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Operating and Maintenance Instructions
Appendix G – Example of Modbus Relay Operation
Initial Configuration Steps
•
BMS Network has slave addresses 1 and 2 available i.e., not taken by any other device
•
Both MDCs have successfully addressed all their respective fans
•
Both MDCs have changed their operating mode to "Monitor & Control" (the control mode is not important)
o See H8 – Operation Mode - Monitor & Control
o This step is purely taken to set all fans to "Modbus" speed control as opposed to their factory
settings of "Analogue 0-10V" speed control
•
MDC1 Modbus Slave address is kept at its factory setting value of "1"
•
MDC2 Modbus Slave address has been changed to "2", see H20 – Advanced Settings – BMS Settings
•
Both MDCs have changed their operating mode to "Modbus Relay", see H11 – Operation Mode - Modbus
Relay
Example of setting the speed of Fan 4 (at Fan Array 1) to 100%: BMS writes "65535" to Holding Register
Address 801 (hex) of MDC1 (Modbus Slave Address 1). This is Holding Register D001 of Fan 4 of MDC1.
Example of reading the Motor Status Register of Fan 3 (at Fan Array 2): BMS reads input register address
40B (hex) of MDC 2 (Modbus Slave Address 2). This is Input Register D011 of Fan 3 of MDC2.
NOTE 1: Do not to read/write more than 9 consecutive "retrieved from fan" registers at a time,
NOTE 2: The controller will respond to the BMS after the communication with the fan has completed.
A Modbus Exception Response will be returned to the BMS if the read/write to the fan has failed or the
210-OMI14229-Iss8.1
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otherwise a Modbus Exception Response will occur.
fan has no comms.
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