FB3000 RTU Instruction Manual
D301851X012
November 2023
62 I/O Configuration and Wiring
3.2.2 Multi-Drop Mode
In multi-drop mode, only the HART digital signaling (FSK – Frequency Shift Keying) is
present and no 4-20mA analog current communication is permitted since the current is
fixed at 4mA. Each device must be set in multidrop mode, drawing a maximum of 4mA to
support HART communications. Each device can support a maximum 4mA bias current.
Because the maximum rated output (sink) current of any 3HRT04 Module channel is
20mA, and each device can draw 4mA, a HART channel in multidrop mode supports a
total of 5 HART devices. With each of the 5 HART devices permitted to draw 4mA, the sum
of the currents is 20mA – the maximum current.
Figure 3-12 shows how to wire a HART channel for multi-drop mode. Multi-drop mode only
supports inputs, you cannot have multi-drop outputs.
You can wire each channel the same way to allow for up to 20 HART devices (5 per
channel).
Figure 3-12. HART Wiring – Multi-Drop Mode on Channel 1
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