Not so obvious

  • Watch out for A/B D+/D- labeling issues. In the early days of RS-485 a group of chip manufacturers used the labels in the OPPOSITE sense to the standard. The world has never been the same again.
  • We use the terminology IH (Idle High) and IL (Idle Low) to unambiguously label the two signal wires. The correct label can be applied by simple observation of the signal on an oscilloscope or even via multimeter on a properly terminated network
  • Ensure that there is a “terminating” resistor to avoid signal reflections that is the same impedance as the characteristic impedance of the cable uses, and although this is normally 120 ohms, it could be 100 ohms or 130 ohms. Check your cable specification.
  • In addition, there should be bias resistors, one from the IL line to ground, and another from the IH signal to +V.  Yes! Termination should be powered!

Obvious

  • Terminate on each end of the bus
  • Use 3-wire, optically isolated devices if at all possible
  • Shield should be connected to site ground at one end only
Collection of BACnet MS/TP Best Practices