The TERM link, switch, or resistor (470Ω), must be on for the first and last devices on the LAN. In a star wiring configuration the TERM link, switch, or resistor, is on for only the devices at the ends of the two longest LAN cable runs. The LAN impedance is 235Ώ. If a LAN is not terminated properly, you can get random devices dropping online and offline frequently and getting weird messages or alarms coming through i.e. Inputs that are not programmed going into alarm, etc.
How to measure
With power disconnected, measure the resistance across the data pair (D+, D-). The ideal range is between 210Ώ and 260Ώ.
Refer to each product’s Installation Guide for further details.