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Operating and Maintenance Instructions
5.6 Modbus relay operating mode
The MDC can be installed on an RS485 Modbus network and configured to act as a relay for a BMS or another
Modbus master device to access a subnetwork of fans as on the example shown below:
Figure 18 – MDCs operating as Modbus Relays connected to a BMS network and sub-network of fans
In Modbus Relay mode, the MDC ceases all control and monitoring operations and becomes a relay for messages
and commands between a Modbus master and each fan within the fan array. See H10 – Operation Mode - Modbus
Relay for how to enter Modbus Relay mode.
This operating mode allows full access to the Modbus registers inside each fan according to the ebm-papst
parameter specification for Modbus enabled motors version 5. A copy of this parameter specification is available
on request by e-mail at info@uk.ebmpapst.com.
In Modbus Relay Mode, it is possible to set a different speed setpoint for each fan, and the BMS (or another Modbus
Master device) is able to read/write more than 6 Modbus registers at a time.
See Appendix G – Example of Modbus Relay Operation for a working example of the Modbus Relay operation.
NOTE: The MDC re-maps all individual fan Modbus registers e.g. fan 1 holding register D000 is MDC's holding
register 200 (hex), fan 2 holding register D000 is MDC's holding register 400 (hex) and so on.
210-OMI14229-Iss2
© ebm-papst UK Ltd 2018
See Appendix E – Modbus Holding Registers for more details.
Operation & Maintenance Instructions
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