Solution - Modbus Integration

Modbus is a master–slave protocol. The PIERRE controller is the master: it asks, the devices answer. A device never sends data on its own. There must be only one master on a line.

Addresses and Line Settings

• Every device on an RS485 line has a unique address (unit ID), 1–247.
• All devices on one line use the same baud rate, parity and stop bits. One device with different settings disturbs the whole line.

Registers

A device exposes its data as numbered registers. The manufacturer's register table lists, for each value, the register type, the address, the data type, the scaling and whether it can be written.

In the PIERRE Partner app every field gets one of four register types: Coil, Holding, Input or Holdings. The type decides which Modbus function code the controller uses to read and write the field, as shown in the table below. Holding and Holdings read the same registers; they differ only in how they write. A field can use a different register for reading and for writing.

Data Types and Scaling

• A register holds 16 bits. Larger values (32-bit integers, floating point) span two consecutive registers. Some devices store the two halves in reverse order; this is corrected with the word swap setting of the device.
• Values are often scaled. A temperature of 21.5 °C is typically sent as 215, so the field is divided by 10.
• Signed values (int16) are used where a value can be negative, for example outdoor temperature.
• Supported data types: uint16, int16, uint32, int32, uint64, int64 and float.

Every Modbus device is described in PIERRE in four levels:

1. Interface – the Modbus RTU line or Modbus TCP connection.
2. Device – one unit on that interface, with its address.
3. I/O – what the device represents: Fan Coil, Temperature Sensor, Energy, Volume and so on.
4. Fields – the values of the I/O, such as Active, Mode, Target or Value. Each field is linked to a register.

With a ready-made driver the fields are already mapped: enter the name and address and the device works. With Generic Modbus you define for every field:

• the register address and data type, including values that span two registers;
• whether the field is read, written or both;
• transformations: divide, multiply, add, subtract, swap (value mapping, e.g. the device's 3 means PIERRE's 2) and mask (selects bits of a register);
• the polling interval and access mode.

Register Addresses

Addresses in PIERRE start at 1. Enter every address exactly as it appears in the manufacturer's register table, without adding or subtracting an offset.

Polling Interval

Every I/O type has its own default polling interval. Keep the default, especially for fan coils. Many units accept a command in one register and report the result in another, with an internal delay. Polling too fast reads the old value before the unit has updated it, so the app shows the setpoint jumping back, and it also loads the bus.

Register Fields

When you add a Generic Modbus device, Select Register Fields offers nine fields. Which of them the controller uses depends on the I/O type. Both are listed in the tables below.

The example after them shows how a register table translates into fields. The addresses are illustrative only; every manufacturer's table is different, so always work from the table of the exact model. Room temperature is not a field of the heating device: read it with a separate Temperature I/O (Value field).

Modbus standardizes how data travels, not what it means. Every manufacturer decides its own register addresses, scaling and value codes, and the same brand can use a different table on a different gateway model.

Example: Midea Indoor Temperature

The value in register 1606 is not in degrees. To get °C the controller applies a chain of transformations: subtract 30, divide by 2, subtract 5. A raw value of 80 gives (80 − 30) ÷ 2 − 5 = 20 °C. Without the manufacturer's documentation this formula cannot be guessed.

What This Means for Every Project

• Only a working system can be integrated. The equipment must be installed, commissioned and running on its own first.
• Complete documentation from the manufacturer: the register table of the exact model and gateway model, the line settings and the unit numbers.
• Nothing is guaranteed before a test on a real unit. Never promise that a device works out of the box.
• A trained partner on site checks every function after the configuration: on / off, mode, setpoint, fan speed and read-back.

Before Quoting the Job

• Confirmation from the supplier that the unit supports Modbus, with the exact manufacturer and model.
• The Modbus register table of that model. A unit with a Modbus port but no documented registers cannot be integrated.
• The unit commissioned and working on its own. An integration cannot fix equipment that is not commissioned.

What You Need

• PIERRE Pro Controller.
• Modbus RTU: one TTL to RS485 converter per line and shielded twisted-pair bus cable.
• Modbus TCP: the unit and the controller on the same local network.

Design Rules

• At most 15 devices per RS485 line, at most 2 lines per controller.
• Reserve the controller pins: pin 7 and pin 8 for the converter RX, one pin from 1 to 6 for each converter TX.
• Plan the bus as one line from device to device. Keep a line under 100 m.
• Group devices with the same line settings on the same line.
• Where several apartments share a circulation pump, plan the pump as a controller output with an automation: the pump runs whenever any valve is open.

An RTU installation has three parts: the controller to converter connection inside the distribution board, the RS485 bus from the converter to the devices, and the mains supply of the devices. The pin assignment and the bus rules are in the tables below, followed by the wiring diagram and cable list for each device type.

Controller to Converter

One Cat 5e run of four conductors: 5 V and GND from the controller, TX and RX to the controller pins. Keep the run short and inside the distribution board.

