Why 27648 Analog Maximum Value in Siemens PLC???
Analog
module in Siemens convert voltage (or other) into integer 16-bit (word
data type), so it have 2^16 = 65536 count. Analog module in Siemens
comes with varies bit-resolution 8-15 .
So for max count (15 bit) it have 2^15=32768 count (0...32767). PLC always convert voltage (or other) into word value regardless
the resolution and its conversion is shown below:
Volt: 0....10 into
Count: 0.....27648
although the rated range is 0-10V (0-27648), PLC also have over-control range (27649-32511), and overflow (32767). Noted that 27648 is not analog maximum value, its the analog rated maximum value. The real maximum value is 32767 (overflow) -> 2^15. So if you give module's input 10.2V it will convert into value bigger than 27648 and not destruct the module (as long as it still below permissible input voltage). But i still don't know why Siemens choose 27648 not 27000 or other (maybe rated range 84% from its max range is good enough).
Resolution: Bigger resolution means smaller voltage which can be read by PLC. For example module with 15-bit resolution. It have 2^15 = 32768 (0-32767) it as same as max value mentioned before. So the smallest change is 32768/32768 =1 equal with 10/27648*1 = 0.00036169 V.
Example again if we have 12-bit resolution. It have 2^12=4096 count. So the smallest change is 32768/4096 = 8 equal with 10/27648*8 = 0.0028935 V
Volt: 0....10 into
Count: 0.....27648
although the rated range is 0-10V (0-27648), PLC also have over-control range (27649-32511), and overflow (32767). Noted that 27648 is not analog maximum value, its the analog rated maximum value. The real maximum value is 32767 (overflow) -> 2^15. So if you give module's input 10.2V it will convert into value bigger than 27648 and not destruct the module (as long as it still below permissible input voltage). But i still don't know why Siemens choose 27648 not 27000 or other (maybe rated range 84% from its max range is good enough).
Resolution: Bigger resolution means smaller voltage which can be read by PLC. For example module with 15-bit resolution. It have 2^15 = 32768 (0-32767) it as same as max value mentioned before. So the smallest change is 32768/32768 =1 equal with 10/27648*1 = 0.00036169 V.
Example again if we have 12-bit resolution. It have 2^12=4096 count. So the smallest change is 32768/4096 = 8 equal with 10/27648*8 = 0.0028935 V
Thank you.
ReplyDeleteI'm getting the 32767 value as the max value when i connect my 4-20 mA transmitter to the AI of S7-400. But the issue now is the value 32767 remains even when i disconnect the transmitter. Is that normal?
Yes. It's normal. It's call wire break. CPU can't process the input.
Deleteplease can any1 tell me when an overflow or underrange can gotten from our analogue input? Thanks
Deleteand the next question is why is 2 used to raise the power of 16 and 15? and why is that 2^15 is the maximum range used since it has been said that analogue module convert digital to 16bits(a word)? please kindly reach me on soughtout336@gmail.com
Deleteplease can any1 tell me when an overflow or underrange can gotten from our analogue input? Thanks
ReplyDeleteand the next question is why is 2 used to raise the power of 16 and 15? and why is that 2^15 is the maximum range used since it has been said that analogue module convert digital to 16bits(a word)? please kindly reach me on soughtout336@gmail.com
ReplyDelete