PLC power supply issue

Greetings everyone,

I'm making a PLC program module that will handle some very important sequences. PLC is one from Siemens S7-300 familly (don't know exactly since I'm creating algorithm, not exactly code specific to some PLC's manufacturer). However, since sequences are important and realised with timer and counter mostly, I wonder what would happen if there is a power lost, even for 1 or 2 sec? Does this depend on power supply unit of PLC or CPU itself? I belive that PLC's program will reset counters and timers. I need to be more specific about this and decide should I anticipate this case.
I know that there are a lot of people here who are working with S7 300 PLCs all time.

Is PLC is going to lose counter and timer data if power supply is lost i.e. should I use some kind of UPS or similar?

Thank you

Siemens do a very good 24v UPS unit I have one fitted in a pumping station, keeps the power to the PLC and ultrasonic level controller (a strans LU on profibus PA) up for at least 6 hours. You need the UPS unit and a seperate battery pack.

UPS part number 6EP1931-2DC21

Also save data between power cycles by writing it into a data block.
Use retentive timers
Configure some non-volatile markers i.e. M0.0 to M0.8 etc.
instead of using a s5 counter increment a data block value.

Data blocks are non volatile.

Thank you for the reply JohnW,
just to summarize:
when left without power Siemens PLC will reset counters and timers and start sequence from the start when power is restored, except if retentive timers and incremental datablock values are used, right?

I assume that similar sitation is with AB or some other manufacturer's PLC...

not all PLCs behave the same, but generally timers and counters reset on a power cycle unless specified as "retentive".

For a sequencer, designate a non volatle word as "sequence count" and then after a power cycle the sequencer can re-start from the same place.

In ABs if you implicitly set a bit then it will stay set after a power cycle (or restart).

Can't remember what happens with Mitsubishi but I thnik that is the same.

Siemens S7-300, all bits reset on start unless designated as "non-voaltile"

There are others in this forum that know a lot more than I do and will enlighten you further in due course, I expect.