Terminating the RS-485 LAN
All Challenger LAN devices (including the panel) use a 470 Ω LAN termination resistor where required. LAN termination resistors are used to set the impedance of the LAN to around 220 Ω in order to minimise noise. The termination resistor may be external or onboard (where a link or DIP switch is used to set the LAN termination to ON).
A Challenger LAN should have only two devices with the LAN termination set to ON:
•In a straight LAN configuration the termination is ON at the Challenger panel and the most distant device.
• In a star LAN configuration the termination is ON at the two devices that are the furthest apart (and OFF at the Challenger panel).
In a completely-connected (but powered down) system, you can check for correct LAN termination by measuring the resistance across the D+ and D– terminals:
• 0 Ω indicates a short circuit in the cabling
• 160 Ω or less indicates that three or more devices are terminated
• 220 Ω is good (two devices are terminated)
• 470 Ω or more indicates that less than two devices are terminated