FM350-2在读取编码器数据时出错，在FC4中出现返回数据，不知是什么意思？怎样解决？手册中有如下信息，帮忙分析一下。
手册信息：Error Messages
Any errors which occurred are displayed in the binary result bit (BR = 0). Possible
errors are:
• Unknown read job (see JOB_RD.UNKNOWN).
• Data transfer error when transferring data with SFC59 “RD_REC”. The error is
indicated at the output parameter RET_VAL (see Reference Manual: System
Software for S7-300 and S7-400, System and Standard Functions).