WB-MRM2-mini/NO

This page describes the latest hardware revision with the latest stable firmware. If your device runs older firmware, update it where possible (see Firmware update).

WB-MRM2-mini NC v.2 in heat-shrink enclosure, before batch v2.4D

Purpose

The WB-MRM2-mini v.2 two-channel relay module is intended for building automation. Control it from a Wiren Board controller or computer over RS-485 using Modbus, or with external buttons connected to the module. Its small size allows installation near loads: in cable ducts, luminaire canopies and deep socket/switch back boxes.

For relay selection, see Relay selection recommendations..

Technical specifications

Parameter	Value
Power supply
Supply voltage	9 to 28 VDC
Power consumption	all relays off — 0.1 W; all relays on — 0.5 W; peak — up to 2 W for 20 ms.
Outputs
Number of outputs	2
Output type	Mechanical relay contacts
Contact configuration	Two-position: normally open (NO version); normally closed (NC version).
Output arrangement	Two groups, two terminals per group
Maximum switching voltage, AC	250 V
Maximum switching voltage, DC	30 V
Rated switching current per output at 230 V AC	NO version — 10 A; NC version — 2 A.
Maximum switching current per output at 230 V AC	NO version — 16 A; NC version — 5 A.
Maximum inrush current (20 ms) at 230 V AC	80 A
Contact resistance	< 100 mΩ
Dielectric withstand voltage between relay coil and contacts	NO version — 4000 VAC (1 minute); NC version — 2500 VAC (1 minute).
Dielectric withstand voltage between open relay contacts	1000 VAC (1 minute);
Operating life	NO version — 100,000 operations at 10 A / 230 VAC; NC version — 100,000 operations at 2 A / 230 VAC.
Relay model	NO version — Hongfa HF32FV-16 NC version — Hongfa HF32F
Inputs
Number of inputs	2
Input type	Dry contact, not isolated from the power supply
Input voltage	3.3 V
Permissible voltage	0 V to +3.3 V
Input activation current flowing to GND	1 mA
Pulse frequency and duration	Up to 10 Hz
Functions	general-purpose inputs; pulse counting; frequency measurement; direct relay output control; simultaneous disconnection of all loads; flexible input-to-relay behaviour using the mapping matrix
Delays
Startup Delay When Power On (s)	0–999 s; default: 0 s
Restart Delay After Output Turn Off (s)	0–999 s; default: 0 s
Indicators
LED	See Indicators
Control
Control interface	RS-485
Interface isolation	Non-isolated
Communication protocol	Modbus RTU; software-configurable address; factory settings are printed on the label
RS-485 interface parameters	Configurable in software; see the register map); default: 9600 bit/s, 8 data bits, no parity (N), 2 stop bits
Ready after power-on	~2 c
Operating conditions
Ambient air temperature	−40 to +50 °C
Relative humidity	Up to 92%, non-condensing
Warranty	2 years
Service life	8 years
Terminal blocks and wire cross-sections
Recommended wire cross-section with insulated wire-end ferrules	control inputs: 0.35–1 mm² — single ferrule; 0.35–0.5 mm² — twin ferrule, power inputs: up to 2.5 mm² — single ferrule; up to 1.5 mm² — twin ferrule
Standard ferrule sleeve length	8 mm
Screw tightening torque	control inputs: 0.2 N·m, power outputs: 0.5 N·m
Dimensions
Dimensions (L × W × H)	51.5 × 23 × 21.5 mm
Weight (including box)	30 g

Operating principle

Indicators

The module has indicators that show its status without connecting to a controller:

S (green) — Modbus communication status: lights up when low-voltage power is applied and flashes during polling over RS-485. On devices with events enabled in Fast Modbus ⚡ the indicator flashes more frequently than with sequential polling because event polling is faster. In bootloader mode it flashes every second; during firmware loading it flashes rapidly. The LED is under the terminal block.

Inputs

Inputs use numbered pluggable terminals and a common GND and operate as dry-contact inputs.

Connect switches for direct output control, or a signal source for pulse counting and frequency measurement.

Both maintained and push buttones are supported. Push buttones support several press types, each of which can be assigned an action for scene control. Press events are also transmitted to the controller over Modbus for processing in software.

The Wiren Board controller does not poll press counters by default. Enable their polling in the device settings if required.

By default, each input controls the output with the same number. Input mode is Latching switch — the relay contacts follow the switch contacts. Change the switch type and input parameters in the settings.

Press-type recognition

The module recognises four press types: short; long; double; and short followed by long.

