Possible cause:
1,the cable is too long 
2,communication device fault
3,cable interference 
4,serial pot settings

Inspection measures:
1,The cable distance between the access controller device and communication convertor exceeds the effective length (1.2km). There are too many branches of the networking cables.
2,Check whether the communication convertor has problems.
3,Line interference leads to unable communicate normally.
4,The communication port or serial port are not corresponding to the actual setting and connection.