Fail-Safe Mode - Siemens SIMATIC NET SPC 4-2 LF Manual

7.2

Fail-safe Mode

An additional mode register bit "Spec-Clear-Mode" (bit 5 in mode register 2),
enables the fail-safe mode. The fail-safe mode is enabled by setting "Spec-Clear-
Mode = 1".
The "IND-N-Cleared" and "IND-U-Cleared" bits are added to the access byte of the
default SAP:
Access byte( 3..0)
Access byte (4)
Access byte (5)
Access byte (6)
Access byte (7)
IND-U-Cleared
Bits 0-
3
Bit 4
Bit 5
Bit 6
Bit 7
Table 7-18 : Fail-safe Mode
82
:= Access-Value
:= RUP-N-Valid
:= IND-N-Valid
:= IND-N-Cleared
:= IND-U-Cleared
Bit Position
7 6 5
IND- IND-
N- N
Clear
Valid
ed
Access value
RUP-N-Valid (only with DEFAULT-SAP)
IND-N-Valid (only with DEFAULT-SAP)
IND-N-Cleared (only with DEFAULT-SAP)
The "IND-N-Cleared" bit can only be evaluated when "IND-N-Valid = 1" is set.
If "IND-N-Valid = 1" is set, then:
The output data in indication buffer N can be adopted.
The output data in indication buffer N must be replaced by the corresponding clear
coding. The last received output data are entered in indication buffer N.
IND-U-Cleared (only with DEFAULT-SAP)
The "IND-U-Cleared" bit must be set by the user on entering the fail-safe status and
reset when the fail-safe status is exited. The SPC 4-2 does not modify this bit. The SPC
4-2 does, however, scan the bit when "Spec-Clear-Mode=1" is set and a "Read-Output-
Data" frame is received in the DP mode.
If the application is not in the fail-safe state; in other words the application uses the
data of indication buffer U as the output data. If "Spec-Clear-Mode = 1" is set, the
user must reset "IND-U-Cleared=0" to update the output data when "IND-N-Valid=1"
and "IND-N-Cleared=0" are set.
If the application is in the fail-safe state; in other words the application uses the clear
coding as output data. The data in indication buffer U are then invalid. If "Spec-Clear-
Mode = 1" is set, the user must set "IND-U-Cleared=1" to update the output data
when "IND-N-Valid=1" and "IND-N-Cleared=1" are set.
4 3 2 1
RUP- Access Value
N
Valid
PROFIBUS Controller SPC 4®-2 LF
Meaning
0
Access-Byte
C79000-G8976-C157-3