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7.6
Memory Expansion to 3 Kbytes
The SPC 4-2 has an extra mode (can be set in mode register 3, see Section
7.1.11), in which the memory segment size of the internal RAM is increased from 8
to 16 bytes. With this mode, access to this RAM using the base pointer can
theoretically address up to 4 Kbytes instead of the previous maximum of 2 Kbytes
(although only 3 Kbytes are implemented.
Figure 7-19
When accessing data structures in this RAM in the new mode, it is therefore vital to
know how the data is shifted when there is a change to the 16-byte segment size.
The following rules apply to individual data structures:Data structures with 16 byte
segments
●
SAP List:
On the SPC4/SPC4-1 as well as on the SPC 4-2, this area begins at address
24 (018
words, the bytes of all 64 SAPs follow on one after the other without gaps. This
structure was selected because this memory area of the internal RAM can be
accessed directly without needing to use a base pointer.
●
Indication blocks:
Each indication block begins at a segment boundary. Before the data, it
contains the response header and the request header making up a total of 8
bytes. The actual data are located in the next free segment. As a result, when
using 16 byte segments on the SPC 4-2, there is a gap of 8 bytes between the
headers and the data that follows. The data itself is stored without gaps.
●
Reply-On-Indication blocks:
Each of these blocks must begin at a segment boundary. In addition to the
data, each of these blocks begins with 2 bytes of header information. In
contrast to the indication blocks, the data follow on without a gap.
PROFIBUS Controller SPC 4®-2 LF
C79000-G8976-C157-3
8 Bit Basepointer-Register (Segmentadresse)
+ 8 Bit Adressbus
= 12 Bit RAM-Adresse
Memory Expansion to 3 Kbytes
). The SAP list is therefore not structured segment-oriented; in other
h
ASIC Interface
87
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