Practical BACnet: exam questions with worked answers

Questions for the "Practical" topic

1. Q01

   When an MS/TP segment is not working, the classic checks are: 120 ohm terminations at both ends, bias resistors (typically 680 ohm) installed on one device, a unique MAC per device, the same baud rate on every device, and correct cable polarity.

   TrueFalse

   22. Practical· MS/TP-Segment Troubleshooting· Hard

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   Correct answer

   True

   Learning tip

   A pragmatic MS/TP troubleshooting checklist runs through six items: (1) 120 ohm terminations present at both physical ends of the segment, (2) bias resistors fitted on a single device (typically the master) so that the idle line is correctly polarised, (3) unique MAC addresses with no duplicates, (4) identical baud rate on every device, (5) correct polarity (e.g. blue/white pairing respected), and (6) a 120 ohm-impedance twisted-pair cable. Around 80 % of MS/TP issues encountered in the field can be traced back to one of these six points.

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2. Q02

   The standard cable for BACnet MS/TP is an AWG 18-22 shielded twisted-pair with a 120 ohm characteristic impedance and capacitance below 30 pF/ft (98 pF/m), covering distances up to 1200 m at 9600 baud.

   TrueFalse

   22. Practical· MS/TP-Kabelspezifikation· Medium

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   Correct answer

   True

   Learning tip

   Typical MS/TP cable specifications are: AWG 18-22 conductors (18 is more robust, 22 is more economical), 120 ohm characteristic impedance (matching standard RS-485), capacitance no greater than 30 pF/ft (a higher value shortens the maximum reach), and either foil or braided shielding (foil is sufficient indoors). Common reference products include Belden 8761 / 9841 / 9842 and Liberty Cable 18AWG/120 ohm, priced at roughly 1-3 euro per metre depending on the brand.

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