Why 27648 Analog Maximum Value in Siemens PLC?

Analog module in Siemens PLC 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

Author: Ashu