Xtralis ICAM Communications Guide
Xtralis ICAM
10
www.xtralis.com
5.3 RS485 Network Setup
Power Supplies
Use galvanically isolated power supplies (in accordance with UL1481) since the RS485 inputs/outputs
are not usually isolated from the equipment internal zero volt line.
The RS232/RS485 converter should also be galvanically isolated.
Cabling
RS485 communication is by current switching between a pair of wires at data rates of up to 250 kbps.
The cable construction is specified as:
•
Twisted pair of nominal 10 turns per metre/yard
•
40 pF to 60 pF metre between conductors
•
100
Ω
to 120
Ω
characteristic impedance
•
Maximum signal attenuation over total length 16 dB at 0.25 MHz
•
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.
Termination
Terminate the network with 120
Ω
resister at both the start and end of the network as shown in the
following diagram.
Figure 5-1: Termination of 485 network
