Schneider Electric Zelio Smart Relay SR3 B261BD


The 6-way programming connector provides VCC and ground, TTL level serial RX and TX, and I2C SDA and SCL. The serial port is used by Zelio Soft to program the device. Presumably the I2C interface is used by the EEPROM backup modules.

I have successfully programmed this device using the Sparkfun FT232R breakout board set to 5V. I suspect the official programming cable just contains a MAX232 level shifter or similar.

In Zelio Soft’s monitor mode the PLC’s state is polled via the serial port in the programming connector. The protocol used looks like MODBUS ASCII at 115200 bps, 7 data bits, even parity, 1 stop bit (7E1).

Programming Connector Pinout

Viewed looking at the front of the smart relay.

1 Vcc
2 Ground
3 Rxd (serial in)
5 Txd (serial out)


Gustavo has been kind enough to provide pictures of the programming cable. Thanks Gustavo!

It looks like two 211/411op-amps are used to convert between the different signal levels.

Pin out DB9 (F) | Color cable
1                NC
2                Black
3                Green
4                Red
5                White
6-9              NC




37 thoughts on “Schneider Electric Zelio Smart Relay SR3 B261BD

  1. Hello,

    Before all, Thanks for the info.
    I tried to connect to a Crouzet Millenium3, because I know that the programming cable is the same like Zelio.
    I used a RS232 to TTL converter and connect the pins like you.
    Power supply is in pin 1 and 2, and 3 and 5 are the comunications ones.
    I set the com port in 115200 7E1, but still no works.
    I buyed 2 FT232, but this part will be at home in 20 or 30 days.
    Could you give me more information about the cable?
    Thanks a lots,

    Diego Luca

  2. Hello,

    I tried to connect to a Crouzet Millenium3.
    I used a RS232 to TTL converter, but I can´t read or write in it.
    I know that Millenium3 and Zelio2 are similar, and the same programming cable can be used in both.
    Can you give me more information about your cable, and the connection that you made?
    Thanks very much,

    • Hi Diego,

      I’m not sure what more information I can give. Have you tried checking that your RS232 to TTL converter is operating at the correct voltages? You could connect RX to TX and do a loopback test to see if your converter is working correctly.

  3. I’ve got the SR3 B261BD interface cable and it has a couple op.amp (411) and voltage comparator (211) and 2 diodes s1j typte.I would like to upload pictures….

  4. i have a problem with this issue
    the VCC is important or not

    because i have serial rs 232 to usb converter and there is only 3 wires connected

    tx rx gnd

    • VCC is not required. Three wires are sufficient: Rx, Tx and GND.
      Be sure that the signals are at 5 V as normal RS-232 levels may damage the PLC.

      • Hi Spencer,
        Thanks for this informative site.

        I have a Zelio SR2 B201BD that I wish to program from within Zelio Soft 4.5 via USB.

        I’m using a “cheap” FT232R -> 5volt TTL device from ebay.
        TxD on FT232R -> pin 5 ( TxD ),
        RxD on FT232R -> pin 3 ( RxD ),
        GND on FT232R -> pin 2 ( GND ).

        The FT232R drivers installs automatically in Win7 and shows up in device manager as USB device : “USB Seriel Converter” and as COM device : USB Seriel Port ( COM8)

        The COM8 is then manually configured to 115200 baud, 7E1. Flowcontrol is set to “None”.

        In Zelio Soft 4.5 the COM8 port shows as an option.
        ( USB connection is of cause not shown. )

        Before connecting to the SR2 I tried to short the TxD & RxD on the FT232R and press the “Test” button.
        I then get a “Connection Successfull” 🙂

        When connected to the SR2 my setup returns an error ( device not found ). 🙁

        I suspect that TxD on the FT232R should in fact go to RxD ( pin 3 ) on the SR2 instead and visa versa, but I’m afraid to kill the module if I’m wrong. :/

        Best regards

  5. i am looking at connection two zelio’s by making a cross over , should just need rx, tx and gnd . then use the serial link function block to transfer data between them . Have you tried this ?

    • I haven’t tried this. I had a look at the help file, and the SLIn and SLOut blocks both require something external to send a message to the Zelio device. The Zelio will not send data out the serial port by itself.

      I would try putting a microcontroller in between the two Zelio devices, and have it query one device then push the result to the other.

      Another option is to connect a standard digital output on one to an input on the other and vice versa. You could then signal one device from the other by counting pulses. It depends on what sort of data you need to share between the devices. What are you trying to do?

  6. Hi,

    I have succesfully hacked SR2A201BD. I have connected ISP (or JTAG, not sure now) programmer using test pins, backed up all FLASH and put there my bootloader. Now I program this PLC in C language over usart. Zelio is really powerfull without buid-in firmware 🙂

    PS: when I have tried to put it to it’s original state from backup, everything works like from box.
    PPS: user program of original firmware is saved in EEPROM (I did play with erase eeprom and then write eeprom from backup)

  7. Interesting writeup and thanks for the info! I managed to program an SR2 with just a PL2303 based USB serial converter set to 5V TTL levels, the optoisolators on the ‘official’ cable look to be a low cost way of providing isolation and level conversion at the same time (neat!). The Zelio software supports selecting arbitrary COM port numbers, it’s pretty much plug’n’play, only need to connect RX->TX, TX->RX and GND->GND.

  8. I used a DIY lead too for the SR2. I used a MAX232 from the PC com1 to the SR2 header, works straight away. If you’re using a laptop with no ground connection charger-type PSU, I recommend using the laptop on battery, or disconnect the laptop mains adapter while you plug into the SR2, just to eliminate any damage the SR2 from any AC mains filter currents that might flow between the devices… opto isolation on a max232!

  9. Hi Spencer, Please i have Zelio Smart Relay SR3 B261BD, and I DON’T the communication cable to upload program onto the device. please which other cable do you recommend i use.

    • You can use any TTL levl serial cable or breakout board. I use the Sparkfun FT232RL board.

      If you are using a homemade cable I recommend you disconnect the smart relay from any mains supply.

  10. i was try to read the firmware using usbasp but it was error cant read it. my be you can gibe us the tutorial how to read it. trims.

  11. Hi. I have a Crouzet Motomate – Brushless motor with integrated logic controller. The integrated controller is the Millenium 2. I DO NOT have the cable needed to program the PLC and I can not find one (Crouzet only sells them in packets of three charging almost 500$).
    Does anyone know for sure if the Zelio programming cable would work?
    The pinout seemed similar, here you can find the motomate pinout:
    Thank you in advance
    P.s. I am kind of desperate because without the programming cable the motor is useless…

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