Newbie with S7 300

Guys please explain this to me, can't seem to find any SIEMENS literature to disprove or support what I am seeing. I am using the timer below.


As I am new ot this stuff I am just playing around, I put the time value in MW1, when I look at a variable table and mointor MW1 and MW2, I see the value decrementing in MW1, but MW2 is also counting...something! All the literature I have read indicates Timers only occupy 1 word, whats going on here?

My guess:


thats right,
1 word = 2 bytes

mw1= two bytes: mb1 and mb 2
mw2= two bytes: mb 2 and mb 3

so when you put the time into mw1, you put the values into mb1 and mb2, and since mb2 is the part of the mw2, you see what you see.

wouldnt mw2=two bytes: mb3 and mb4?

check out this example from plcsim
'right' byte of the mw1 = 'left' byte of the mw2, because it's the same byte - mb2



It's so in S7.

MwX = MbX and Mb(x+1)



mw2 would be mb3 and mb4 in some other software like CoDeSys for example

and I believe there is a same story with double words: md4 would be mw4 + mw6, i.e. mb4+mb5+mb6+mb7

When identifying a memory address in S7 there are three things to take in to account -

A) The first letter (or first two letters) tell you where the memory address is located : M, I, Q, DB

B) The final letter indicates how much memory is being addressed on this occasion: B, W, D for byte, word or double-word (i.e. 1, 2 or 4 bytes in length)

C) The number specifies the starting byte address of the data.

Example 1 : DBD24 is in a data block, it's a double-word length address, and it starts at byte 24. This means it uses bytes 24, 25, 26 and 27.

Example 2 : MW1 is in marker memory, is one word long and starts at byte 1. Therefore it uses bytes 1 and 2

Beware overlapping addresses -
MB49, MW48 and MD46 will all affect the same byte number 49

Regards

Ken

Thanks so much for all your help, its as clear as mud now! LOL Just kidding, I understand now but this very different than many other PLC's, when in Rome...

Thanks Again

Surely you mean "when in Munich ..."!

Ken

Right you are Ken, when in Munich indeed, although after playing with this PLC I feel like I'm in the land of Oz!!!