1756-HSC and storing the present value

[Gary Burton 0]

When the PLC loses power the counter is reset to "0". Any idea on how to retain the value? I know you cannot write to the present value tag... The manual is not helpfull at all. I tried "store and continue" mode but no dice. Basically I'm trying to remember a position when the PLC loses power.

[Ken Moore 23]

Just thinking out loud, so this may not be a viable option. Each scan, record the present value to a scratch register. Then on the first scan of a restart, move the scratch value to the the preset register, then reset the card, then clear the preset back to it's original value. The card should start counting at the new value from the reset. You will lose the counts that occur between scans, but better than nothing.

[Contr_Conn 4]

You probably need an absolute encoder, like SSI type with 1734-SSI module Ken: you will loose count between 2 PRIs as well. Counter sends input data based on RPI. So if PRI is 20ms and scan time is 5, then 25ms of pulses will be lost. Edited 17 Sep 2006 by Contr_Conn

[Gary Burton 0]

If "store and continue" is selected as the storage mode, does the controller automatically store the current position in the preset register?

[Contr_Conn 4]

No, store command saves it to the internal HSC memory that has no battery backup. Cycling the power will clear it. And even it would have a non-volatile memory, you will not store every pulse, so stored value will be always off. Controller input tags don't save anything as well, they are direct reflection of HSC memory with delay equal to RPI. You can reat these tags in periodic task with rate = RPI, but you still have an error because of up to 2xPRI delay. Edited 17 Sep 2006 by Contr_Conn