Field reference
Short references for installation, commissioning, and troubleshooting.
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Protocols
BACnet
BACnet is the standard protocol for building automation. Objects (AI, AO, BI, BO, AV, BV, MSV) contain the point data. Devices communicate on BACnet/IP or MS/TP. If a device does not discover, examine the device instance numbers for conflicts first.
Modbus
Modbus is common on meters, VFDs, and small equipment. It operates as RTU (serial, RS-485) or TCP. Examine the baud rate, the parity, and the register offset (0-based or 1-based). An incorrect offset is the most common cause of bad values.
LonWorks
LonWorks is an older protocol, but it is common in legacy systems. Network variables (NVs) bind point-to-point. Make sure that the neuron ID and the bindings are correct before you replace a device.
Proprietary / OEM
Carrier, Johnson Controls, Siemens, Honeywell, and other manufacturers each have their own bus for native controllers. A supervisor usually connects these buses to BACnet.
Troubleshooting checklist
- Examine the physical layer firstExamine the power, the wiring polarity, the termination resistors, and the shield grounds. These cause more "software" problems than the software does.
- Confirm the point value at the deviceCompare the raw input or output at the controller with the operation of the field device. Do this before you examine the logic.
- Examine addresses and instances for conflictsDuplicate device instances or MAC addresses are a frequent cause of intermittent communication losses.
- Examine recent changesMany problems start after a program download, a point override, or a schedule change. Examine the audit trail first.
- Isolate the loopRecord trends of the process value, the setpoint, and the output together. The chart shows if the problem is in the tuning, the mechanical equipment, or the sensor.
Conversions
°F to °C: (°F − 32) × 5/9 · °C to °F: (°C × 9/5) + 32
1 psi = 27.7 in. W.C. · 1 in. W.C. = 0.0361 psi
CFM = Velocity (FPM) × Duct Area (sq ft)