Xtralis VESDA VFT-15-C, Руководство по эксплуатации продукта

Страница: 66 / 70

VESDA VFT-15 Product Guide
VESDA by Xtralis
60 www.xtralis.com
D.4.2 Cable Material
RS485 communication is established by current switching between a pair of wires at data rates of up to 250
kbps.
The cable construction is specified as:
l
Twisted pair of nominal 10 turns per meter or yard
l
40 pF to 60 pF meter between conductors
l
100 Ω to 120 Ω characteristic impedance
l
Maximum signal attenuation over total length 16 dB at 0.25 MHz
l
Overall screen (to be connected at one end only)
The cable materials are not specified, but in order to meet the low capacity specification
polyethylene/polyolefin/polypropylene is used as the insulator, with PVC/flame retardant PVC as the overall
sheath. Other materials can be used if the impedance and capacity requirements are met.
Installation requirements may add other features, such as fire resistance, flame retardance, UL approval, low
smoke and fume etc.
Also remember to check that the electrical characteristics of the cable used is appropriate for the wiring
distance required for the installation.
Common Mode
The RS485 inputs/outputs are not usually isolated from the equipment internal 0 V line. The maximum
potential difference between the 0 V lines of all instruments connected to an RS485 communication cable is
limited by the specification to between -7 V and +12 V. If this limit is exceeded, communications will be lost.
Interference Induced onto Cable
Twisted pair construction and cable screens do not completely shield the communication link from external
electromagnetic interference (EMI). Good practice should be followed when installing communication cables
to eliminate EMI.
RS485 cables should not be installed in the same duct/tray as power cables, or switching cables. When
RS485 cables cross power or switching cables, this should be at right angles. RS485 should not be installed
in areas of high interference, where this is not possible, then electro-magnetic screening must be adopted for
example, metal conduit or cable tray could be used.
The total length of the cable run must not exceed 1200 m (4000 ft). It is only possible to exceed this length by
use of repeater units to re-generate the signal.
Terminating Cables
It is imperative that the cable structure is maintained to a position as close as possible to the termination. This
is to maintain the interference protection and reduce losses due to changes in cable impedance.
The outer sheath and screen must not be cut-back for a distance of more than 100 mm (4 in.) from the
termination.
The twisted pair must not be untwisted for a distance of more than 25 mm (1 in.) from the termination.
The recommended cable manufactured to RS485 specifications is: BELDEN 9841/9842 (or equivalent) - 24
AWG, polyethylene insulation, PVC sheath, UL style number 2919.
l
9841 Single twisted pair: communications data only
l
9842 Double twisted pair: 1 pair for communications data, 1 pair for 24 V supply.
Ensure that electrical characteristics are sufficient for the distance of connection between devices.