In the hardware configuration for the PLC, there are "system memory bits", one of which is the first scan bit.

Typically the default is M0 and M1 bits.  If you did not set this up and already used M0 and M1, you have a choice.  Change the existing tags to new M ranges, or make the system memory bits non-default.  The choice is yours. 

aliaslam7163

Hey guys, really new to plc programming, coming from web engineering background, iam stuck on how the step 7 s7-1500 works. My previous project had me using rslogix5000 where the program itself internally provided a first scan bit. However step 7 does not and it provides a startup block. When i create a OB100 block, it is completely empty meaning I have to code it myself. Can anyone provide me with some help on what this code will do? Really stuck here.

Standard things to perform in OB100 may be things that establish a known system state at startup.  If you have any latched (set) bits, you may choose to unlatch (reset) them. If you have connection status for various devices, you may want to force the connection to re-establish.

If you have process status that moves through various values or bits, you may want to set these values to a known startup state.

You may also want to interrogate the I/O to see the physical state of your system and set appropriate startup status within your program logic.

At startup, there are many reasons from power outage, bad logic causes system stuck in bad state (what you find during commissioning and warranty period), downloading logic updates, normal startup, and restart after E-Stop.

Each situation often requires specific and sometimes different startup considerations. 

For example, for a power outage, you need to wait for drives to get stable.  In that case, perhaps signify a delay for drive startup.  If an E-Stop occurred, you may require certain manual operations be performed prior to automatic mode being allowed.

These all depend on your process and your level of automatic recovery and error handling. 

Duris_I

Hello,

you can imagine that OB100 like f.ex. in one Form application - Form Load Event (translated to your previous experiences), if you have to init some variables to start values, this is right place for it.

Duris_I

Hello Huggy,

thnx for this info. Yes, absolutelly agree with you, all depends in prj logic, some bad SET/RESET f.ex. can cause big problem. But this is programmer's side to guard his logic and what he put to OB100. But your info is really helpful to all.

OFF TOPIC

It would have been nice if there was some guidelines put down that could guide program designers as to the format of handling some standard tasks. But if major brands cannot agree even on a standard for industrial communication.. how then for setting guidelines for coding structures and methodology.

w