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RI-DMUX

RI-DMUX is a Modbus RTU multiplexer.

Its main feature is allowing two masters on a single RS-485 bus.

How does it work?

Requests from two Modbus RTU masters are queued and forwarded to Modbus RTU slave devices ensuring no data transmission conflicts. Once response is received it is sent to master that sent the request.

Second master does not receive requests nor responses initiated by first master.

Typical application

Diagram 1 - typical application
Diagram 1 - typical application

Specifications
Power supply: Stabilised +5 VDC, min 100 mA
Enclosure: DIN Rail mounted,
2 modules wide (35 mm)
Connectors: Terminal blocks,
raster 5.08 mm, 30-12 AWG
Operating temperature: from -20°C to +65°C

Configuration and registers

Modbus functions

Supported read functions:

  • Read Holding Registers (FC=03 — 0x03)

Supported write functions:

  • Preset Multiple Registers (FC=16 — 0x10)

Note

Above functions apply to configuration registers only.

RI-DMUX supports forwarding other Modbus RTU functions as well.

Data types

Configuration registers use following data types:

  • u16 - unsigned 16 bit integer
  • u32 - unsigned 32 bit integer

Unlocking configuration registers

Making configuration changes requires unlocking the device.

  • To unlock the device write maintenance or administration password to 0x000C register.
  • Writing incorrect password to 0x000C register will change access level to 0 (guest).

Reading Enter Password register will return current access level:

Setting Default value Unit Register Address Type
Enter Password 0 enum:
0 - guest
2 - maintenance
4 - administrator		 0x000C u16

Note

Device will automatically return to 0 (guest) access level within 24 hours.

Passwords

Passwords can be changed using following registers:

Setting Default value Register Address Type
Set maintenance password 0x0001 0x000D u16
Set admin password Hidden 0x000E u16

Unit number

Mobus unit number ("address") is by default set to 252 (0xFC).

Note

Requests sent to this address will not be forwarded to OUT port.

Setting Default value Register Address Type
Unit number
(Address)		 252
(0xFC)		 0x0007 u16

Response delay

Setting Default value Unit Register Address Type
Minimal response delay 100 ms 0x000B u16

RS-485 port configuration

To change setting of currently used port (IN1 or IN2) use following registers:

Setting Default value Unit Register Address Type
Baudrate 9600 bps1 0x0008, 0x0009 u32
Mode 1 (8N1) enum2 0x000A u16

1 - Supported baudrate values: 9600, 19200, 38400, 57600, 115200

2 - Mode enum values: 1: 8N1, 2: 8E1, 3: 8O1, 4: 8N2, 5: 8E2, 6: 8O2
Note: When reading the register value, discard the leftmost reserved byte (RRRR MMMM) to get the mode enum value.

Each port can also be configured independently using following registers:

Port IN1 (Master 1)

Setting Default value Unit Register Address Type
Baudrate 9600 bps1 0x0140, 0x0141 u32
Mode 1 (8N1) enum2 0x0142 u16
Max wait3 2000 ms3 0x0143 u16
Enable 1 enum4 0x0144 u16
Debug messages5 1 enum5 0x0145 u16

3 - Max wait specifies how long the IN master waits for the response. Allowed values: 500 – 5000 ms.
For the best performance, max wait should be:

  • IN Ports - slightly smaller than a time that a Modbus master waits for the response before sending another request on the given IN port.
  • OUT Port - slightly greater than the max response time of the slowest Modbus device connected to the OUT port.

4 - 0: Port disabled (Modbus requests are not forwarded to the OUT port), 1: Port enabled

5 - Debug messages are sent by the RI-DMUX device on behalf of the Modbus target devices to the Modbus masters when the communication with the given Modbus target device is currently not possible. Values:

  • 0 – no debug messages, the device just does not respond.
  • 1 – 0x06 Modbus error code is sent if the OUT port is busy and the Modbus request hasn’t been forwarded.
  • 2 – 0x0B Modbus error code is if the target Modbus device is filtered out.

Port IN2 (Master 2)

Setting Default value Unit Register Address Type
Baudrate 9600 bps1 0x0160, 0x0161 u32
Mode 1 (8N1) enum2 0x0162 u16
Max wait3 2000 ms3 0x0163 u16
Enable 1 enum4 0x0164 u16
Debug messages5 1 enum5 0x0165 u16

Port OUT

Setting Default value Unit Register Address Type
Baudrate 9600 bps1 0x0180, 0x0181 u32
Mode 1 (8N1) enum2 0x0182 u16
Max wait3 2000 ms3 0x0183 u16

Querying port number

Master can read which port it is connected to by reading following register:

Setting Default value Unit Register Address Type
Port ID N/A enum: 1 - port 1, 2 - port 2 0x0100 u16

Port priority

RI-DMUX can be configured to prioritize one of the IN ports.

