MODBUS Break Condition & Checksum


Thread Starter


I'm trying to communicate with a power meter using MODBUS RTU. The cable is only 1 meter long. I have a RS-485 to RS-232 converter connected to a RS-232 to USB converter. I use MODBUS Poll and receive Write Error: Break Condition and then Checksum Error.

The electric meter has a bias resistance option. Without Bias resistance the RX light on the meter is illuminated indicating an open circuit. When I enable Bias resistance the RX light on the meter turns off but simultaneously the RX light on the RS-232 to USB converter illuminates.

The RS-232 USB converter also has a light to indicate TX. When I use MODBUS poll the TX light will flash so I know I'm transmitting but the RX light always remains illuminated and I receive the errors mention previously.

Thank You for any help you can provide,
Maybe try a different converter (or your current setup with another slave). FTDI sells some that have the conversion circuit in the USB connector and flying leads on the other end. I think you can buy them off the site but you can also get them at Mouser and so on.

That way you'd get from USB to 485 in one shot and have less pieces to try to coordinate.

Lynn August Linse

Having the RS-485 +/- reverse (or swapped) will usually look like a BREAK condition if blindly converted to RS-232, which is just a serial line being in the non-idle state for 'too long' between bytes. So if your app sees the break ... you have a FANCY USB->232 converter :). Most cheap micro/pic ports can't even detect break conditions anymore.

- Lynn
I agree that the fewer converters you have transferring the signal, the better. Connecting one RS-485 to USB converter should simplify the matter. One such model, the I-7563, is an ICP DAS unit and contains a self-tuner, which is a chip auto-tunes the baud rate and data format to the RS-485 network.