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Common tests, results, and potential root causes of issues on BACnet MSTP bus.

 

1. Check Resistance (Set to Ohms)

Common Issues

 O

 

Shield

Normal Reading

0 Ohms

Greater than 3 Ohms: Shield is not grounded.

 O

 

Earth Ground

Less than 3 Ohms when ground is removed: You are grounded in more than one place. Find the extra ground and eliminate it. Only ground once on your network.

 


2. Check DC Voltage | A- and B+ (Set to DC)

Common Issues

 O

A-

Normal Reading

200 MV or 0.2 VDC

Less than 200 MV: A- and B+ are shorted on network.

 O

 

B+

Greater than 500 MV: Either 1) A- and B+ are not connected on one side of the network; or 2) an end of line (EOL) switch is not turned on.

 


3. Check DC Voltage | B+ and Earth GND (Set to DC)

Common Issues

 O

Earth Ground

Normal Reading 2.0 - 2.5 VDC

Less than 700 MV: B+ is shorted to the shield, potentially by a stretched or crushed wire.

 O

B+

Greater than 3 VDC: A NetSensor (SSP or ZS) is shorted on the B+ side.

 


4. Check DC Voltage | A- and Earth GND (Set to DC)

Common Issues

 O

Earth Ground

Normal Reading 2.0 - 2.5 VDC

Less than 700 MV: A- is shorted to the shield, potentially by a stretched or crushed wire.

O 

A-

Greater than 3 VDC: A NetSensor (SSP or ZS) is shorted on the A- side

 


5. Check AC Voltage | B+ and Earth GND (Set to AC)

Common Issues

 O

B+

Normal Reading

Less than 2.0 VAC

Greater than 2 VAC: Some source of voltage is shorted to the COMM.

 O

Earth Ground

Around 24 VAC: A controller is mis-phased.

 


6. Check AC Voltage | A- and Earth GND (Set to AC)

Common Issues

 O

A-

Normal Reading

Less than 2.0 VAC

Greater than 2 VAC: Some source of voltage is shorted to the COMM.

 O

Earth Ground

Around 24 VAC: A controller is mis-phased.