fdag1

Hi,

How is logo executing a program.

How can we know in whitch order instruction blocks are executed.

I have not seen a proper explanation of how the LOGO executes it's blocks so I have developed this based on observation and testing.

Here are my rules for block execution:

1. Blocks are examined (not executed) from lowest to highest repeatedly until all blocks execute.

2. A block will not execute if any of the blocks connected to it's INPUTS have not been executed and will only execute once per cycle.

3. PARAMETERS are updated immediately when a block executes but they do not do not affect execution order.

When you are referencing parameters this explains why sometimes it does not execute things in the order you want.

Toeternietoe,

We are in agreement.

When I say block input from another block I am referring to a connection from one block to the input connection on another block.  I am not referring to an input from an input register.

My rules apply to your step 2.

Let me explain by example:

Let's say we have 3 blocks (Blocks must be numbered as shown):

  Block 001 is a Mathematical Instruction block with V1=5 

**IMPORTANT: Set OUTPUT   When En="0", output is:  Last Value.  (It will not work without this.)

  Block 002 is an AND(EDGE) (This allows us to simulate a single cycle)

  Block 003 is a Mathematical Instruction block with V2=10

**IMPORTANT: Set OUTPUT   When En="0", output is:  Last Value.  (It will not work without this.)

We connect them as follows:

  Input I1 is connected to the first input of Block 002

  The input EN of Block 001 is connected to the output of Block 002

  The input EN of Block 003 is also connected to the output of Block 002

  The P1 PARAMETER of Block 003 is connected to the Aq PARAMETER of Block 001

  The output of Block 001 is connected to AM1

  The output of Block 003 is connected to AM2

Run the following test:

  1. Start the simulator and press the power button to reset all blocks to 0

  2. Turn Input 1 on then off.

AM1 reads 5 and AM2 reads 10.  On Block 003 P1 is 5, P2 is 10 and Aq is 10.

The reason it calculates this way can be explained by my rules.

It goes like this:

  Block 001 is examined and does not execute because 002 has not executed.

  Block 002 is examined and executes because it is only attached to Input registers.  The output goes HI

  Block 003 is examined and executes because 002 has already executed and it's output is HI.  Since Parameter Aq of Block 001 is still 0, Block 003 adds 0 and 10 and gets 10.

  Since this is the last block the LOGO starts examining again starting with 001.

  Block 001 is examined and executes because 002 has already executed and it's output is HI.  Block 001 adds 5 and gets 5.

  Since all blocks have been executed, the cycle is ended.

To further test the rules, change the program as follows:

  Create a Block 004 AND(EDGE)

  Delete Block 002

  Make the following connections:

     Input I1 is connected to the first input of Block 004

     The input EN of Block 001 is connected to the output of Block 004

     The input EN of Block 003 is also connected to the output of Block 004

Run the following test:

  1. Start the simulator and press the power button to reset all blocks to 0

  2. Turn Input 1 on then off.

AM1 reads 5 and AM2 reads 15.  On Block 003 P1 is 5, P2 is 10 and Aq is 15.

  This is because Blocks 002 and 003 did not execute on the first examination because Block 004 had not executed yet.  Block 004 did execute on the first examination right after 003 was examined.

On the second examination 002 executed and then 003 executed because 004 already was executed on the first examination.

These rules can be used to predict how the blocks will execute.  The LOGO probably uses a more sophisticated approach to actually process the blocks but this is how they behave.

I noticed that in my post above I forgot to say that the Mathematical Instruction Blocks need the setting "OUTPUT When En="0", output is:" set to "Last Value".  The example will not work without this setting.

I have modified the post to include these instructions.  Sorry about that if you tried to run it and it did not work for you.