Check the control card power supply is normal, voltage is 5V, control card is exclusive of a power connection port.
Check whether the running lights on the control card are flashing (green light or yellow light). If there is no flash, please turn off the screen and restart the display. After the restart, the running lights still don't flash. Please change the control card.
Check whether the serial line is 235 straight through line.
If you have USB to use the serial port line, please check whether the USB will turn the serial line for USB to turn 232. Check whether the USB turn serial driver is installed correctly.
Check whether the computer serial port is normal.
Check whether all serial port interfaces have water or rust, if there is a replacement.
If the control card can not be detected in the above operation, the control card is bad, please change the control card.