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function block is creating some illogical problem !

Created by: naviid at: 6/6/2009 9:22 PM (4 Replies)

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6/6/2009 9:22 PM	Rate (0)
naviid Posts: 10 Rating: (0)	hello, i have attached a jpg image of the problem i m facing . i have made a fc block for extracting the LSB bit of a number(eg. the LSB of 3 is 1 in binary) , but when i m calling the block it is creating the attached problem.plaease tell me that wat may be the mistake or wat may be the problem?? please help me out of it ! and one thing i mention that i have atached the simulation of the block. and one more thing that i have called this function multiple times in the ob1. on first call it is working perfectly but on the second and third call it is creating the problem... thnaking in anticipation. Attachment fc block.JPG (153 Downloads)
naviid Posts: 10 Rating: (0)
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6/6/2009 10:19 PM	Rate (1)
Sydney Gold Member Joined: 1/4/2008 Last visit: 9/2/2024 Posts: 703 Rating: (96)	Hello, a few points about your block: - it isn't function block (FB), but the function (FC) - you loose the LSB at first instruction while shifting - I haven't tried to think what is the result of that code, but probably you should read some manual about programming of S7, addressing is always with byte number so if you use the MW104 it doesn't mean the word number 104 but word from address 104 so itaffects byte 104 and byte 105. - what have you assigned as actual parameter for the input"number1" in OB1? Isn't it some memory area from the address of 104? - for this type of comparison is better to use the STL logic, for the evaluating of bit use the bitwise AND instruction with desired mask and then read the CC1 from the status word. It's more simple, for example for the LSB you can use this code: L MW 104 L W#16#1 AW A >0 = #result // write result If you want to specify which bit you like to check, the code could be: L 3 // the third bit (bit 2) L MW 104// number you want to test SRW// shifts desired bit to CC1 A >0// test CC1=1 = #result // write result Regards Sydney
	Last edited by: Sydney at: 6/6/2009 10:23 PM
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6/9/2009 6:20 AM	Rate (0)
Prats Posts: 272 Rating: (4)	Hi, i think you should not usr shift right as the LSB will get lost and will be replaced by zero, and this is happeneing in your program. please use shift lfet and then mask it. you will get your answer.
Prats Posts: 272 Rating: (4)
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6/9/2009 1:11 PM	Rate (0)
Sydney Gold Member Joined: 1/4/2008 Last visit: 9/2/2024 Posts: 703 Rating: (96)	Hello, as I wrote before, it will loose the LSB within the shift right instruction, but it's replaced by the second bit, not zero. Why shift left? Isn't it enough to mask directly? Regards Sydney

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6/9/2009 3:09 PM	Rate (0)
Niranjan Prasad Posts: 691 Rating: (37)	You can go with modulus function i.e. number mod 2 (number %2) L #number L 2 mod T #ret_val just trial
Niranjan Prasad Posts: 691 Rating: (37)	Last edited by: Niranjan at: 6/12/2009 8:49 AM
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