我们车间是用416F的CPU通过SEW的PROFIBUS模块控制SEW的电机，但是有一天，一台电机无法动作。从程序上看，对应的Q点有输出，而且检查过SEW电机进电正常，PROFIBUS模块正常。在排除所有可能的故障点后，仍无法动作，直到把PLC断电再启动后才恢复正常。请问是什么原因？以下附上诊断缓冲区的诊断信息：
Event 1 of 120: Event ID 16# 38C4
Distributed I/Os: station return
Address of the affected DP slave: station number: 87
DP master system ID: 2
Log. base address of the DP slave: Input address: 16280
Log. base address of the DP master: 16382
Requested OB: Rack failure OB (OB86)
Priority class: 25
External error, Outgoing event
04:12:15.820 PM 03/08/2010


Event 2 of 120: Event ID 16# 39B1
I/O access error when updating the process image input table
P area, byte access, access address: 877
process image partition no.: 0
not user relevant(Z1): 2942
Requested OB: Program execution error OB (OB85)
Priority class: 25
External error, Incoming event
04:12:15.789 PM 03/08/2010


Event 3 of 120: Event ID 16# 39B2
I/O access error when transferring the process image to the output modules
P area, byte access, access address: 877
process image partition no.: 0
not user relevant(Z1): 2943
Requested OB: Program execution error OB (OB85)
Priority class: 25
External error, Incoming event
04:12:15.787 PM 03/08/2010


Event 4 of 120: Event ID 16# 39B1
I/O access error when updating the process image input table
P area, byte access, access address: 877
process image partition no.: 0
not user relevant(Z1): 2942
Requested OB: Program execution error OB (OB85)
Priority class: 25
External error, Incoming event
04:12:15.741 PM 03/08/2010


Event 5 of 120: Event ID 16# 39B2
I/O access error when transferring the process image to the output modules
P area, byte access, access address: 877
process image partition no.: 0
not user relevant(Z1): 2943
Requested OB: Program execution error OB (OB85)
Priority class: 25
External error, Incoming event
04:12:15.739 PM 03/08/2010