Open Modbus Gateway

This software is an Open Source alternative to Teltonikas' Modbus Gateway (modbusgateway).

NOTE: This software is in no way affiliated with Teltonika.

Background

The modbusgateway software of the Teltonika RUT is the critical component of a product package at the company that I work for.

Due to limitations in the original software, it's impossible to read or write to coils or read from discrete inputs. I wanted to fix this. But due to budget constraints, I could not push through a request to replace the software.

So, I made an Open Source version in my spare time.

Protocol

A controller publishes a message in the format below on a request topic. The software interprets the message and performs a Modbus request based on instructions from the message. The software then replies on the response topic.

Request message

0 <COOKIE> <IP_TYPE> <IP> <PORT> <TIMEOUT> <SLAVE_ID> <MODBUS_FUNCTION> <REGISTER_NUMBER> <REGISTER_COUNT/VALUE> <DATA>

Explanation:

• 0: must be 0, which signifies a textual format (currently the only one implemented).

• Cookie: a 64-bit unsigned integer in range [0..2^64]). A cookie is used to distinguish which response belongs to which request. Each request and the corresponding response contain a matching cookie: a 64-bit unsigned integer.

• IP type: host IP address type. Possible values:

  • 0: IPv4 address;
  • 1: IPv6 address;
  • 2: hostname pointing to an IP address.

• IP: IP address of a Modbus TCP slave. IPv6 must be presented in full form (e.g., 2001:0db8:0000:0000:0000:8a2e:0370:7334).

• Port: port number of the Modbus TCP slave.

• Timeout: timeout for Modbus TCP connection, in seconds. Range [1..999].

• Slave ID: Modbus TCP slave ID. Range [1..255].

• Modbus function:

  • 1: read coils
  • 2: read discret inputs
  • 3: read holding registers
  • 4: read input registers
  • 5: force/write single coil
  • 6: preset/write a single holding register
  • 15: force/write multiple coils
  • 16: preset/write to multiple holding registers

• Register number: number of the first register (in the range [1..65536]) from which the registers will be read/written.

• Register count/value: this value depends on the Modbus function:

  • 1, 2, 3, 4: coil/register count (in range [1..125]). Must not exceed the boundary (first register number + register count <= 65537)
  • 5: coil value (in range [0..1])
  • 6: register value (in range [0..65535])
  • 15: register count (in range [1..123])
  • 16: register count (in range [1..123]). Must not exceed the boundary (first register number + register count <= 65537)

• Data: this value only exists for Modbus functions 15 (coil) and 16 (register). A series of coil/register values separated with commas, without spaces (e.g., 0,1,1,0,0,1 or 1,2,3,654,21,789). There must be exactly as many values as specified in register count. Each coil value must be in the range of [0..1]. Each register value must be in the range of [0..65535].

Response message

A particular response message can take one of the following forms:

For functions 5, 6, 15 and 16

<COOKIE> OK

For function 1, 2, 3 and 4

COOKIE> OK <VALUE> <VALUE> <VALUE>

Where are the values read.

For failures

COOKIE> ERROR: <message>

Where is the error description.

Examples

Reading 5 coils

Request

0 16468394968118163995 0 10.0.0.126 5020 5 1 1 1 5

Response

16468394968118163995 OK 1 1 1 1 1

Reading 3 input registers

Request

0 9958479625634 0 10.0.0.126 5020 5 1 4 1 3

Response

9958479625634 OK 1234 5678 9101

Sending to few holding register values

Request

0 565842596387 0 10.0.0.126 5020 5 1 16 1 3 1234,5678

Response

565842596387 ERROR: INVALID REQUEST