Field reference
Quick-hit references for install, commissioning, and troubleshooting. No fluff.
Protocols
BACnet
Standard for building automation. Objects (AI, AO, BI, BO, AV, BV, MSV) carry points; devices communicate over BACnet/IP or MS/TP. Check device instance numbers for conflicts first when a device won't discover.
Modbus
Common on meters, VFDs, and smaller equipment. RTU (serial, RS-485) or TCP. Watch baud rate, parity, and register offset (0 vs. 1 based) - the most common source of bad readings.
LonWorks
Older but still common in legacy retrofits. Network variables (NVs) bind point-to-point. Confirm neuron ID and binding before assuming a device is dead.
Proprietary / OEM
Carrier, Johnson, Siemens, Honeywell, and others each have their own bus for native controllers, usually bridged to BACnet at the supervisor.
Troubleshooting checklist
- Physical layer firstPower, wiring polarity, termination resistors, and shield grounding cause more "software" problems than software does.
- Confirm the point value at the deviceBefore chasing logic, verify the raw input/output at the controller matches what the field device is actually doing.
- Check for address or instance conflictsDuplicate device instances or MAC addresses are a classic cause of intermittent dropouts.
- Review recent changesMost issues trace back to a recent program download, point override, or schedule edit - check the audit trail before anything else.
- Isolate the loopTrend the PV, setpoint, and output together to see if it's a tuning issue, a mechanical issue, or a sensor issue.
Handy 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)