A Field Guide / Written by a Working BAS Technician
Tridium's documentation tells you what Modbus is.
This guide tells you how to actually make it work.
Written by a working BAS technician through multiple real-world integrations — including a 5-boiler Weil-McLain plant running a master/shadow RS-485 architecture with bitmapped status registers and undocumented scaling quirks. Every technique in here was figured out on live equipment, not in a lab.
Tridium's own Modbus documentation is notoriously sparse on edge cases. The forums have fragments. This puts it together in one place — from wiring the bus to decoding a bitmapped register in BQL to understanding why your ExprLogic keeps returning null.
"If you're a BAS tech, controls contractor, or facilities engineer staring down a Modbus device and a blank Niagara station — this was written for you."
What's inside
Understanding your Modbus device, RTU vs TCP, RS-485 wiring fundamentals, getting a usable register map — and what to do when the manufacturer's map is wrong.
ModbusAsync vs ModbusClient — which one and when. Serial port configuration on the JACE. The failure points that kill every first connection.
Register types, the 0-vs-1-based address offset trap, data types, word order for 32-bit values, and scaled integers.
Full walkthrough of ExprLogic setup, the BStatusNumeric .value unwrap problem, floating point bit isolation, the if() string output bug, and complete working expressions from a real Weil-McLain integration.
No response, wrong values, intermittent comms — and a dedicated BQL debugging section for when everything returns null and Niagara won't tell you why.
Master/shadow boiler addressing in Niagara, actual register addresses for a 5-boiler Weil-McLain plant, documentation habits that pay off on the next job.
Complete mapped point tables for master and shadow boilers, bitmapped decode tables with working BQL expressions, verified on live equipment.
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