Encoding and decoding of KNX telegram structures in cEMI format, as used in KNXnet/IP.

The Telegram struct is used as a model of KNX telegrams. When sending a message to the KNX network, you create a Telegram struct, encode it and send it using the KNXnetIP.Tunnel.

Examples

When you create a telegram, you must specify source and destination. source is a physical address and should always be "0.0.0" when sent from a tunnel client to the tunnel server. destination is the the group address that you want to interact with. Note: currently the library only supports 3-level group addresses, e.g. “1/2/16” (main, middle and subgroup).

To read the value of a group address, you’ll need a telegram of type :request and service :group_read:

read_request = %Telegram{
  type: :request,
  service: :group_read,
  source: "0.0.0",
  group_address: "0/0/2" # replace with the group adress you need to read
  value: <<0::6>> # a read request always has the value zero
}

Once encoded and sent to the KNX network, you will later receive a read response that contains the value of the group address:

read_response = %Telegram{
  type: :indication,
  service: :group_response,
  source: "1.1.4",
  destination: "0/0/2",
  value: <<0x41, 0x46, 0x8F, 0x5C>>
}

The value field now contains the value of the group address, encoded as a KNX datapoint. To decode datapoints, see KNX.Datapoint.

To send a group write, the type is still :request but the service is now :group_write:

write_request = %Telegram{
  type: :request,
  service: :group_write,
  source: "0.0.0",
  destination: "0/0/2",
  value: <<0x42, 0x69, 0x22, 0xD1>>
}

Again, the value is the binary representation of a KNX datapoint with the value you want to write. See KNX.Datapoint for how to encode a value.

Whenever you send a telegram to the KNX bus, you will receive a telegram back of type :confirmation. This confirmation telegram simply echoes the telegram you sent back to you - only the source and type fields will be different. Example confirmation for a group read:

confirmation = %Telegram{
  type: :confirmation,
  service: :group_read,
  source: "1.0.53",
  destination: "0/0/2",
  value: <<0::6>>
}