InteliLite4 AMF20 Global Guide
57
On board RS485 description
Balancing resistors
The transmission bus into the RS485 port enters an indeterminate state when it is not being transmitted to.
This indeterminate state can cause the receivers to receive invalid data bits from the noise picked up on the
cable. To prevent these data bits, force the transmission line into a known state. By installing two 620Ω
balancing resistors at one node on the transmission line, a voltage divider is created that forces the voltage
between the differential pair to be less than 200mV, which is the threshold for the receiver. Install these
resistors on only one node. The figure below shows a transmission line using bias resistors. Balancing
resistors are placed directly on the PCB of controller. Use the jumpers PULL UP/PULL DOWN to connect the
balancing resistors.
Image 4.55 Balancing resistors
Image 4.56 RS485 on board
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