Modbus to BACnet gateways: connecting Modbus devices to a BACnet BMS

Most new building management systems (BMS) use BACnet/IP, while many devices in plant rooms still speak Modbus: energy meters, fan coil controllers, lighting modules, heat pumps and remote I/O. A Modbus to BACnet gateway closes that gap without a custom driver in the BMS. This article covers how the gateway works, register mapping, BACnet/IP versus MS/TP, and which ICP DAS gateways and BACnet/IP I/O modules fit which direction of data flow.

How a Modbus to BACnet gateway works

The gateway has two roles. Towards the BMS it is a BACnet server: one BACnet device with a list of objects that the BMS discovers and reads. Towards the field it is a Modbus master (RTU) or client (TCP) that polls the configured registers of each device in a cycle and stores the latest values in the matching objects. When the BMS writes to an output or setpoint object, the gateway passes the value on as a Modbus write.

Every point has to be configured in the gateway, and values are only as fresh as the polling cycle, so keep the point list on a slow RS-485 segment to what is actually used. Some projects need the opposite direction, where a Modbus-only SCADA system reads BACnet controllers. ICP DAS has gateways for both cases.

Mapping Modbus registers to BACnet objects

A consistent mapping keeps the BMS view readable:

For each register, check:

• Addressing: manuals mix 1-based notation (40001) and 0-based offsets. An off-by-one error gives plausible but wrong values.
• Data type: signed or unsigned 16-bit, or 32-bit values over two registers with the correct word order.
• Scaling and units: many devices send 215 for 21.5 °C. Decide where scaling happens and set the engineering unit on the object.
• Names and instances: use object names the operator understands, and a unique BACnet device instance per gateway.

The GW-2492M and GW-2493M offer simple data translation between the protocols. Check that it covers the conversions your registers need.

BACnet/IP and BACnet MS/TP

BACnet/IP runs over Ethernet and UDP, by default on port 47808. Device discovery uses broadcasts, which stay inside one IP subnet. Across subnets, a BACnet Broadcast Management Device (BBMD) forwards them, and devices elsewhere can register with it as foreign devices.

BACnet MS/TP runs on an RS-485 twisted pair with token passing between masters. It is common for room and VAV controllers at field level: cheaper to wire than Ethernet, but much slower. All devices on a segment use the same baud rate, and both ends need termination.

When a Modbus TCP system has to reach MS/TP devices, the GW-2139M acts as MS/TP master on one side and Modbus TCP server on the other. It supports 9600, 19200, 38400 and 76800 baud and has a built-in 120 Ω terminating resistor enabled by jumper.

Worked example: one RS-485 segment in a plant room

A plant room has six SC-6104-W5 fan coil controllers, four LC-101H lighting control modules, two PM-3133 power meters (Modbus RTU version) and two M-7015P-G RTD input modules, all on one RS-485 segment. The BMS uses BACnet/IP, so one GW-2492M goes into the plant room panel.

The GW-2492M polls up to 32 Modbus RTU stations and transfers up to 180 points each of DI, DO, AI and AO, leaving room for additions. Set the same baud rate and data format on all 14 devices, give each a unique Modbus address, and check every object with a BACnet browser.

The BIBBs listed for the GW-2492M cover ReadProperty(Multiple) and WriteProperty(Multiple) but not change of value (DS-COV-B), so plan for the BMS to poll, preferably with ReadPropertyMultiple.

ICP DAS BACnet gateways and BACnet/IP I/O

All four gateways support the object types AI, AO, AV, BI, BO, BV, MSI, MSO and MSV, mount on a DIN rail, consume 5 W at 24 VDC and operate from -25 to +75 °C. The GW-2492M and GW-2493M are configured in a web browser, the GW-2139M with an ICP DAS utility.

The GW-2439M lets a Modbus master read and write object values in BACnet/IP controllers. It receives messages forwarded by a BBMD, and its utility scans for BACnet servers and generates the Modbus mapping table.

For new hardwired signals, the BNET-2310M connects directly to the BMS: analog inputs from ±1 V to ±10 V at 16-bit, analog outputs up to 0–10 V or ±10 V, dry-contact inputs and open-collector outputs. It can register with a BBMD as a foreign device, and its BIBB list includes DS-COV-B.

Checklist before you order

• Direction: BMS reads Modbus devices (GW-2492M, GW-2493M), or a Modbus master reads BACnet devices (GW-2139M, GW-2439M)
• Register list per device with data type, scaling and access
• Stations per gateway and points per type
• IP subnets and BBMD use

Send your device list and BMS requirements to our technical sales team for a gateway proposal.