| ??? 06/30/06 17:36 Read: times |
#119466 - grounding differential signals Responding to: ???'s previous message |
Kai Klaas said:
But it seems, that the connecting of shield at only one end is from the old days. Seems, that connecting it at both ends to chassis ground (protection earth) tends to prevail.
So, the cable shield can be hard grounded (connected to safety ground, chassis) and the signal ground can be soft grounded. As it can be seen with PROFIBUS applications. Exactly. In a balanced system, the shield is only the shield, and not a signal conductor. This means that the shield is simply an extension of the enclosure. A balanced system (especially digital, like RS485) can deal with shield current due to ground differences because noise from the shield current is common mode (with proper wiring) and as such is cancelled by the receiver. (Ground currents in the shields are typically at mains frequency or a multiple so they're easily cancelled by the receiver.) Also, proper wiring means that shield noise is drained directly into the chassis and NOT into the circuit ground. Now, if the ground differences between the driver and receiver are so great that you have "Excessive" shield current, then you're probably going to exceed the receiver's input voltage range anyways. -a |
