Analog-Value Representation Of The Nck Analog Input/Output Values - Siemens SINUMERIK 840D sl Function Manual

1.2.6

Analog-value representation of the NCK analog input/output values

Conversion of analog values
The analog values are only processed by the NCU in a digital form.
Analog input modules convert the analog process signal into a digital value.
Analog output modules convert the digital output value into an analog value.
Analog-value representation
The digitized analog value is identical to input and output values with the same rating range
(e.g., voltage range ±10 V DC).
The analog values are coded in the PLC interface as fixed-point numbers (16 bits including
sign) in the two's complement (see table below).
Table 1-1 Digital coding of analog values at the PLC interface
Resolution
Bit number
Significance of the bits
Sign
The sign (SG) of the analog value is always located in bit 15.
Applies to SG: "0" → +
Resolution less than 15 bits
The analog value can be finely adjusted depending on the resolution of the digital-to-analog
converter.
If the resolution of the analog module is less than 15 bits, the analog value is entered left-
justified. The free least significant positions are filled with zeroes.
Using a bit pattern, the table below illustrates how zeroes are written to the free bit positions
for a 14-bit and a 12-bit analog value.
With a resolution of 14 bits (including sign), the minimum increment is 1.22 mV (10 V: 8192).
In this case, both least significant bits of the analog value (bit 0 and bit 1) are always 0.
With a resolution of 12 bits (including sign), the increment is 4.8 mV (10 V: 2048). Bits 0 to 3
are always 0.
Extended Functions
Function Manual, 01/2008, 6FC5397-1BP10-3BA0
Binary analog value
High byte
15 14 13 12 11
SG 2 2 2 2
14 13 12
"1" → -
Low byte
10 9 8 7
2 2 2 2
11 10 9 8 7
Digital and analog NCK I/Os (A4)
1.2 NCK I/O via PLC
6 5 4 3
2 2 2 2
6 5 4 3
2 1 0
2 2 2
2 1 0
49