Siemens SIMATIC NET SPC 4-2 LF, Manual

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PROFIBUS Controller SPC 4®-2 LF
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C79000-G8976-C157-3
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-Value
Access byte (4) := RUP-N-Valid
Access byte (5) := IND-N-Valid
Access byte (6) := IND-N-Cleared
Access byte (7) := IND-U-Cleared
Bit Position Meaning
7 6 5 4 3 2 1 0
IND-U-Cleared IND-
N-
Clear
ed
IND-
N
Valid RUP-
N
Valid Access Value Access-Byte
Bits 0-
3 Access value
Bit 4 RUP-N-Valid (only with DEFAULT-SAP)
Bit 5 IND-N-Valid (only with DEFAULT-SAP)
Bit 6 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.
Bit 7 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.
Table 7-18 : Fail-safe Mode