smart-energy-monitor/box-pc/application/inputs/snap7_connect.py

import snap7
import time
import struct

class SiemensCPU(snap7.client.Client):

  cpu_start_time = None
  cpu_last_time = None
  local_start_time = time.time()
  db = 1
  interval = 0.05

  cpu_db_value_count = 100

  def get_ints(self, start, count):
    raw = self.db_read(self.db, start*4, count*4)
    data = struct.unpack(">" + "i" * count, raw)
    return data

  def get_timestamp(self, cpu_time):
    if self.cpu_start_time:
      cpu_diff = cpu_time - self.cpu_start_time
      return self.local_start_time + cpu_diff
    else:
      raise Exception("call synchronize first")

  def synchronize(self, duration = 3):
    start = time.monotonic()
    self.cpu_last_time = None
    intervals = []
    while time.monotonic() - start < duration:
      cpu_time = self.get_ints(0, 1)[0] / 1000
      if cpu_time != self.cpu_last_time:
        self.cpu_start_time = cpu_time
        self.local_start_time = time.time()
        if self.cpu_last_time:
          intervals.append(cpu_time - self.cpu_last_time)
        self.cpu_last_time = cpu_time
      time.sleep(0.05)
    if len(intervals) > 0:
      self.interval = sum(intervals) / len(intervals)
    return self.interval

  def read(self):
    data = self.get_ints(0, self.cpu_db_value_count + 1)
    cpu_time = data[0] / 1000
    if not self.cpu_last_time:
      self.cpu_last_time = cpu_time - self.interval
    if cpu_time == self.cpu_last_time:
      return []
    inc_time = (cpu_time - self.cpu_last_time) / self.cpu_db_value_count
    points = []
    for i, val in enumerate(data[1:]):
      val_time = self.get_timestamp(self.cpu_last_time + inc_time * (i+1))
      points.append((val_time, val))
    self.cpu_last_time = cpu_time
    return points

  def read_continous(self):
    while True:
      points = self.read()
      for point in points:
        yield point