For improved electrical noise protection, it is recommended that the commons (M) of the different power supplies be connected.我的翻译是：作为进一步的电气干扰防护，推荐使用不同的电源共同连接的方式。
Connecting an external 24 VDC power supply in parallel with the S7-200 24 VDC sensor supply can result in a conflict between the two supplies as each seeks to establish its own preferred output voltage level.我的翻译是：将一个外部的24 VDC电源与S7-200 24 VDC 检测用电源并联，可能在两个电源之间导致因根据各个请求来确定自己首选的输出电平而产生的冲突。以上翻译，不知对错，请高手指正。