather skirts around it.

I have been looking into this same issue for over a month now and have come to the conclusion that the best option is to either purchase, for around $30:

SIM908 Shield

or make, for around $20 (+$60 for the modules)

Modular SIM908/900 shield

a shield that uses the SIM908. The SIM908 comes with both GPRS/GSM functionality, and GPS functionality, whereas the SIM900 only has GPRS/GSM. Now, there is no longer an issue with pin conflicts, as instead of two separate shields, only one is required.

The only disadvantage is that if you already have a GPRS/GSM shield, then that purchase will become redundant.

As an aside there is an alternative of a 3G (WCDMA and HSPA) and GPS, using both assisted-mobile (A-GPS) and mobile-based (S-GPS) modes, combined shield, from Cooking Hacks. A bit more pricey though, at $160.

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