Engineering Documentation

Chapter 2 – Network Electrical Systems
RS-485 MS/TP Communications
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Network Loading
The RS-485 specification allows 32 full load devices on a section of network cable
before a repeater is required. Most Siemens BACnet nodes are 1/8 load devices, so,
in theory, you could place 256 on a network section. Response times normally limit the
maximum number of devices on a network to lower values of around 96 devices.
The PXC Modular, PXC-36, and P1 BIM have 1/4 or 1/8 load interfaces, which would
allow for a maximum of 128 devices on a network section. These limits are strictly
electrical load limits, please check the network manager/next higher controller specs
for limits on the total number of addressable nodes on a network.
The network distance for a fully or partially loaded network is 4000 feet (1220 meters)
at a maximum network speed of 76.8K bps. Lower speeds do not mean longer
network sections are possible. The maximum network section is 4000 feet. Network
repeaters can be used to extend this distance.
To determine how many devices can be on a network section, add up all the loading
numbers and do not exceed 32. Many third-party devices have full load interfaces.
Check the manufacturer’s literature for network loading information.
Network Cable Sharing and Distances from Higher Power Cables.
Network cable installed environment
Never run network cabling closer than 5 feet to a Variable Frequency or Variable Speed Drive except at
the point where the network must connect to the VFD/VSD. Network entry into a VFD must be through
separate conduit and all network wiring must be kept as far as possible for high power cabling in the drive.
Never run network cable closer than 5 feet from circuits carrying 100KVa or greater. Always cross high
power cables (at a distance of 5 feet) at a 90 degree angle.
Network run in open cable trays with circuits carrying over 20 amps should be no closer than 26 inches to
the higher power cables
Network run in enclosed trays with circuits carrying over 20 amps should be no closer than 18 inches to
the higher power cables.