Set/ Rst Problem In S7 - 200

Hi!

I am not a regular with Siemens S7-200. Currently I am doing a project with S7-224 & analog outputs.

While testing my program on line I observed following feature-

I have put the mode selector on RUN mode & can STOP or RUN the execution thru the PC( ie remotely ). In the program if I set
flag (V0.0) & turn on output Q0.0 from V0.0 & then put the CPU in RUN so that V0.0 is SET & Q0.0 is on. Now I put the CPU in STOP mode thru PC , I observe that only Q0.0 is turned OFF BUT the
flag V0.0 is still SET in STOP mode. This seems to be quite different than other PLCs I have used where all the devices ( except retentive ) are RST in STOP mode.

In CPU 224 I have not configured any device to be retentive ( In System Block , I have made all selections as 0 - 0 for all 6 retentive range selection & downloaded this to the PLC )

According to the HELP menu the non retentive flags are not RESET in STOP mode but only if programmed for reset or at POWER OFF.

I would like to know the logic behind this feature as already retentive range selection sufficient. Moreover such kind of NON RESETING flags can play havoc during ON SITE edits & tryouts!

Any explanation welcome !

Thanking all of you in advance.

Regards

reachasp

I'm not really sure what you are asking.

SET and RESET flags in many PLC's are retentive, often even through power cycling. That is one reason that SET/RESET instructions should be used with great care.

When in Stop mode, the outputs are <usually> hard disabled, but that doesn't always have to be the case, but internal bits, words, data blocks (in Siemens) don't re-initialize themselves.

In Siemens S7-xxx, the selelection in CPU config for 'Retentive Data Areas' only applies to what areas should be maintained by battery when the system is powered off.

The 'logic' behind this is things such as memory words, or data blocks are often used to hold things like calibration values, or operator setpoints, and the like. It wouldn't be very good if all the calibration values for a machine were lost every time the PLC was put in stop, or powered off.

The CPU really doesn't know the difference between bits (bools), words, ints, dwords, reals, etc...a bool can be a directly addressed bit in something defined as a real in a data block. It can't therefore automagically clear things selectively.