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Case Analysis of interference of -PROFINET Communication

香港嘉玮科技有限公司   作者:SystemMaster 发布日期: 2016-05-30 15:49:17   点击率:810

I went to a customer site a few days ago to help solve a problem involving PROFINET and failsafe. In the scene accidentally found a PROFINET network interference phenomenon, here to share with you.

At that time, when checking the switch status information, found that there are switch statistics show that one of the ports will have an error message, such as the following figure of port 2, there will be error packet statistics, and the number will slowly increase. But the site connected to port 2 did not drop off, nor did it have any other problems, so the customer did not pay attention to the site. Therefore, at the beginning to the scene to see the phenomenon and did not have in-depth inspection, and so on to deal with customer concerns, before turning back to continue to deal with. 
Ethernet port statistics data show that there is an error message even if it is a CRC error. Generally, the data is disturbed in the transmission process, and a test error occurs. Although it does not drop the station, it is risky to the network. Upon inspection, the site connected by port 2 and the switch are in the same electronic control cabinet, the upper and lower two rows, the length of the network wire is not more than two meters, also using Siemens industrial Ethernet wire and metal connectors. 
It is normal to use a special tool to check the wire, and no obvious errors have been found in the wire and connector. For further investigation, a number of substitution experiments were carried out. 
Change the port on the switch, the fault is still there. 
Replace the cable, the fault is still. 
Replace the distributed IO site interface module, failure as before. 
Change the firmware of the switch, the fault is still the same. 
Later, with the help of multimeter detection, it was found that the peripheral metal part of the RJ45 interface of the distributed IO module is not connected with the installation backplane when not plugged into the network cable, and only when the Internet wire is plugged in, can the metal part be turned on. 
This indicates that the equipotential connection between the switch and the IO site is realized through the shield layer of the network wire. This will cause when the two ends of the line potential is not equal, there will be a current through the shield, and then cause data line interference, data error. But why the module RJ45 interface metal periphery punches the part and the backplane does not open, originally is installs the guideway to cause. A metal shrapnel is connected to the installation guide rail on the back of the interface template, and the metal shrapnel is conductive to the peripheral metal part of the RJ45 interface. But there is a coating on the installation guide, which is insulated, which is very difficult to see from the naked eye, thinking that it is a common aluminum rail. There does seem to be a scepticism about exclusion, even where it is taken for granted. 
In this case, although there will be an error message, it will not cause the station to drop. This is because the PNIO site has a watchdog time set for off-site decisions, and the default is to allow three lost PNIO cycles to be accepted, meaning that the watchdog time is three times the refresh time. This way, as long as it is not a continuous error packet, it will not cause the station to drop.


前些日子去某客户现场帮助解决一个涉及PROFINET和故障安全的问题。在现场意外发现一个PROFINET网络受干扰的现象,在这里与大家分享一下。

当时在检查交换机状态信息时,发现有交换机的统计信息显示其中某端口会有错误报文,如下图中端口2,就有错误报文统计,而且数量会缓慢增加。但是连接在端口2上的站点并没有掉站,也没有出过其它问题,因此客户也没有关注到相关站点。所以一开始到现场看到该现象时并没有深入检查,等处理完客户关心的问题后,才回过头来继续处理。

以太网端口统计数据显示有错误报文犹其是CRC错误,一般提示数据在传输过程中受到干扰而发生较验错误,尽管不掉站,但是对网络而言是有风险的。经检查,2号端口所连接的站点与该交换机就在同一电控柜内的上下两排,网线长度不超过两米,用的也是西门子的工业以太网线和金属接头

检查网线和接头并没有发现明显的错误,用专用工具检查网线,也是正常的。为了进一步排查,做了许多替换实验。

在交换机上更换端口,故障依旧。

更换网线,故障依旧。

更换分布式IO站点接口模块,故障依旧。

更换交换机的固件,故障依旧。

后来借助万用表检测,发现分布式IO模块RJ45接口的外围金属部分在不插网线的时候与安装背板是不导通的,只有当插上网线的时候才会导通。

这说明交换机和IO站点的等电位连接是通过网线的屏蔽层来实现的。这就造成当连线两端电位不等时,就会有电流从屏蔽层上流过,进而造成数据线受到干扰,数据出错。而为什么模块RJ45接口的金属外围冲部分与背板不导通,原来是安装导轨造成的。在接口模版的背面有金属弹片与安装导轨相连,该金属弹片与RJ45接口的外围金属部分是导通的。但是安装导轨上有涂层,是绝缘的,这从肉眼上很难看出来,以为是个普通的铝轨。对于排除问题来讲,看来还真是要有怀疑一切的态度,即使在那些认为理所当然的地方。

在这个案例中,尽管会有错误报文,但并不会造成掉站。这是因为PNIO站点的掉站判断都是有看门狗时间设定的,缺省值是允许接受丢失PNIO周期通信三次,即看门狗时间是刷新时间的三倍。这样只要不是连续的错误包,并不会造成掉站。