Communication · wired
RS-485 and Modbus RTU
When meters need to connect directly to a controller, gateway or building automation system, RS-485 with Modbus RTU is the most direct route. Both the H2O and the DELTA III are available with this interface.
How it works
Line, termination, request and response
- M-Bus: one cable, many metersThe master supplies the two-wire line with about 40 V. All meters are connected in parallel; polarity does not matter.
- M-Bus: readingThe master selects a meter by its secondary address. Only that meter replies, by modulating its current draw.
- RS-485: line with terminationA twisted pair A/B runs as a line from device to device, with 120 Ω at both ends. RS-485 is the physical interface.
- Modbus RTU: request and responseThe protocol layer: the master polls a slave address (1–247) with a function code; the meter replies with a CRC-16 checksum.
- LoRa: radio to the gatewayThe meter sends short uplinks on 868 MHz. Every gateway in range receives them, with no cable to the metering point.
- LoRa: server and applicationGateways forward the data to the network server, which passes it on to the application. Range and transmission interval limit data volume and timeliness.
What is RS-485, what is Modbus RTU?
RS-485 only defines the electrical layer: data is transmitted as a voltage difference between two twisted wires, A and B. This makes the line robust against interference and allows long distances. RS-485 does not define a protocol.
Modbus RTU is the protocol that runs on it. A master, such as a gateway or controller, polls the slaves in turn. Each meter has a slave address from 1 to 247. The response contains the requested registers and a CRC-16 checksum. Telegrams are separated by a pause of at least 3.5 character times.
Topology and termination
RS-485 is wired as a line, from device to device, with stubs kept as short as possible. A 120 Ω termination resistor is fitted at each end of the line. Missing or surplus terminations are the most common cause of transmission errors. A common reference conductor (GND) and a screen improve noise immunity.
Typical function codes
Meters are usually read with function code 03 (read holding registers) or 04 (read input registers). Which values are held in which registers is defined in the register list for each meter.
Commissioning checklist
- Set all devices to the same baud rate and parity
- Use each slave address only once
- 120 Ω at both ends, not in between
- Connect A and B consistently throughout
- Give the register list to the system integrator
The register list for the H2O and DELTA III is available on request via the contact form.
System design
Three typical routes into building automation
Directly to the controller
The controller or DDC unit is the Modbus master and reads the meters itself. Suitable for a small number of meters in one plant room.
Via a gateway
A gateway converts Modbus RTU to Modbus TCP or BACnet IP. The EMU M-Center reads Modbus devices via two RS-485 interfaces and passes the values on via Modbus TCP, BACnet IP, MQTT or CSV.
Combined with M-Bus
The DELTA III is also available with both M-Bus and RS-485 (ABMM), so one meter serves billing and control in parallel.
H2O with RS-485
DN15 to DN100 and DN150, suffix ABMO, for example WACI.20C.ABMO for DN25.
DELTA III with RS-485
DN15 to DN600, suffix ABMO. The communication card can be replaced later.
Questions about RS-485 and Modbus
Is RS-485 the same as Modbus?
No. RS-485 is the electrical interface, Modbus RTU the protocol. Other protocols can also run over RS-485.
Where do the 120 Ω resistors go?
At the two physical ends of the bus line, not at every device.
Can I convert Modbus RTU to Modbus TCP?
Yes, with a gateway. The EMU M-Center reads Modbus RTU devices and provides the values via Modbus TCP and BACnet IP, among others.