Question :
Why does the M580 Real Time Clock (RTC) fails switching from Summer time to Winter time when I test the Daylight Saving Time (DST) feature ?

Answer :
- The local time DST feature works actually correctly !
- The user is given the wrong impression in the 'PLC Screen\Real Time Clock' window by the local time to UTC conversion feature.
- To test rapidly the DST feature, the user must go through the following steps :

1) determine the 2 UTC dates and times few seconds before the local clock must disrupt between EST (standard time) and EDT (daylight saving time)
e.g. Boston MA USA 2021
Sun, 14 Mar, 02:00    EST → EDT    +1 hour (DST start)    UTC-5h -> UTC-4h => 1:59:55 + 5h = 6:59:55
Sun, 7   Nov, 02:00    EDT → EST    -1 hour (DST end)      UTC-4h -> UTC-5h => 1:59:55 + 4h = 5:59:55

2) open an animation table showing %S50, and %SW50 to %SW53, side by side with the RTC window and follow these steps

Spring DST disruption standard time -> daylight saving time (+1 hour)
set %S50 to 1 to stop the %SW50-53 update and enable modification
set %SW50 to 16#5500 (seconds)
set %SW51 to 16#0659 (hours-minutes)
set %SW52 to 16#0314 (month-day)
set %SW53 to 16#2021 (year)
set %S50 to 0 to set the RTC and enable the %SW50-53 update from RTC
look in the animation window, the local time will jump from 01:59:59 to 03:00:00

Autumn DST disruption daylight saving time -> standard time (-1 hour)
set %S50 to 1
set %SW50 to 16#5500 (seconds)
set %SW51 to 16#0559 (hours-minutes)
set %SW52 to 16#1107 (month-day)
set %SW53 to 16#2021 (year)
set %S50 to 0
look in the animation window, the local time will jump from 01:59:59 to 01:00:00

figure 1 : Automn local time disruption testing


here, after %S50 has been reset, the automn local time disruption is just about to occur (in 3 seconds)

3) Basics :
- the M580 RTC is running UTC time (Coordinated Universal Time) so that no time disruption ever occur internally.
- the displayed local time is calculated from the RTC UTC time
- the automn Daylight Saving Time (DTS) ends at 02:00AM.
- when setting the local time before 02:00:00 (say 01:59:55) using the control expert real time clock window, this time has to be converted to UTC before setting the RTC.
- the conversion only computes EST (standard time) to UTC.
- the only way to witness the disruption using the animation screen is to set the local time to 00:59:59 (forcing DST) and to wait for an hour to see the automn local time disruption.

Publié pour: Schneider Electric Belgium