# R2 Arduino Uno and an R3 Ethernet Shield mismatch

I bought an R3 Ethernet shield for my R2 Arduino Uno, but I noticed that it has a few more pins than my R2 Uno has sockets. Do I need to buy a different shield?

Arduino UNO

Arduino Ethernet Shield

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Post links to devices and we'll be able to comment. – AndrejaKo Jul 20 '12 at 22:17
It's probably cheaper to buy a new r 3 arduino. The shields I've seen are more expensive than the arduino itself. – SimpleCoder Jul 20 '12 at 23:45
Hi, sorry for the delay. The links are link and link. Unfortunately, I think that @SimpleCoder might be right. Thank you guys for your time! – cjohnson318 Jul 21 '12 at 3:15
@jippie, that is terrific explanation. Thanks so much! – cjohnson318 Jul 23 '12 at 13:36

When I take a look at the schematic for R3,

it appears that the extra pins at the top left:

carry the same signal as pin A4 (AD4/SDA) and pin A5 (AD5/SCL).

The extra two pins at the bottom left are respectively unused and attached to +5V according to the same schematic diagram. Unsure why the printing IOREF on the PCB gives other impression.

As a solution, you should be able to simply wire the top left two connections to the bottom right ones (A4 and A5).

A multimeter should be able to confirm my theory. The new connections on R3 have probably been introduced to allow for smaller shields still using power supply and I2C bus.

As a reference the R2 looks like this:

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