Siemens Simatic S7 Series Manual page 18

Putting it Together. The.user.memory.of.a.PLC,.such.as.the.S7-200.PLC.shown.in.
the.following.illustration,.includes.space.for.the.user.program.as.
well.as.addressable.memory.locations.for.storage.of.data..The.
amount.of.program.and.data.space.available.depends.on.the.
CPU.model.
User.program.space.stores.instructions.that.are.executed.
repetitively.as.part.of.the.PLC.scan..The.user.program.is.
developed.using.a.programming.device,.such.as.a.personal.
computer.(PC).with.programming.software,.then.loaded.into.the.
user.program.memory.of.the.PLC.
A.variety.of.addressable.memory.locations.are.used.for.storage.
of.data.that.is.available.to.the.user.program..Among.other.
things,.this.includes.memory.locations.for.variable.data,.discrete.
inputs.and.outputs,.analog.inputs.and.outputs,.timers,.counters,.
high-speed.counters,.etc.
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