Siemens SIMATIC NET SPC 4-2 LF, Manual

Page: 28 / 178

PROFIBUS Controller SPC 4®-2 LF
28
C79000-G8976-C157-3
4.3 Assignment of the Parameter Registers
It is only possible to access the internal parameter registers (in other words, the
memory cells that intervene directly in the controller) using the address window
300h to 3FFh on the SPC 4-2.
These cells can either only be read or only be written and have different functions.
In the Motorola mode, the SPC 4-2 swaps addresses for access to the address
range starting at 300H (word register); in other words, it swaps the address bit 0
(generates an odd address from an even address and vice versa).
The SPC 4-2 has an 8-bit data interface. When the byte registers are accessed via
this interface, it does not matter whether the SPC 4-2 is in the Intel or in the
Motorola mode.
When the word registers are accessed (two-byte registers), the SPC 4-2
distinguishes between the Intel and Motorola mode.
Example: INT-MASK-REG
Intel mode: Write access with address 300
⇒
INT-MASK-REG (7..0) is written (little endian)
Motorola mode: Write access with address 300
⇒
INT-MASK-REG (15..8) is written (big endian)
This means that the high byte and low byte are addressed with different addresses.
The meaning of the individual bits in the registers is explained in greater detail in
the following sections.
Since the addresses of the parameter registers are not fully coded, the registers
appear again every 64 bytes. This simplifies the implementation since different
addresses (names) can be assigned for read and write access.