Honeywell HC900 User And Installation Manual page 167

If the RS-485 network to slave devices is intermittent or fails completely over short durations and recovers
after the controller is power cycled, use an external isolator with additional port biasing. See Figure 81.
Note: Legacy Controllers do not provide biasing.
.
For XYR 5000 base stations, if the RS-485 network is intermittent or fails completely over short durations
and recovers after the controller is power cycled, the need for additional port biasing may be required.
Figure 82 shows recommended installation procedure for using an external isolator and additional port bias.
Revision 6
21 April 2017
Figure 81 - RS-485 Modbus slave wiring with isolation
HC900 Process & Safety Controller User and Installation Manual 153