Siemens S7/TIA v18: When the PLC's unplugged, will it retain wall clock time...?

[Mas01]

The PLC in question is a S7-1212C.

The PLC will be transportable on a wheeled trolley and used wherever it's needed in the factory.

I've been told that when it's not being used, it will be unplugged and kept in a store room until needed. Typically 3-4 months.

QUESTION: Will the PLC retain wall-clock time after that period of time?

Maybe I should just allow the operator to set the time when it's next powered up.

Thanks

 

[Puddle]

I wouldn't have thought so. There are batteries available to maintain the clock, but I don't if it would last for months.

You can set the time via HMI relatively easily, although setting the date is quite a frustrating process.

Could always set the time via NTP and just tell them to plug it into a network on start up.

 

[Mas01]

Cheers - what do you mean by 'set the time via NTP'?
Sorry, I don't understand that.

 

[Mas01]

EDIT: I think I understand.

I'm not sure if this PLC will be allowed to be plugged into a network, but I'll ask.

 

[Puddle]

Just checked the manual, the battery board (BB 1297) should last for a year. It specifies a CR1025 battery though, which aren't rechargeable.

 

[Mas01]

Just checked the manual, the battery board (BB 1297) should last for a year. It specifies a CR1025 battery though, which aren't rechargeable.

Many thanks for this.

 

[JesperMP]

I don't know what the default values the clock will revert to, but I would check if the year has reverted to the default value (i.e. 1995 or whatever) and in that case prompt the operator to set the clock via the HMI.

 

[Mas01]

I don't know what the default values the clock will revert to, but I would check if the year has reverted to the default value (i.e. 1995 or whatever) and in that case prompt the operator to set the clock via the HMI.

I like it - that's not a bad idea.

Default year is 1990 - so that could be used to prompt the operator.

 

[JesperMP]

Thinking about it, I suggest that anything less than 2023 should prompt the operator to set the clock.

 

[Mas01]

Thinking about it, I suggest that anything less than 2023 should prompt the operator to set the clock.

Makes sense.

 

[Lare]

There is two different situations you need to take account, time accuracy and backup time, both are sayed on manual.



Time of day clock
The CPU supports a time-of-day clock. A super-capacitor supplies the energy required to keep the
clock running during times when the CPU is powered down. The super-capacitor charges while
the CPU has power. After the CPU has been powered up at least 24 hours, then the super-
capacitor has sufficient charge to keep the clock running for typically 20 days.


20 days typ./12 days min. at 40 °C (maintenance-free Super Capacitor)





Real time clock accuracy
+/- 60 seconds/month.




Even whit external battery or powered on clock can have several minuts difference after couple months.

 

[Mas01]

Real time clock accuracy
+/- 60 seconds/month.

That's when it's powered 24/7?
If so, that's really poor.

 

[Lare]

That's when it's powered 24/7?
If so, that's really poor.

Yes. PLCs don't have accurate clock oscilators.
Not sure why when even cheapest watch can keep time.

 

[mk42]

That's when it's powered 24/7?
If so, that's really poor.

I would say "eh it's the 1200" but, honestly, the 1500 is similar.


I was going to say the built in cap that keeps the clock going lasts more than a day but less than a month, and that tracks with the 480hr (20 days) in the spec.


I did a couple projects where the time really mattered (had to calculate the position of the sun for solar trackers) and we used GPS clocks to sync to out in the middle of where ever. I doubt that complexity is worth it for a an infrequently used skid.


The battery SB seems like the best idea if you aren't already using the SB slot, but make sure you change them proactively. Otherwise, the fallback of "make the operator type it in when you roll it out" isn't the worst, but I'm not sure what you need the time for anyway. Most of the time I want good time for the alarms, and only care on general principles otherwise. Is there an HMI that can keep the time for you, and then you don't need to care about the PLC as much?

 

[JesperMP]

Is there an HMI that can keep the time for you, and then you don't need to care about the PLC as much?

Siemens HMIs can be the master of the PLC clock.
But the HMI clock is just as 'accurate' as the PLC clock.


The bit triggered alarms on the HMI uses the HMI clock.
The diagnostics buffer in the PLC uses the PLC clock.
If the clocks must match, then it could make sense to sync them.