有这样一系统：s7400的系统，用ET200M,走PROFIBUS总线，以前都是用的好好的，最近老是出现某一个站（12号站）通讯不正常，老是一会能通讯，一会又不能通讯，对dp头，et200m和通讯缆也都更换过的了，还是会出现这样的问题，有时候能又能正常几个小时。之后也试过重启cpu，重新下载过组态，把12号站跟11号站的dp头和通讯缆调换，原来是11号站的dp头是终端电阻的，现换成12号站的dp头，出现的情况是又能用几个小时，之后还是出现一 会能通讯，一会不能通信的问题，我把12号站的dp头的开关打到off的位置，再打回on，又能正常一段时间。
如果出现通讯不行的话，cpu上的报BUS2错误和外部错误
请教各位大侠，这个问题是怎么引起的，怎么才能解决这个问题？


其中硬件诊断的信息节选如下：Diagnostic buffer of module CPU 414-2 DP
Order No./ Des cription Component Version
6ES7 414-2XG04-0AB0 Hardware 2
- - - Firmware V 4.1.0
Rack: 0
Slot: 3
Event 1 of 120: Event ID 16# 38C4
Distributed I/Os: station return
Address of the affected DP slave: station number: 12
DP master system ID: 1
Log. base address of the DP slave: Input address: 8185
Log. base address of the DP master: 8191
Requested OB: Rack failure OB (OB86)
Priority class: 25
External error, Outgoing event
12:39:09.641 AM 01/13/2010
Event 2 of 120: Event ID 16# 2943
I/O access error, writing
P area, word access, Access address: 640
FB number: 31
Module address: 3426
Requested OB: I/O access error OB (OB122)
Priority class: 1
External error, Incoming event
12:39:09.637 AM 01/13/2010
Event 3 of 120: Event ID 16# 2943
I/O access error, writing
P area, word access, Access address: 642
FB number: 31
Module address: 3322
Requested OB: I/O access error OB (OB122)
Priority class: 1
External error, Incoming event
12:39:09.637 AM 01/13/2010
Event 4 of 120: Event ID 16# 2943
I/O access error, writing
P area, word access, Access address: 644
FB number: 31
Module address: 3218
Requested OB: I/O access error OB (OB122)
Priority class: 1
External error, Incoming event
12:39:09.637 AM 01/13/2010
Event 5 of 120: Event ID 16# 2943
I/O access error, writing
P area, word access, Access address: 646
FB number: 31
Module address: 2594
Requested OB: I/O access error OB (OB122)
Priority class: 1
External error, Incoming event
12:39:09.637 AM 01/13/2010
Event 6 of 120: Event ID 16# 2943
I/O access error, writing
P area, word access, Access address: 648
FB number: 31
Module address: 2490
Requested OB: I/O access error OB (OB122)
Priority class: 1
External error, Incoming event
12:39:09.637 AM 01/13/2010
Event 7 of 120: Event ID 16# 2943
I/O access error, writing
P area, word access, Access address: 638
FB number: 31
Module address: 2386
Requested OB: I/O access error OB (OB122)
Priority class: 1
External error, Incoming event
12:39:09.637 AM 01/13/2010
Event 8 of 120: Event ID 16# 2943
I/O access error, writing
P area, word access, Access address: 634
FB number: 31
Module address: 2282
Requested OB: I/O access error OB (OB122)
Priority class: 1
External error, Incoming event
12:39:09.636 AM 01/13/2010