Siemens MOBY FC 56, Technical Description

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Introduction
1-5
FC 56
J31069-D0155-U001-A0-7618
1.2.2 Configurable memory sizes
MDS units are available with memory chips (RAM/EEPROM) of various sizes.
When initialising (see section 4.2) the “filehandler” must be told what type of MDS
is involved so that the correct data management system can be created.
The following table shows how the filehandler structures the various types of MDS.
Table 1-1 Memory sizes which can be formatted
Typ
(Hex)
MDS-capacity
in bytes (gross)
Max. no. of files User data bytes
(nett)
Number of
blocks
Block size in
bytes
Enter this
parameter
when
initialising
Total MDS memory
capacity
Max. number of
files that can be
created
Max. no. of user
bytes that can be
written to the MDS
The product of no. of blocks and block
size gives the user data length (net) in
bytes
04H
84H
2.045
1.778
1
16
16
1.680
1.440
105
90
16
16
05H
85H
8.189
7.154
1
32
32
7.456
6.464
233
202
32
32
06H
86H
32.765
28.658
1
64
64
31.488
27.520
246
215
128
128
1 ...with ECC operation (MOBY I)
1.2.3 Check sum
To minimise the time it takes the SLG to process an MDS, a check sum (2 bytes
long) is maintained in the system area of the MDS. Use of the check sum
mechanism is
essential in dynamic read and write operations
(see section 7.1).
Each time the contents of the directory or the FAT are modified (execution of write
commands e.g. CREATE, WRITE, ATTRIB etc.), the filehandler calculates a check
sum on the two areas and places this in the MDS system area. If two consecutive
MDSs have the same check sum, their data structures (file names, file lengths
etc.) will be totally identical.
The filehandler does not therefore need to read the directory and FAT of the
second MDS (additional time requirement), but can continue to operate with the
current (internal to filehandler) parameters.
Note:
The check sum is created from the directory and FAT only, and not from the file
contents.