Inductive interference and Radio Frequency (RF) interference can adversely affect MS/TP applications, causing poor bus performance and frequent device offline occurrences. Experience has shown that installing a properly grounded shielded bus cable in MS/TP applications greatly reduces the impact of ambient inductive noise and RF interference. Applications installed without shielded cable are much less tolerant to ambient interference.
We recommend installing MS/TP bus applications using shielded cable. In applications using shielded cable, it is very important to ground the cable shield properly. Improper shield grounding can also result in poor bus performance and frequent device offline occurrences.
To properly ground the cable shield on an MS/TP application, the cable shields on each bus segment must be connected in a daisy-chain as shown in Figure 1. Each daisy-chained segment must be connected at one point (only) to a hard ground connection. We recommend connecting the cable shield to hard ground close to the bus supervisor’s bus terminations. In metal panel or enclosure applications, connect the cable shield to ground where the bus cable enters the panel or enclosure that contains the bus supervisor. On bus segments without a bus supervisor, the best practice is to connect the cable shield to hard ground at a bus device near the middle of the bus segment.
In certain environments with high ambient inductive interference or strong radio frequency transmissions, an MS/TP application may require the addition of soft ground connections along the bus segments to enhance bus performance and reduce device offline occurrences, or possible device damage.
Examples of potential inductive interference include large motors, contactors, relays, welding equipment, high-voltage conductors that are not in metal conduit or raceways, other high wattage devices within 10 m (30 ft) of the bus cable, and areas of frequent lightning.
Examples of potential radio frequency interference include locations near airports, hospitals, radio or television transmission towers, police and fire stations, or factories. Mobile transmitters in police, fire, emergency, and fleet vehicles are also potential sources of radio frequency interference.
Soft ground connections should be made within 2 inches of the bus terminations of any bus device that experiences frequent offline occurrences resulting from high ambient inductive or RF interference (Figure 1).