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I have a question about shielding an ethernet cable. I have to go from point A to point B with a CAT5 or CAT6 ethernet cable, the cable is used for a RS422 link (two pairs). On the sides of the cable there are two DB9 type connectors.

The cable also has a shield, the question is, does the shield have to be connected to ground? If yes, should it be connected on both sides or only on one side of the cable?

Federico Massimi
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  • It depends on many things in your system which you don't mention. Why do you ask, i.e. what do you want to achieve with the shield, because the answer will depend on how the whole system will look like, not just the fact that there are two unknown systems connected with cable. What devices they are and do they have manuals how to install the RS-422 between them? – Justme Jul 19 '23 at 10:14
  • @jonathanjo that section 4.5, or the whole document, does not say a single word about how to connect a cable shield. – Justme Jul 19 '23 at 10:30
  • @justme Of course you're right about that ... I elaborated what I was thinking in an answer and deleted my comment. – jonathanjo Jul 19 '23 at 11:05

2 Answers2

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RS-422 and its close cousin RS-485 have a number of issues which look simple but really depend on the installation details.

In any case, make sure you solve any grounding questions before your shielding one.

From AN-1057 Ten Ways to Bulletproof RS-485 Interfaces
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You might be intersted in Bob Perrin's comments The Art and Science of RS-485, where he suggests that shielding is often unnecessary, and discusses the pros and cons.

For the basics, I can also recommend: RS-422 and RS-485 Standards Overview and System Configurations

jonathanjo
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  • So, based on the resources in the answer, the connection of the shield depends on application, and as the application is unknown, how to connect the shield is also unknown, until more information about the application is given. – Justme Jul 19 '23 at 11:44
  • @justme Rather than give a "it all depends" type answer, I thought it could help the OP to read something more discursive about what things the answer depends on. You can see from OP's comments elsewhere ("I was thinking of using the cable shield as ground between the two boards") that grounding and shielding are not as clearly separated concepts as they might be. – jonathanjo Jul 19 '23 at 11:55
  • Finally, a well thought out reply to CAT5/6 cable shielding. Pointing out that 'just connect both ends to ground' can make things much worse, especially when the equipment is distributed. – Chris Knudsen Jul 19 '23 at 12:08
  • Yes, but how to connect (or how not to connect) it may be defined elsewhere. While RS-422 (or RS-485) standard does not define the shielding or wiring in any way, different standards that are built on top of these can define the interface wiring in great detail. Many standards mandate that the shield must only be a shield, not a ground wire for currents between devices. – Justme Jul 19 '23 at 12:09
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There are multiple options which depend on how your system looks like.

In general, the shield of a cable is not generally intended for any other purpose than shielding, so it generally is not used as a ground conductor between two devices.

For example, if you have two fully isolated pieces of battery operated equipment, with both having metal chassis, then the shield may be connected to chassis on both ends of the cable and then it can be seen as an extension of the chassis and the two devices operate inside this metal bubble. As the metal case may not be bonded to system ground on the devices, there needs to be a common ground wire between devices where signal ground currents may flow, so that the return currents do not flow in the cable shield.

The scenario will be different if the two devices have e.g power supplies with mains inlet or other interfaces that connects the metal case or system ground to mains earth (e.g. USB to earthed desktop PC). To avoid ground loop currents from flowing in the cable shield, it is best to connect the shield only on one end. Even if the devices now get ground reference between them through other connections, it might be that in some cases they don't (one desktop PC replaced with battery operated laptop), the common ground return wire must still be between equipment. But at least the shield can't carry current, and it may be best to have direct shield connection on one device and to break the DC ground loop and allow AC signal return currents, the other device can capacitively couple the chassis to shield.

Justme
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