If the RX light is on solid on your modbus serial port and:
The port is in RS485 mode- This is usually indicative of polarity being reversed (ie. positive is connected to negative instead of positive to positive). Try swapping the wiring to pins 1 and 8 on our port.
The port is in RS232 mode- This suggests that a constant electrical signal is hitting our receive pin which should never occur during normal operation. We have seen this on occasion with customer USB to serial converters which are not set up correctly, we strongly recommend trouble shooting your USB to serial converter if this is the case. If there is no converter involved, we recommend verifying the wire connected to our RX pin is connected to the correct pin on the other device.
If none of these common causes seem to be causing your RX to be on solid, please call Prosoft Technical Support to trouble shoot your issue further.