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2.4.3
Special Data Blocks
DB 0
DB 1
DX 0
DX 1
DX 2
CPU 948 Programming Guide
C79000-G8576-C848-04
On the CPU 948 data blocks DB 0, DB 1, DX 0, DX 1 and DX 2 are
reserved for special functions. They are managed by the system
program and you cannot use them freely for other functions.
• • Data block DB 0 (see Section 8.3.2)
Data block DB 0 contains the address list with the start addresses
of all blocks that are located in the data block RAM of the CPU.
The system program generates this address list during initialization
(following each POWER UP or OVERALL RESET) and it is up-
dated automatically when you use a programmer to change data
blocks or generate a new data block.
• • Data block DB 1 (see Section 10.1.6)
Data block DB 1 contains the list of digital inputs/outputs (P periphe-
ral with relative byte addresses from 0 to 127) and the interprocessor
communication (IPC) flag inputs and outputs that are assigned to the
CPU. If applicable, the block may also contain a timer field length.
DB 1 can have parameters assigned and be loaded as follows:
to reduce the cycle time in single processor operation, since
only the inputs, outputs or timers entered in DB1 are updated.
DB 1 must be assigned parameters and loaded as follows:
a) for multiprocessing
b) when IPC flags exist with CPs
• • Data block DX 0 (see Chapter 7)
If you assign parameters to data block DX 0 and load it, you can chan-
ge the defaults of certain system program functions (e.g. the start-up
procedure) and adapt the performance of the system program to your
particular application.
• • Data block DX 1
Reserved.
• • Data block DX 2
Reserved for the second serial interface.
Data Blocks
2 - 41
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