Bias Resistor
Bias Resistor
Figure 2.4 Transceiver with Bias Resistors
Example 1. 10 node, RS-485 network with two 120 Ω termination
resistors
Each RS-485 node has a load impedance of 12KΩ. 10 nodes in parallel
give a load of 1200 Ω. Additionally, the two 120 Ω termination resistors
result in another 60 Ω load, for a total load of 57 Ω. Clearly the termination
resistors are responsible for a majority of the loading. In order to maintain at
least 200mV between the B and A line, we need a bias current of 3.5 mA to
flow through the load. To create this bias from a 5V supply a total series
resistance of 1428 Ω or less is required. Subtract the 57 Ω that is already
part of the load, and we are left with 1371 Ω. Placing half of this value as a
pullup to 5V and half as a pulldown to ground gives a maximum bias resistor
value of 685Ω for each of the two biasing resistors.
18
RS-422/485 Application Note
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