Setting Default value Unit Register Address Type
Port priority 0 (equal) enum: 0 - equal, 1 - port 1, 2 - port 2 0x0101 u16

Modbus filtering

When RI-DMUX receives response on one of the IN ports (instead of OUT port) it will add Modbus unit number (address) to filtering list.

All subsequent requests addressed to filtered devices will NOT be forwarded to OUT port.

This feature enables more advanced RS-485 bus topologies where e.g.:

  • only part of RS-485 bus is shared with second master
  • each master has private devices, which can use same unit numbers (addresses)

When there is no response from filtered device on any of IN ports for more than 120 seconds it is removed from filtering list.

Filtering time can be adjusted by writing to 0x0103 register.

Setting Default value Unit Register Address Type
Modbus filtering 120 s6 0x0103 u16

6 - Allowed values: 10 - 65535

Advanced filtering configuration

We advise to use default automatic filtering.

Setting Default value Unit Register Address Type
Filter mode 1 (auto) enum:
0 - disabled
1 - auto
2 - manual		 0x0102 u16

Note

Automatic filtering is supported for function codes 0x03, 0x04 and 0x10 only.

However if required, filtering can be configured manually using 256 bit mask to mark units that should be filtered.

Mask is divided into eight 32 bit registers. E.g.: Writing 0x00000012 to register 0x0104 will disable forwarding of requests to addresses (unit numbers) 0x01 and 0x04.

Setting Register Address Type
Filter mask 0x0104 u32
Filter mask 0x0106 u32
Filter mask 0x0108 u32
Filter mask 0x010A u32
Filter mask 0x010C u32
Filter mask 0x010E u32
Filter mask 0x0110 u32
Filter mask 0x0112 u32

Note

In automatic mode filter mask registers can be read to view current filtering status.

Modbus statistics

RI-DMUX provide following bus statistics that can be used in troubleshooting.

Each register contains count of messages (requests or responses).

Note

Statistics reset every 24 hours.

Port IN1

Message type Register address Type
Correct 0x01A0 u16
Filtered 0x01A1 u16
Rejected 0x01A2 u16
Invalid request 0x01A3 u16
Invalid response 0x01A4 u16
No response 0x01A5 u16

Port IN2

Message type Register address Type
Correct 0x01B0 u16
Filtered 0x01B1 u16
Rejected 0x01B2 u16
Invalid request 0x01B3 u16
Invalid response 0x01B4 u16
No response 0x01B5 u16

Device information

Name Register Address Type
Status 1 0x0000 u16
Status 2 0x0001 u16
Protocol version 0x0002 u16
Serial number 0x0003, 0x0004 u32
Hardware version 0x0005 u16
Firmware version 0x0006 u16

Factory reset

Warning

Writing to (or reading from) this register will reset the device to factory settings.

Name Register Address Type
Factory reset 0x001F u16

Timing over RS-485 with Latency Considerations

Overview

The performance characteristics and timing calculations for Modbus RTU communication over RS-485 include:

  • Modbus RTU frame construction
  • Request-response timing at various baud rates
  • Impact of hardware-induced delays (slave processing and converter buffering)
  • Practical recommendations for optimizing performance

Setup Description

Consider typical system with single Master:

sequenceDiagram
    participant Master1 as Master 1
    participant Converter as RI-DMUX
    participant Remote as Modbus Slave

    Master1-->Converter: RS-485 Port IN1

    Master1->>Converter: Request to Slave Device
    activate Master1
    activate Converter

    Converter-->Remote: RS-485 Port OUT

    Converter->>Remote: Request to Slave Device
    activate Remote

    Converter-->Remote: Request processing time
    opt
        Converter-->Remote: Response delay<br>(default depends on device model)
    end

    note over Converter,Remote: #9888;<br>Port OUT Max Wait configuration<br>may require adjusting.<br>(default: 2000 ms)

    Remote->>Converter: Response from Slave Device
    deactivate Remote

    opt
        Converter-->Master1: Response delay<br>(default: 100 ms)
    end

    note over Converter,Master1: #9888;<br>Port IN1 Max Wait configuration<br>must match Master timeout.<br>(default: 2000 ms)

    Converter->>Master1: Response from Slave Device
    deactivate Master1
    deactivate Converter

Example operation:

  • Master: Sends a Modbus RTU request using Function Code 0x03 (Read Holding Registers).
  • Slave: Adds a 100 ms processing delay before responding.
  • RI-DMUX: Introduces an additional 100 ms delay before forwarding the slave's response back to the master.
  • Protocol: Modbus RTU, 8N1 format (1 start bit, 8 data bits, 1 stop bit).
  • Register Count: 128 registers (256 bytes of data).

