Siemens SINUMERIK 840D sl Function Manual page 1049

2.7.10 PLC machine data
General
The user has the option of storing PLCspecific machine data in the NCK. The user can then
process these machine data after the power-up of the PLC (OB 100). This enables, for
example, user options, machine expansion levels, machine configurations, etc., to be
implemented.
The interface for reading these data lies in the DB 20. However, DB20 is set up by the basic
program during powerup only when user machine data are used i.e. sum of GP parameters
"UDInt", "UDHex" and "UDReal" is greater than zero.
The size of the individual areas and thus also the total length of DB20
is set by PLC machine data:
MD14504 MAXNUM_USER_DATA_INT
MD14506 MAXNUM_USER_DATA_HEX
MD14508 MAXNUM_USER_DATA_FLOAT
is set and specified to the user in the GP parameters:
"UDInt", "UDHex" and "UDReal"
.
The data is stored in the DB 20 by the BP in the sequence:
Integer-MD, Hexa-Fields-MD, Real-MD.
The integer and real values are stored in DB 20 in S7 format.
Hexadecimal data are stored in DB20 in the order in which they are input (use as bit fields).
Figure 2-14
Basic logic functions: PLC Basic program powerline (P3 pl)
Function Manual, 11/2006, 6FC5397-0BP10-2BA0
DB 20
2.7 Structure and functions of the basic program
Detailed description
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