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4.2.5
Addressing Using the Memory Window
When addressing the SPC 4-2, the physical address of the integrated RAM is
formed via the second address window (200h to 2FFh) from the base pointer, the
segment address for the indication queue and the lower 8 bits of the address bus.
The base pointer is added to the address of the address that is shifted by three or
four bit positions, see Figure 4-5.
In the extended SPC 4-2 mode, the base pointer is shifted by 4 bits instead of by 3
bits to the left so that the segment size is then 16 bytes instead of 8 bytes.
New segment address = base pointer + AB7..3 - end pointer + begin pointer
8-byte segment in the compatibility mode
New segment address = base pointer + AB7..4 - end pointer + begin pointer
16-byte segment in the SPC 4-2 mode
Along with the 3 or 4 least significant address bits, the result forms the physical 11-
bit or 12-bit address for the internal RAM. With the calculator, the FLC can address
up to 256 data bytes once the base pointer is loaded without having to reload the
base pointer or worry about the wrapping at the queue limit.
Figure 4-5
26
Base point register
Address bus bits 7 ... 0
A10
11-bit RAM address
Calculating the Physical RAM Address in the Compatibility Mode
A3
Byte address within
a segment
PROFIBUS Controller SPC 4®-2 LF
C79000-G8976-C157-3
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