Modbus RTU Request Details

To read 128 registers starting at address 0x0000 from unit ID 1, the command is:

01 03 00 00 00 80 C4 0B
Field Description
01 Slave Address
03 Function Code (Read Holding Reg)
00 00 Start Address
00 80 Quantity of Registers (128)
C4 0B CRC16 (Lo Hi)
Length 8 bytes total

Response Frame

The expected response includes:

  • 1 byte slave address
  • 1 byte function code
  • 1 byte byte-count field (256)
  • 256 bytes of register data
  • 2 bytes CRC

Total size: 261 bytes

Request-Response Timing by Baud Rate

Each byte (in 8N1 format) transmits as 10 bits. Time per byte = 10 / Baud Rate.

Baud Rate Byte Time (ms) Request (8B) Response (261B) Inter-frame Gap Ideal Total Time
9600 1.0417 8.33 ms 271.88 ms 3.65 ms ~284 ms
19200 0.5208 4.17 ms 135.94 ms 1.82 ms ~142 ms
38400 0.2604 2.08 ms 67.97 ms 0.91 ms ~71 ms
57600 0.1736 1.39 ms 45.32 ms 0.61 ms ~47 ms
115200 0.0868 0.69 ms 22.66 ms 0.30 ms ~24 ms

These are ideal-case numbers assuming no hardware latency and immediate response.

With Real-World Latency

Now consider:

  • 100 ms slave processing delay
  • 100 ms RI-DMUX buffering delay

Total Transaction Time at 9600 bps

Component Time
Master transmit 8.33 ms
Slave processing 100 ms
Slave transmit 271.88 ms
Converter delay 100 ms
Inter-frame gap 3.65 ms
Total ~484 ms

Total at 19200 bps

Component Time
Master transmit 4.17 ms
Slave processing 100 ms
Slave transmit 135.94 ms
Converter delay 100 ms
Inter-frame gap 1.82 ms
Total ~342 ms

Throughput Implications

Baud Rate Max Transactions/sec (ideal) Max Transactions/sec (with delays)
9600 ~3.5 ~2.0
19200 ~7.0 ~2.9
38400 ~14.0 ~3.8
115200 ~41.6 ~5.4

The 200 ms artificial latency limits scaling, even at high baud rates.

Recommendations

  • Use higher baud rates to minimize protocol overhead.
  • Avoid large register reads if system introduces latency — break requests into smaller blocks (e.g., 64 registers).
  • Tune slave firmware to reduce processing delay, if possible.
  • Reduce RI-DMUX delay configuration if delay is not needed.

Timeout Scenario: Slave Not Present or Unresponsive

When a slave device is not present on the RS-485 bus or fails to respond (e.g., due to power failure, configuration error, or hardware fault), the Modbus master waits for a response until the configured timeout elapses. In most systems, this timeout is set to 2000 ms (2 seconds) by default.

In such cases:

  • The master blocks for 2000 ms, expecting a reply that never arrives.
  • No useful data is returned, and a timeout exception or error code is typically triggered in the SCADA/HMI/logging system.
  • The transaction time balloons from hundreds of ms to 2 seconds, severely degrading throughput if retries or polling continue.
  • If multiple unresponsive devices are polled sequentially, this can cascade into seconds of wasted time per cycle.

Impact

At 9600 baud with a 2-second timeout per failure, polling just five missing slaves could waste 10+ seconds, severly reducing maximum polling rate.

Multi-master operation

All considerations above apply to multi master operation:

sequenceDiagram
    autonumber

    participant Master1 as Master 1
    participant Master2 as Master 2

    participant Converter as RI-DMUX
    participant Remote as Modbus Slave


    Master1->>Converter: Request to Slave Device
    activate Master1
    activate Converter
    Note left of Converter: RS-485 Port IN1

    Converter->>Remote: Request to Slave Device
    activate Remote
    Note right of Converter: RS-485 Port OUT

    Master2-->>Converter: Request to Slave Device
    activate Master2
    Note left of Converter: RS-485 Port IN2

    Remote->>Converter: Response from Slave Device
    Note right of Converter: RS-485 Port OUT
    deactivate Remote

    Converter->>Master1: Response from Slave Device
    Note left of Converter: RS-485 Port IN1
    deactivate Master1

    Converter-->>Remote: Request to Slave Device
    activate Remote
    Note right of Converter: RS-485 Port OUT
    Remote-->>Converter: Response from Slave Device
    Note right of Converter: RS-485 Port OUT
    deactivate Remote
    Converter-->>Master2: Response from Slave Device
    Note left of Converter: RS-485 Port IN2
    deactivate Master2
    deactivate Converter