connect to modbus device failed

hi all,
   mapper connects to modbus devices in modbus-TCP mode,can connect to a device,But the HoldRegister data type read failed.
The questions are as follows:
"read tcp ip:port->device IP:502 i/o timeout",ping device ip address is OK,the ip address of the telnet device and port 502 are normal.The error location is twindata.go line 127