Specifications

Figure 14: FC Bus and SA Bus Terminal Block Wiring Details
Grounding the Bus Cable Shield
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 14. 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.
Important: Ensure that the cable shield is connected to hard ground at only one point on the bus segment
and is completely isolated from hard ground at all other points. Multiple hard ground connections on a
bus segment can create ground loop currents in the cable shield, resulting in poor bus performance
and frequent device offline occurrences.
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.
37MS/TP Communications Bus Technical Bulletin