Command Chaining - Siemens SIMATIC FC 45 Function Manual

3.2.2

Command chaining

Command chaining permits various address areas of the MDS to be processed by starting
just one command. The advantage of command chaining is the optimum speed at which
commands can be processed on the ASM.
A command chain is set up by the user by storing a succession of UDT 20s in a DB. The
chained commands must all be command type 4x. The last command in a chain must be
type 0x.
The FC 45 uses this to detect the end of a command chain.
Example:
An MDS is to process 4 data records. The command structure is stored in DB 47. The MDS
data are stored consecutively in DB 48.
Read
Read
Read
Write
Execution steps:
1. Define structure
Table 3-15
Partial command
Partial command
Partial command
Partial command
2. Specify the "starting values" in the data view
FC 45
Function manual, Release 03/2006 , J31069-D0167-U001-A2-7618
MDS address
MDS address
MDS address
MDS address
DB 47 - declaration view
STRUCT
ARRAY [1 ... 4]
UDT 20
END_STRUCT
0000 hex Length 600
1000 hex Length 100
1200 hex Length 1
1200 hex Length 1
Parameterizing
3.2 MOBY commands
Note:
1 ... 4 = No. of partial commands
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