Third party device does not provide a place to terminate the shield on the comm bus cable.
Comm cable shielding is required for MSTP and Infinet buses but a controller does not provide a termination point for the shield.
- The shield on a comm cable works something like a drain, by redirecting any electrical interference to ground. rather than allowing it to degrade the data signal.
- If there is more than one grounding point on a shield conductor, often there is the potential that the two ground points have difference of electric potential.
- Such a situation could cause the shield to actually introduce electrical interference as the current passes from the point of greatest potential, through the cable and to the point of lowest potential.
- For this reason a shield should always be grounded on one end but never be grounded at both ends.
- These days many field bus controllers include non grounded shield terminations that simply serve to connect all shield wires as one continuous conductor that are drained by the master controllers shield termination point which is grounded.
- Where there is no shield termination, simply make sure that the shields from two comm cables are connected together.