Decoder for SML (Smart Message Language) telegrams.

SML (BSI TR-03109-1) is the protocol German electricity meters ("moderne Messeinrichtung", eHZ) use on their optical interface. A meter typically pushes a telegram every one to four seconds that contains the current readings, e.g. the energy counter (1-0:1.8.0) and the current power (1-0:16.7.0).

Reading values

{:ok, bezug_wh, 30} = Sml.value(telegram, :bezug)
{:ok, leistung_w, 27} = Sml.value(telegram, :leistung)

The third element is the DLMS unit code (30 = Wh, 27 = W), see Sml.Units. Values are scaled with the scaler sent by the meter: the result is an integer if the scaler is not negative and a float otherwise.

Keys can be an alias atom (see Sml.Obis for the list), an OBIS string like "1-0:1.8.0" or "1-0:1.8.0*255", an Sml.Obis struct or the raw 6 byte OBIS binary.

All functions accept either the raw telegram binary or an already decoded Sml.File. When reading several values from one telegram, decode it once:

{:ok, file} = Sml.decode(telegram)
{:ok, bezug, _unit} = Sml.value(file, :bezug)
{:ok, einspeisung, _unit} = Sml.value(file, :einspeisung)

Inspecting telegrams

decode/2 returns the full structure: an Sml.File with a list of Sml.Messages whose bodies are the structs in Sml.Messages. Since everything is a plain struct, pattern matching works as expected:

{:ok, %Sml.File{messages: messages}} = Sml.decode(telegram)

for %Sml.Message{body: %Sml.Messages.GetListResponse{val_list: entries}} <- messages,
    %Sml.ListEntry{obis: obis, unit: 27} = entry <- entries do
  {to_string(obis), Sml.ListEntry.scaled_value(entry)}
end

Reading from a serial port

Telegrams arrive in arbitrary chunks. Sml.Transport.extract/1 splits a buffer into complete telegrams and returns the rest for the next read:

{telegrams, buffer} = Sml.Transport.extract(buffer <> chunk)