Presses can be processed on the controller, which receives a separate counter for each type over Modbus, or locally in the module to control its outputs.

Press-recognition parameters can be changed in the settings.

Outputs

Two internal relays switch the two corresponding outputs:

NO version — Hongfa HF32FV-16;
NC version — Hongfa HF32F.

Relay outputs use screw terminals. TVS diodes protect the contacts against overvoltage. For permitted load power and type, see Relay selection recommendations.

Safety Mode

Safety Mode can stop a process if control over the module is lost, for example if the RS-485 cable is severed. Account for loss of controller communication when designing control systems (see Configuring Safety Mode)

Actuator Control

This function is available only on the normally open model WB-MRM2-mini/NO.

Despite the wide range of actuators with digital control, mains-switched actuators remain common for curtains, blinds, shutters, projector screens, air dampers, ball valves and similar devices. Their main control requirement is to prevent simultaneous activation of Open and Close.

The module's Actuator Control combines two relays into one control and uses a software interlock to prevent both relays being energised simultaneously. It also inserts a 500 ms delay when reversing direction and energises the relays for the configured duration (10 seconds by default).

To protect the actuator, commands through the relay coil registers and input/output mapping matrices are ignored in this mode. These outputs can only be controlled through the Actuator Control registers and the actuator mapping matrix.

Actuator Control is disabled by default. Enable it in the settings before use. Configure delays and control from the module inputs there as well.

Delays

Startup Delay When Power On (s) starts when the device firmware boots, approximately 2.5 s after low-voltage power is applied. It switches outputs on sequentially to protect supply circuits from combined inrush currents.

Restart Delay After Output Turn Off (s) starts when the relay switches off. It protects loads sensitive to frequent switching, such as compressors and pumps.

Delays are configured separately for each relay output.

Output timer functions

Timer functions switch module outputs on or off according to a defined timing sequence.

Direct control and timer functions can be used in the same module. Each output can have only one timer function. Outputs are configured independently; changing one output does not affect the others. More about relay module timer functions.

Each relay output supports these timer functions: On Delay, Off Delay, Switch on for a time, Switch on for a time (with command reset), Switch off for a time, Switch off for a time (with command reset), On/Off Cycle.

Configuring output timers.

Installation

WB-MRM2-mini can be installed in back boxes, cable ducts and similar spaces. The 3.5 mm pitch V+ GND A B block connects power and RS-485. Correct RS-485 wiring is essential for reliable communication; see RS-485 bus wiring and connection.

If devices on the RS-485 bus use different power supplies, their GND terminals must be interconnected. See Grounding Wiren Board devices and connecting GND and iGND.

Use flexible stranded wire with insulated wire-end ferrules (NSHVI).

Strip the insulation to the sleeve length, or strip a little more and trim the protruding wire after crimping. Use a ferrule crimping tool. A crimped ferrule protects the strands from the screw clamp and secures the connection.

Do not overtighten the terminal screws: excessive force damages the screw terminal.

For power over a long cable, consider voltage drop along the cable.

Digital inputs activate when connected to GND. The current is small, approximately 2 mA, so ordinary pushbuttons or switches can be used. Cable length and cross-section are not critical to activation, but twisted pair with GND on the second conductor is recommended to reduce interference from nearby power wiring.

Conductor cross-section at relay output screw terminals must suit the load. The terminals accept 2.5–4 mm².

Install the relay module in accordance with electrical safety requirements and prevent accidental contact with live high-voltage terminals. Operate it under the specified environmental conditions.

Installation example

The diagram shows load connections to WB-MRM2-mini/NO.

The module is installed in a back box and the wiring before and after it is protected by a 6 A circuit breaker. Select breaker rating for conductor size and the module's maximum switching current.

The diagram connects a fan and an incandescent lamp to K1 and K2.

Outputs in the example can be controlled by switches and RS-485. To change input functions, see Control from switches.

For switch types and other module settings, see Configuration.

Connecting curtain actuators

Curtain actuator connection example for WB-MRM2-mini/NO

Before connecting a curtain actuator, enable Actuator Control for the outputs used! Incorrect configuration can energise both relays simultaneously and may damage some actuators.

A curtain actuator uses two relays, typically connected to mains phase L and the actuator's Open and Close wires. The actuator is also connected to neutral N and protective earth PE. Always check the proposed wiring against the actuator documentation.

One curtain actuator can connect to WB-MRM2-mini/NO. Any input can control it using the four press types. Input-based actuator control is disabled by default.

