In favour
- Control systems and automation speak Modbus anyway
- Robust against electrical interference in industrial surroundings
- Direct register access, including for instantaneous values
- No IT infrastructure needed in the field
Products/ EMU Professional II/ RS-485 / Modbus RTU
Interface · EMU Professional II
RS-485 and Modbus RTU on the EMU Professional II
The differential two-wire line is at home in industry and forgives electrical interference – but not topology mistakes. This page explains what distinguishes RS-485 from Modbus RTU, what matters when building the bus, and what two RS-485 interfaces on the M-Center give you.
Two things that are often confused
What is the difference between RS-485 and Modbus RTU?
RS-485 describes only the electrical layer: a differential two-wire line, usually labelled A and B, in half-duplex operation. Differential means the signal is transmitted as the voltage difference between the two conductors. Interference hits both conductors equally and cancels out when the difference is taken – which is why RS-485 is so widespread in industry.
Modbus RTU is the protocol that runs over that line. A master asks, the slaves answer; only one may speak at a time. Each slave has an address from 1 to 247, every message ends with a CRC16 checksum, and a gap of at least 3.5 character times between two telegrams marks the frame.
Data is read from 16-bit registers, usually with function code 3 (holding register) or 4 (input register). Larger values such as energy readings are spread across several registers, as a 32-bit integer or floating-point number. Which value sits at which register address, and in which byte order, is listed in the register table of the manual – the part you actually need during setup.
Terms
A / B
The two signal conductors. Unlike with M-Bus, the assignment matters here – if they are swapped, the bus will not work.
Half duplex
Transmitting and receiving over the same pair of conductors, so never at the same time.
Termination resistor
120 Ω at both ends of the bus against reflections. Only at the ends, not on every device.
Unit load
Load measure of a node. Classically a segment tolerates 32 unit loads; modern drivers draw fractions of that.
Slave address
1 to 247, set on the device. Each address may occur only once per segment.
Silent interval
A gap of 3.5 character times that marks the start and end of a telegram.
Step by step
How does a Modbus RTU query over RS-485 work?
Using an EMU M-Center as master with an EMU Professional II and the Helvatron H2O and DELTA III meters as an example: bus layout, telegram, response and typical mistakes.
- One line, two endsThe meters are connected to one continuous line with short stubs. Each end has a 120 Ω termination resistor.
- Differential signalThe signal is the voltage difference between A and B. Interference hits both wires equally and cancels out.
- Master requestThe M-Center sends the slave address, function code 3 or 4, the register and a CRC16 checksum.
- Pause as a frameA pause of at least 3.5 character times marks the start and end of a telegram.
- Slave responseOnly the addressed meter responds, with the register values and its own CRC. Only one device talks at a time.
- Typical mistakesEach address from 1 to 247 only once, never swap A and B, 120 Ω only at the two ends.
- Passing on the dataThe M-Center passes the values on simultaneously via MQTT, BACnet IP, Modbus TCP or CSV.
Building the bus
What matters when building an RS-485 bus?
A line, not stars
RS-485 wants a continuous line from one end to the other, with the shortest possible stubs to the devices. Star wiring is the most common reason for buses that run at a low baud rate and no longer at a high one. Anyone coming from M-Bus, where stars are expressly allowed, regularly falls into this trap.
Termination at exactly two points
120 Ω belongs at the two physical ends of the line. A third termination resistor somewhere in the middle loads the bus without improving anything. With no termination at all, reflections arise that show up as sporadic CRC errors – the bus runs, but not reliably.
Baud rate against length
The longer the line, the lower the possible baud rate. As a rule of thumb, the product of length and baud rate is constant; in building practice 9600 or 19200 Bd over a few hundred metres is uncritical, whereas 115200 Bd runs cleanly only over short distances.
Ground and screen
A common reference between the nodes prevents the common-mode voltage from running out of the permissible range. The screen is bonded at one end only, otherwise equalising currents arise.
Typical mistake number one
A slave address assigned twice. Two devices answer at the same time, the master sees only CRC errors and reports both as unreachable. Keep an address list before commissioning.
Typical mistake number two
A and B swapped. Unlike M-Bus, RS-485 is not polarity-independent. A single device connected the wrong way round can disturb the whole segment.
Check before commissioning
- Line topology, short stubs
- 120 Ω at exactly both ends
- Addresses unique and documented
- Baud rate, parity and stop bits identical on all devices
- Screen bonded at one end
Use case
How do the Professional II and the M-Center work together?
The EMU M-Center has two RS-485 interfaces and reads up to 64 Modbus RTU devices over them. Two separate interfaces are more than double the capacity: they let you keep two segments with different baud rates or different device manufacturers cleanly apart, instead of forcing everything onto one line.
The M-Center takes the most tedious part of Modbus RTU – the register table – off your hands with device drivers stored in Device Management. Instead of typing in register addresses, you pick the device and get named measured values.
From there on, the same applies as with M-Bus: the M-Center reads cyclically, logs failed readings and passes the values on to MQTT, BACnet IP, OPC UA, Modbus TCP, (s)FTP, email or Joulio-Web. A Modbus RTU segment that was previously read only by a building management system can then serve billing and the cloud at the same time.
RS-485 on the M-Center
Interfaces
2 × RS-485
Devices
up to 64, licence stages 32 and 64
Protocol
Modbus RTU
Drivers
through Device Management, instead of register addresses by hand
Forwarding
MQTT, BACnet IP, OPC UA, Modbus TCP, CSV, JSON, Excel
Where it fits
When is Modbus RTU over RS-485 the right choice?
Against
- Sensitive to star topology and missing termination
- Not polarity-independent – installation mistakes have consequences
- Mixed media such as water and heat run more easily over M-Bus
- Register tables have to be maintained where no driver exists
Documents
Manuals and data sheets to download
This page explains the principle. Register lists, telegram structure, terminal assignment and the exact parameters of each version are in the interface manual.
PDFModbus interface manual1485 · register table · English PDFProfessional II data sheet1441 · English PDFM-Center manual2151 · master side · English
Frequently asked questions
Frequently asked questions about the interface
What is the difference between RS-485 and Modbus RTU?
RS-485 is the electrical layer: a differential two-wire line in half-duplex operation. Modbus RTU is the protocol on top of it, with master-slave operation, addresses from 1 to 247, 16-bit registers and a CRC16 checksum. You need both, and both are often named in the same breath.
Where do the termination resistors belong?
120 Ω at the two physical ends of the line, nowhere else. An additional resistor in the middle loads the bus without doing any good; missing termination causes reflections that show up as sporadic CRC errors.
May I wire it as a star?
Better not. RS-485 wants a continuous line with short stubs. Star wiring is the most common reason for buses that run at a low baud rate and fail at a higher one. Anyone coming from M-Bus, where stars are allowed, overlooks this easily.
How many devices can the M-Center read over RS-485?
Up to 64 Modbus RTU devices over two RS-485 interfaces. The 32 and 64 device stages are unlocked by licence. Electrically, the limit on unit loads per segment applies on top of that.
Where do I find the Professional II register table?
In the Modbus interface manual, document 1485. It lists register addresses, data types and byte order. On the M-Center this is usually not needed, because device drivers are stored in Device Management.
Two RS-485 interfaces – what for?
To separate two segments that do not fit together: different baud rates, different manufacturers, or two physically separate areas. Forcing everything onto one line is the most common cause of faults that are hard to track down.
Not sure whether RS-485 / Modbus RTU fits?
Send us the number of measuring points, the distances and the target system. We will tell you which version makes sense – and when another interface would be the better choice.
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