EN-IEC 62056-21, Part 21: direct local data exchange, 2002-05

- Requires python2+
- Install deps
  # pacman -S python-requests python-pytz
- Create script
  # nano /root/dsm_reader.py && chmod +x /root/dsm_reader.py
- Create config
  # echo "auth_token='abcdefg'" > /root/dsm_reader_config.py
- Start
  # systemctl enable dsm_reader.service && systemctl start dsm_reader
- ToDo: 
  1. modify service file to a template unit file (name@string.service), where the serial device becomes variable
  2. use this name@ to create a separate backlog file per serial device?
  3. auto insert backlog to influxdb
  4. implement retries, with backoff? like 
     - solution 4 from https://izziswift.com/can-i-set-max_retries-for-requests-request/
     - https://github.com/pvl7/emu-to-pvoutput/blob/2edf7fff8d1af180fa203968b2d33acb80485dec/rainforest-to-pvoutput.py#L88
  5. also backlog pvoutput temp. errors, note: The date parameter must not be older than 14 days from the current date