Available output functions
Actuator	Outputs
1	K1 - open, K2 - close

Configuration

Configuration methods

Set the parameters in the Wiren Board controller web interface under Settings → Configs → Serial devices driver configuration. If these items are unavailable, check your access level.
Write settings to the Modbus registers of the module from the controller console using the utility modbus-utils-rpc or modbus_client.
Without a Wiren Board controller, use the Wiren Board device web configurator or other software; see Using Wiren Board Modbus devices without a controller. You will need a computer with an interface converter: USB-RS485.

Our devices are continually improved. If settings described in this manual are missing, update the device firmware and controller software.

Contact-closure and press counters

Input closures to ground are counted by Input X counter. These counters are available in all modes for counting pulses and processing dry-contact signals.

When operating with the mapping matrix , counters are available for these press types:

Single Press Counter — single short presses,
Long Press Counter — long presses,
Double Press Counter — double short presses,
Shortlong Press Counter — short-then-long presses.

Enabling polling of press-type counter channels

These count recognised press types, rather than input closures, and are intended for push-button processing. To use press types in wb-rules enable polling of the corresponding channels.

Counters Counter and Single Press Counter serve similar purposes but process contact closures differently:

the counter Counter increments as soon as the input closure is accepted after the configured debounce time;
the counter Single Press Counter increments when a short press has been detected without a second press. The response time is the short-press duration plus the second press waiting time.

Therefore, when using Single Press Counter to count digital signals, you may need to reduce the second press waiting time. For pulses and reed-switch activations, use Input Counter; for push-buttons, use the press counters.

All counter values are held in the relay microcontroller's RAM and reset when power is removed.

Control from switches

The parameter Mode lets you select the connected switch type, disable press actions, or select a mode based on the mapping matrix.

In matrix modes, input states and event counters can still be sent to the controller with input actions disabled, depending on channel polling settings.

Matrix modes are universal; switch modes are retained for compatibility with older projects.

When you select the mode Operate according to button mapping-matrix lets you assign output actions to each of the four press types for scene control and multi-location switching. See Press-type recognition.

Available settings:

Timing diagram for press events, with debouncing disabled

Press counters for input 1 in the Wiren Board controller web interface

Long Press Time (ms) — distinguishes long and short presses. A press lasting longer than this value is long; otherwise it is short.
Second Press Waiting Time (ms) — opens a time window after a short press ends. A second press within this window is treated as a double press; otherwise the first press is treated as a single press.
Debounce Time (ms) — the input signal must remain stable for this duration before it is accepted and processed by the press-recognition algorithm. For press recognition, set it to one fifth to one tenth of the second press waiting time.
Polling interval for each counter — leave this at the default value; the driver manages polling automatically.

Double-press detection delays short-press detection. For example, with the default 300 ms setting, the short-press event occurs 300 ms after the button is released. If double presses and short-then-long presses are not required, set Second Press Waiting Time (ms) to 0 ms. The short-press event will then occur immediately after button release.

Press recognition takes place after debouncing. With Debounce time set to the default 50 ms, presses shorter than 50 ms are ignored. Button release is also registered 50 ms after physical release. Choose the debounce time for the particular button to minimise latency while preventing false events, such as a double press being detected instead of a single press.

Default input settings:

each input controls the corresponding relay: for example, input 2 controls relay 2;
input mode — latching switch.

Useful resources:

Example of processing press counters in wb-rules.

Selecting the mode for relay input 1
Assigning actions to press types

Debouncing

For any relay input, configure debouncing with the parameter Debounce Time (ms). Range: 0–2000 ms; default: 50 ms.

Output states at power-on

The following power-on behaviours are available:

Safe state (default) — after power-on, outputs follow the setting Output Safe State;
Restore last state — saves the last output states and restores them at power-on;
According to inputs — after power-on, outputs follow the input states (input N closed → output N on, where N is the input/output number). This only works with inputs in Latching switch mode.

Safety Mode

Safety Mode can stop a process if control of the module is lost, for example because the RS-485 cable is severed. Account for communication loss when designing control systems.

The fail-safe timer restarts after every successfully received and processed Modbus packet. The module enters safe mode when the configured time expires. Normal operation resumes when communication returns. The default timeout is 10 seconds.

Each output has a configurable fail-safe state and can be configured to enter it on communication loss.

Input control behaviour can be configured:

Don't change — previously enabled input control remains enabled.
Block in safe mode — inputs cannot control the relays.
Allow only in safe mode — input control blocked in normal operation becomes available in safe mode.

