Last month I did buy a 2kW photovoltaic system for my house. To connect the pv panels to the grid I bought the Sunny Boy 2100TL  inverter from SMA. Unfortunately, it comes without any communication interface in standard configuration. The price for the so called RS485 piggyback interface card is around 90 Euros. On sales pictures it seems to contain only a DC/DC converter and some optocouplers for galvanic isolation and the RS485 driver itself.

I did some research and was able to find out the pinout of the 14 pin and 10 pin header. The 14 pin header is on the “hot” side of the inverter and provides an UART interface with RXD, TXD and RTS signals. For first tests I decided to build a simple galvanic isolated UART adapter using Analog Devices iCouplers. Together with a FTDI USB to TTL adapter I will be able to test everything very fast.

Later I like to use mbed or Arduino to log data on sd-card.

At this point I need to warn you: There is risky high voltage inside of the inverter! Only open it after 15min after it has been disconnected from the pv panels and the grid! 


The first PCB was designed for two 2 channel iCouplers of the type ADUM1201. One for the RXD and TXD and one for RTS and CTS signals. Later I found out, that only RXD and TXD is used for communication. Below you can see the PCB of the first version.



Complete assembled PCB and connector for the FTDI  TTL-232R-3V3 USB adapter.

PCB RS232 piggyback SMA


I have updated the PCB design and attach the Eagle design:

UART piggyback sma schematic



UART piggyback SMA PCB



Eagle project files: SMA piggyback UART schematic and PCB

Schematic: UART_PiggyPackV2.sch

PCB: UART_PiggyPackV2.brd

Have fun!



10 Responses to Sunny Boy UART (RS232) PiggyBack

  1. Bojan says:


    whether the inverter sunny boy 1200 has the same pinout ?

    can i connect pins 3,5 and 9 directly to RS232 port on my PC ?

    Thank you

    • adelkido says:

      Hi Bojan,

      the Sunny boy 1200 has the same pinout. My adapter only provides TTL output level. To connect it to the PC you have to use a TTL-RS232 or an TTL-USB adapter. I’m using the TTL-232R-3V3 adapter from FTDI without problems.


      • Bojan says:

        Did you mean that i need connect MAX232 converter on pins 3,5 and 9 on SV1 connector (that is on sunny boy inverter)?

        Or i need connect max232 on yours adapter ?

        Thank you

        • adelkido says:


          I have included an extra pin header to have easy access to RX,TX and GND. This header is also pin compatible to the TTL-232R-3V3 FTDI adapter. You could connect the MAX232 converter directly to this pin header. Alternative, you could connect it on pins 3,5 and 9.

          • Bojan says:

            Must be pin 10 connected to +5V ?

          • adelkido says:

            This adapter needs external +5V to power the iCoupler as shown as on the diagram. I used the FTDI USB adapter which uses the +5V from the USB. Please check with an Ohmmeter which Sunny Boy pin is connected to the PCB pin 5.

  2. Bojan says:

    Sorry ,

    but if i connect max232 directly to inverter pins 3(Recive),5(transmit),9(gnd) , i don’t need any additional voltage on inverter pins ?

  3. adelkido says:

    You still need an additional +5V supply for the IC4.

  4. Bojan says:


    i want try connect max232 directly to inverter (pins 3,5,9) without iCoupler…is that ok ?

  5. adelkido says:

    IC4 is the iCoupler and is responsible for the galvanic isolation. There is up to 600V on the “hot” side.

    You can connect the MAX232 to pin 3,5 and 9 and the +5V power supply.

Leave a Reply

Set your Twitter account name in your settings to use the TwitterBar Section.