Figure 77 - Rs-485 Modbus Slave Wiring With Isolation - Honeywell HC900 Installation And User Manual

Communications Installation - Connecting the HC900 Controller to Modbus device(s)
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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.
Revision 19
06/14

Figure 77 - RS-485 Modbus slave wiring with isolation

HC900 Process Controller Installation and User Guide 143