Communication

The NODA Modbus Gateway communicates with the NODA MQTT network over a 4G mobile network.

Communication is secured using standard client-based certificates where only a holder of a valid certificate can connect to the network.

Each client is further locked down using ACL (access control lists), declaring which topics (communication channels) a specific client is allowed to subscribe and publish.

A certificate can easily be black-listed (by NODA) if, for any reason, the certificate is compromised.

All communication over this MQTT channel is encrypted.

Communication protocol

The NODA Modbus Gateway relies on software written by the manufacturer (Teltonika) of the Gateway hardware. Teltonika developed a simple protocol for transferring Modbus requests over MQTT. The protocol works by;

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 discrete 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 five coils

Request

0 16468394968118163995 0 10.0.0.126 5020 5 1 1 1 5

Response

16468394968118163995 OK 1 1 1 1 1

Reading three input registers

Request

0 9958479625634 0 10.0.0.126 5020 5 1 4 1 3

Response

9958479625634 OK 1234 5678 9101

Sending too 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