Step7

Is OB1 the fundamental block in a program that is used to call other functions back to it. Then Function blocks used to perform the tasks of the program and data blocks to store the information for the program. what other blocks exist.
Can anyone verify this as correct? I have the siemens manuals but they tend to tell you how rather why on most things.
Tom

OB1 is invoked automatically by the firmware. It can be used to call other program blocks.
In Step7, you can have FC (functions) and FB (function blocks).
As a rough guide, I use FCs for general this and that, while FBs can be used to perform a more complex commonly repeated task.
Both, however, can be parameterised i.e. the code can be used again and again and 'slotting in' parameters, the difference with an FB is that it can have a DB (data block) associated to it so the data is stored. This is called an Instance data block.
An FCs local data (as it's called) is valid for that one execution of the block only.

Data blocks exist without being tied to an FB, and as you say can be used to store info.