On every converter:

• Switch the 120 Ω termination DIP switch ON. The converter is one end of the line.
• Use a converter with built-in pull-up and pull-down resistors.
• Fit two 470 Ω resistors: one between RX and VCC, one between TX and GND.

Device Settings

• PIERRE Modbus thermostats: address, baud rate and parity are set in the thermostat menu.
• PIERRE temperature and humidity sensor: 9600 baud, no parity, 1 stop bit.
• Third-party units: as given in the manufacturer's documentation, matched to the line.

Mains

Bring L and N to each 230 V thermostat or unit from the distribution board on 1.5 mm² NYM-O. Mains never runs in the bus cable.

Modbus RTU

1. Open the controller and go to the Configurable I/O tab.
2. Tap Add and choose Modbus RTU. Fill in Name and Role, and set baud rate, parity and data format to match the devices. Save.
3. Open the Modbus RTU interface and add the devices. Choose a ready-made driver where one exists, otherwise Generic Modbus.
4. For each device enter Name and Address (the unit ID from the device documentation).
5. For a Generic Modbus device tap Add new I/O type, pick the I/O type, then map each field: register address, data type, transformation and access.
6. Save. The device appears in the Smart PIERRE app as the chosen I/O type.

Modbus TCP

1. Open the controller and go to the Services tab.
2. Tap Add and choose Modbus TCP. Fill in Name, Role, and the IP address and port of the unit. Save.
3. Add devices and map the fields as for Modbus RTU.

Operating the Equipment

Once configured, the unit is operated from the unit card in the Smart PIERRE app, from a PIERRE display or thermostat in the room, from scenarios (for example Leaving Home lowers every setpoint) and from automations (schedules, a window contact that stops heating).

1. Bus check. Add one device first and confirm it shows live values before adding the rest of the line.
2. Read. Open each device in the Partner app: room temperature, setpoint and fan speed show live values. An empty or zero field means a wrong address, register or line setting.
3. Write. Change the setpoint in the Smart PIERRE app and confirm the unit follows. Wait for the next polling cycle before judging the read-back value.
4. Read back. Change the setpoint on the unit or thermostat and confirm the app updates.
5. Offline. Disconnect the internet and change the setpoint from a display. Regulation must continue.

Three rooms with underfloor heating, one PIERRE Modbus thermostat per room, one valve actuator per room on a controller output, and a circulation pump.

1. Converter. TX on pin 1, RX on pin 7, wired and prepared as described in Wiring.
2. Bus. Shielded 2 × 2 bus cable from the converter to thermostat 1, then 2, then 3: A+ and B− on one pair, GND reference on the second. Terminator at thermostat 3.
3. Mains. L and N to each thermostat from the distribution board.
4. Thermostats. In each thermostat menu set address 1, 2 and 3, and the same baud rate on all three.
5. Outputs. Valve actuators on outputs 2 to 4, pump relay on output 5. Pins 1 and 7 are taken by the converter.
6. Interface. Configurable I/O → Add → Modbus RTU, named Heating Bus, with the baud rate from step 4.
7. Devices. Add three PIERRE Modbus thermostats – Living Room, Bedroom, Kitchen – with addresses 1, 2 and 3, each in its room group.
8. Valves and pump. Add the valves on pins 2 to 4 and link each to its thermostat as the heating device. Add the pump on pin 5 with an automation: active when any valve is active.
9. Test. Raise a room setpoint: the valve opens and the pump starts. Lower it: the valve closes, and the pump stops when the last valve closes.

Air conditioners are the most demanding Modbus integration. The AC unit and the smart home speak different languages: the Modbus gateway is the interpreter, the PIERRE configuration is the dictionary, and every AC type needs its own dictionary. It does not depend only on PIERRE: the unit, the gateway and the documentation come from third parties, and some systems cannot be integrated at all.

An AC integration is therefore taken on only after a site survey, an engineering assessment and a test on a real unit, and nothing is guaranteed before that test.

Steps

1. Site survey. Answer every question on the checklist below. A missing item means the integration cannot be assessed yet.
2. Engineering assessment by PIERRE. If the system cannot be integrated, the process stops here.
3. Test on a real unit. The integration is confirmed only when the test passes on the installed system.
4. Handover.

Site Survey Checklist

• AC brand and exact model.
• Is there a Modbus gateway, or can one be fitted, and which model.
• How many indoor units, and do all of them need control.
• Will the wall remote stay in use.
• The manufacturer's address table for that exact gateway model is in hand.
• Line settings and indoor unit numbers are known.
• A powered, reachable unit is available for testing.

Technical Points

• The register table that counts is the one of the gateway model, not of the AC unit.
• Every indoor unit is a separate device with its own number and configuration.
• If the wall remote stays in use, the unit can change state without the controller; the app shows it after the next poll.
• Unit-specific behaviour, such as the Daikin delays, is listed in Troubleshooting.

When You Contact PIERRE Support

Send with every request:

• Brand, model and gateway model.
• The indoor unit's number or name.
• What the app shows, and what the wall remote shows.
• What the customer pressed, and when (date and time).
• Whether the wall remote was used in the meantime.