PLC ladder programs are processed left to right, top to bottom, this needs to be understood by PLC programmers so all intended operations occur. It also needs to be understood when the HMI writes to the PLC, generally this is in between each scan of the program but not with all PLC/HMI systemsThat is something to keep in your mind very well: all lines in the ladder logic plc program act like they are happening together and at once.
So don't think: it first does rung/line 1, then after that the line below that.
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by Dale Wilson