Mitsubishi Electric MELSEC iQ-R Series Programming Manual page 1108

Operation error
Error code Description
(SD0)
3405H The connection number specified by (s1) is a value other than 1 to 16.
When completed with an error, an error code is stored in the completion status (s2)+1.
For the error code stored in the completion status (s2)+1, refer to the following.
 MELSEC iQ-R Ethernet User's Manual (Application)
When the receive status signal does not turn on in TCP fixed-length receive mode, the data received as of the
current time can be read with the SP.SOCRDATA instruction to check whether the data sent from the external
device is missing.
8 BUILT-IN ETHERNET FUNCTION INSTRUCTIONS
1106
8.2 Socket Communications Instructions