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
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 integeru32- unsigned 32 bit integer
Unlocking configuration registers
Making configuration changes requires unlocking the device.
- To unlock the device write maintenance or administration password to
0x000Cregister. - Writing incorrect password to
0x000Cregister will change access level to0(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
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 –
0x06Modbus error code is sent if the OUT port is busy and the Modbus request hasn’t been forwarded. - 2 –
0x0BModbus 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 |
Example connection diagram
