Analogue input ADC

Hello everyone,

I have a question that follows me for a long time and I didnt't find an answer even by asking my older and much more experienced coleaques.

In the Siemens PLCs, when trying to scale an analogue input you have to select the limits of the numeric provided by ADC of the analogue input card as 0 to 27648.

My question is where does this number come from? I used before Bosch Rexroth PLCs and there the numeric that the analogue input card provided was 0 to 32768 or 0 to 65535 numbers that come from the resolution and number of bits of the analogue card 2^15, 2^16.
Here I am a little confused.

Thank you in advance and sorry if it is a silly question.

Here's how the 27648 is derived..

Hello Vlad;

This is a questioon which seems to perplex most beginners in S7 PLCs (and some experiienced users i have known) so don't feel alone.

There is a manual that gives an answer to your query: S7-300 Module Data, in chapter 4, Analog Modules (look for it on Siemens' website).

Basically, to answer your second question first (about resolution), Siemens has standardized all analog representation on 16 bits, and left-justifying the data from modules of lesser resolution; maybe this picure will explain better:


That way, all data is standardized on a single range (0-32767 for unipolar values, -32768 to 32767 for bipolar signals) that can be used to present all values from all its analog modules, independantly of the module's specific resolution. Here is how this works in percentage values:


The value 27648 is simply a percentage of the full range; it allows the display of analog values in terms of acceptable ran=ges, and also under/overflow status of the signal. Then, depending of the nature of the input signal, 4-20 mA, 0-10V, J-thermocouple, RTD..., the percentage is simply applied to the specific engineering range of the module.

There is a lot more information in the manual I suggested earlier; I really recommend you spend a few hours on that chapter.
Hope this helps,
Daniel Chartier