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time.md

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Setting and Updating Time

The BSM-WS36A tracks time with its internal clock as epoch time. Time information needs to provided externally and updated at a certain interval to maintain valid epoch time on the device. The device starts up with an invalid epoch time and after setting time information, it needs to be updated every 24 to 48 hours.

Warning: Updating the time at intervals smaller than 24 h reduces the lifetime of the device.

There are two data points for getting, setting, and updating this time information:

  • Epoch representing the seconds since 1970-01-01
  • TZO the current time zone offset to UTC in minutes

Both data points are located in the single instance of the BSM model:

Model Data Point Type Unit Start Address
bsm Epoch uint32 s 40261
bsm TZO int16 min 40263

Initially setting epoch time information, both data points needs to be written. Later updates may only set either Epoch or TZO.

Daylight Saving Time

The BSM-WS36A does not support daylight saving time rules. Epoch time information needs to be updated externally at the beginning and end of daylight saving time observation.

Terminology

Setting Time

Setting the time refers to initially setting the time from an invalid state or providing time information which differs more than the accepted cone of drift since the last update or setting of epoch time (see user manual for more details).

Setting time gets registered at two data points:

Model Data Point Type Unit Start Address
bsm EpochSetCnt uint32 40264
bsm EpochSetOS uint32 s 40266

where EpochSetCnt counts the number of times epoch time has been set and EpochSetOS provides when this happened in operating seconds of the device.

Updating Time

Updating time refers to providing time information within the accepted cone of drift since the last updating or setting of time.

Updating time does not get registered.

Initially Setting Time

After powering up the device, epoch time information needs to be set by providing both, epoch time Epoch and the timezone offset TZO. For setting the device's time to 2021-01-01 10:00:00 UTC+01:00, the following values need to be written to the device:

  • Epoch = 1609491600 s = 0x5feee490 s
  • TZO = 60 min = 0x3c min

To set them with the BSM Tool, invoke it as follows:

$ bsmtool set bsm/epoch=1609491600 bsm/tzo=60

This will result in the Modbus write request starting at the protocol address 40260 = 0x9d44 (data model address 40261) and an acknowledge from the device:

--> 2a 10 9d44 0003 06 5feee490003c d4b0
<-- 2a 10 9d44 0003 e9aa

This gets recorded at the epoch time set counter EpochSetCnt and modification timestamp EpochSetOS.

Updating Time

For updating epoch time information, only the epoch seconds need to be provided while it is still possible to write epoch seconds and the same timezone offset.

After initially setting time, it could be updated 24 hours later to

  • 2021-01-02 10:00:00 UTC+01:00
  • Epoch = 1609578000 s = 0x5ff03610 s

by keeping the already set timezone offset and just writing the epoch seconds wit the BSM Tool:

$ bsmtool set bsm/epoch=1609578000

--> 2a 10 9d44 0002 04 5ff03610 8855
<-- 2a 10 9d44 0002 286a

This update is assumed to be within the accepted cone of drift and therefor does not get registered at EpochSetCnt and EpochSetOS.

Starting to Observe Daylight Saving Time

To start observing daylight saving time with 2021-03-28 03:00:00 UTC+02:00 you could keep the actual epoch seconds on the device and just set the new timezone offset at this point in time:

  • Epoch remains 1616893200 = 0x605fd510
  • TZO needs to be updated to 120 min = 0x78

This could be done with the BSM Tool as follows:

$ bsmtool set bsm/tzo=120

--> 2a 10 9d46 0001 02 0078 0eec
<-- 2a 10 9d46 0001 c9ab

Changing the timezone offset is considered setting the time and gets registered at EpochSetCnt and EpochSetOS.