Configuring frequency measurement

Frequency measurement settings in the Wiren Board controller web interface

To measure the frequency of a signal at an input, configure that input as follows:

Select the operating mode Control disabled / Frequency measurement. This disconnects the input from output control.
Set Debounce time to 0 ms to measure the highest supported frequencies. For low-frequency signals, the debounce time may instead be set below the pulse period.
Disable input-state polling to reduce bus load.

For details of frequency register updates, see Updating frequency values in registers.

Actuator Control

On the Actuator Control tab, select the appropriate output mode. Settings then become available for opening/closing times, reversal delay, and actions at power-on and on entering Safety Mode. The input tabs also allow an actuator action to be assigned to each of the four press types.

Actuator configuration examples
Configuring actuator control for K1 and K2
Configuring actuator control from input 1

Configuring protective delays

In the device settings, open Protective Functions and set the Startup Delay When Power On (s) or Restart Delay After Output Turn Off (s) for the corresponding relay output.

Configuring output timers

In the device settings, open Timers, select a function and configure its duration for the corresponding relay output.

Protective delay and timer configuration examples
Configuring protective delays in the Wiren Board controller web interface
Configuring relay timers in the Wiren Board controller web interface

Wiren Board controller web interface

Selecting a template

To display the device on the Devices tab of the Wiren Board controller web interface, add it automatically by scanning for Wiren Board devices on RS-485 or manually by selecting a template:

for WB-MRM2-mini/NO — WB-MRM2-mini/NO;
for WB-MRM2-mini/NC — WB-MRM2-mini/NC.

Controlling the device and viewing values

The web interface allows you to control device outputs and view readings. To change the displayed channels, edit the device configuration in the wb-mqtt-serial driver settings.

Module widget in the Wiren Board controller web interface
Module controls and indicators
Actuator Controls in Actuator Control

Modbus operation

Wiren Board devices use Modbus RTU. The physical connection uses the RS-485.

interface. All main commands for reading and writing one or multiple registers are supported. See the list in the Modbus protocol description.

The module can be configured:

in the Wiren Board controller web interface;
using the Wiren Board device web configurator;
using third-party software.

Default port settings

Value default	Parameter name in the web interface	Parameter
9600	Baud rate	Baud rate, bit/s
8	Data bits	Number of data bits
None	Parity	Parity
2	Stop bits	Number of stop bits

In current firmware, setting Stop bits is optional: the device also operates correctly when its number of stop bits differs from the Modbus master setting.

For a faster response, we recommend increasing the baud rate to 115200 bit/s; see Communication parameter settings

Modbus address

Each device on the bus must have a unique address from 1 to 247. The factory-assigned address is printed on labels on the enclosure (top cover, side or underside). Devices in a production batch are assigned different addresses, so devices in one order will usually have different addresses.

For instructions on finding, changing or resetting the address, see Wiren Board device Modbus address.

Fast Modbus extension

Fast Modbus extends standard Modbus with rapid discovery of Wiren Board devices on RS-485 and event polling.

These features are enabled by a special command from the master, so Wiren Board devices remain compatible with third-party equipment.

Not every register supports event generation. Check the register map: event-capable registers are marked with ⚡. See Fast Modbus.

Register map

Relay module register map

Firmware updates and resetting settings

Wiren Board devices support firmware updates over Modbus. This allows bugs to be fixed at the installation site and, where technically possible, new features to be added. Settings stored in the device are retained during a firmware update.

Instructions:

New firmware releases are listed in the firmware changelog.

Known issues

Hardware revisions

Revision history

Troubleshooting

Interference from relays and contactors

Relays and contactors can interfere with nearby devices. When switching loads, especially inductive loads, the contacts generate high-frequency transients with amplitudes of several kilovolts, which may couple into other circuits. Snubber networks suppress these transients, reducing interference and improving equipment stability. See Interference when switching inductive loads

Device images and drawings

CorelDRAW 2018 (font: Ubuntu): File: WB-Library.cdr.zip

Visio:

Wiren Board devices: File: WB-Visio-Lib.zip.
Enclosures, circuit breakers, contactors and other third-party symbols.

SVG: File: WB-Library.svg.zip

AutoCAD Electrical schematic symbol library: File: Wirenboard-AE-base.zip

Module dimensional drawing (DXF): File: WB-MRM2-mini-NO.dxf.zip

Module dimensional drawing (PDF): File: WB-MRM2-mini-NO.dxf.pdf

Module dimensional drawing (DXF): File: WB-MRM2-mini-NC.dxf.zip

Module dimensional drawing (PDF): File: WB-MRM2-mini-NC.dxf.pdf