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3.4.5 Multidrop using RS485 / RS422 cable
Terminating resistors T
R
each have a typical value of 120Ω
MintDrive X7
Master
Controller
GND
GND
T
R
MintDrive X7
RX-
DGND
RX+
TX+
TX- 6RX-
5DGND
1RX+
4TX+
9TX-
6RX-
5DGND
1RX+
4TX+
9TX-
Twisted pairs
T
R
Figure 24 - 4-wire RS485 multi-drop connections
The MintDrive at the furthest end of the network from the transmitter should have
its RS485 DIP switch (located on the front panel of the MintDrive) set to the ’On’
position. This will connect a termination resistor, used to match the impedance of
the load to the impedance of the transmission line (cable).
Each TX/RX network requires a termination resistor at the final RX connnection, but intermediate
devices must not be fitted with termination resistors. (An exception to this rule is where repeaters are
being used which may correctly contain termination resistors).
Unmatched impedance causes the transmitted signal to not be fully absorbed by the load. This causes
a portion of the signal to be reflected back into the transmission line (noise). If the Source impedance,
Transmission Line impedance, and Load impedance are all equal, these reflections (noise) are
eliminated. Termination does increase load current and sometimes changes the bias requirements and
increases the complexity of the system.
ON