Siemens SINUMERIK Series Function Manual page 47

Comparator signals as digital NCK inputs
All NC functions that are processed as a function of digital NCK inputs can also be controlled
by the signal states of the comparators. The byte address for comparator byte 1 (HW byte
128) or 2 (HW byte 129) must be entered in the machine data associated with the NC
function ("Assignment of hardware byte used").
Example:
"Multiple feedrates in one block" NC function
Entry in channel-specific machine data:
MD21220 $MC_MULTFEED_ASSIGN_FASTIN = 129
This activates various feedrate values as a function of the status of comparator byte 2.
Functional sequence
The functional sequence for comparator input byte 1 is represented schematically in the
following figure.
Extended Functions
Function Manual, 03/2013, 6FC5397-1BP40-3BA1
A4: Digital and analog NCK I/Os for SINUMERIK 840D sl
1.2 Access via PLC
47