With RTU in particular I've found that there are a lot of devices that don't follow the spec with regard to parity, stop bits, etc. Since its RTU I'd verify the settings from your device's spec sheet and check them against the modbus setting. Section 2.5.1 here http://www.modbus.org/docs/Modbus_over_serial_line_V1_02.pdf  Bits per Byte: 1 start bit, 8 data bits, 1 bit for parity completion, 1 stop bit Even parity is required, other modes ( odd parity, no parity ) may also be used. In