RS-485 Shield Grounding
Improper shield grounding on RS-485 cable can cause ground loops, EMI pickup, and signal corruption. Correct grounding improves reliability on BACnet MS/TP trunks.
Common Causes
- Shield grounded at both ends (creates ground loop)
- Shield floating (no ground reference, poor noise rejection)
- Shield connected to equipment chassis at every tap
- Unshielded cable in noisy environments (VFDs, motors)
Fix Steps
- Use single-point grounding: connect shield at one end only (typically controller/router).
- At other end, tape or trim shield; do not connect to chassis or ground.
- If cable runs between buildings, use shielded cable and ground at one building only.
- For very long or noisy runs, consider shielded cable with drain wire to single ground.
- Verify no shield continuity between nodes that could form a loop.