Siemens SINUMERIK 828D Function Manual page 31

The array elements as of index 6 contain the user data:
Index
Note
Overwriting of user data
Because of the ring buffer structure, the oldest user data is overwritten as soon as all free
array elements of a $AC_FIFO variable have been assigned.
Configuration
Number per channel
Number of array elements
The maximum number of array elements per $AC_FIFO variable can be set via the machine
data:
MD28264 $MC_LEN_AC_FIFO = <maximum number of array elements>
Start of $AC_FIFO variable range
The R parameter as of which the $AC_FIFO variable range is to start, can be set via the
machine data:
MD28262 $MC_START_AC_FIFO = <number of the start R parameter>
R parameters above the start R parameter cannot be written in the part program.
Synchronized actions
Function Manual, 07/2012, 6FC5397-5BP40-3BA0
Meaning
6 Write/read: The 1st array element for user data is addressed
7 Write/read: The 2nd array element for user data is addressed
n Write/read: The nth array element for user data is addressed
2.3 System variables for synchronized actions
Detailed description
31