RS-485 Resistor Calculator
Using equations from
RS-485: Passive failsafe for an idle bus
by Thomas Kugelstadt, Senior Applications Engineer at Texas Instruments.
Step 1: Calculate R
_{FS}
Supply voltage:
V
Tolerance:
5%
10%
Noise margin:
mV
R
_{T1}
:
For proper termination, use 100 ohm for Cat5 cable, 120 ohm for standard RS-485 cable. If your cable is short enough so you don't care about reflections, you can try higher values.
Calculate R
_{FS}
Step 2: Calculate R
_{T2}
R
_{FS}
:
Calculate R
_{T2}
Step 3: Determine max. number of devices
R
_{T2}
:
Calculate