Programmatically Enable/Disable DP Slaves with SFC12 D_ACT_DP on S7-300/400

This reference details how to use the SIMATIC SFC12 D_ACT_DP instruction to dynamically activate and deactivate configured PROFIBUS DP slaves from the S7 user program. It is written for engineers who must deploy a single standard program that supports many DP slaves but only commissions a fraction of them per project, and who must avoid spurious Report System Error entries when slaves are not connected at startup.

The behavior described in this document was verified on a 6ES7 315-2FH10-0AB0 CPU running firmware V2.3.3 and was cross-checked against the 6ES7 315-2FH13-0AB0 firmware V2.5.1 release notes.

1. Problem Statement

When a STEP 7 hardware configuration (SDB container) contains more DP slaves than are physically present, the CPU raises the following symptoms at startup or during operation:

• Bus fault (BF) LED on the CPU/master
• Report System Error entries for the missing station
• Diagnostic interrupts (OB82) repeating each power cycle
• WinCC alarm log flooded with slave failure messages

The objective is to mark configured slaves as logically disconnected in the S7 user program so that the diagnostic state is recognized as a deliberate operator action rather than a network fault.

2. Prerequisites

3. SFC12 D_ACT_DP Technical Reference

The D_ACT_DP instruction is documented in the Siemens function block help and in the Siemens Support entry 105020938. The instruction signature is identical in STEP 7 Professional V13.1 onward (see Siemens Support entry 109011420):

3.1 MODE Byte Encoding

4. LADDR Parameter Rules

Three rules govern the LADDR input. Misapplication of any of these is the dominant cause of SFC12 timeouts in field installations.

1. Data type: The parameter must be of type WORD. Passing an INT constant (for example 100) is rejected at compile time or returns error W#16#8090 at runtime.
2. Format: The value must be expressed in hexadecimal. A diagnostic address of 100 decimal must be entered as W#16#0064.
3. Address selection: The address may be either:
   • The diagnostic address of the DP slave (slot 0 in HW Config), or
   • The I/O address of any module in the station.

5. LADDR Lookup Using SFC5 (GADR_LDB)

The PROFIBUS address (1..125) is not the same as the logical address. To obtain the correct LADDR from a PROFIBUS node number, call SFC5 GADR_LDB first:

Use SFC5 to translate a station's slot-0 diagnostic address into a PROFIBUS address when building an HMI-driven selection list, or in reverse when commissioning a station from its physical PROFIBUS node number.

6. SCL Implementation Example

The following SCL (Structured Control Language) block wraps SFC12 with a state machine, idempotency, and Report System Error suppression:

FUNCTION_BLOCK FB1000 "DP_Slave_Control"
VAR_INPUT
    i_Enable  : BOOL;        // 1 = activate, 0 = deactivate
    i_LADDR   : WORD;        // W#16#nnnn (hex)
    i_Req     : BOOL;        // rising edge to execute
END_VAR
VAR_OUTPUT
    o_RetVal  : INT;         // SFC12 return value
    o_Busy    : BOOL;        // SFC12 BUSY
    o_Status  : BYTE;        // 0=inactive, 1=active, 2=unknown
    o_Error   : BOOL;        // 1 = last call returned error
END_VAR
VAR
    sfc12_BUSY : BOOL;
    sfc12_RET  : INT;
    mode_set   : BYTE;
END_VAR
BEGIN
    o_Error := FALSE;

    IF i_Req THEN
        IF i_Enable THEN
            mode_set := B#16#02;  // activate
        ELSE
            mode_set := B#16#01;  // deactivate
        END_IF;

        // Only one SFC12 job per slave at a time
        IF NOT sfc12_BUSY THEN
            D_ACT_DP(
                REQ    := TRUE,
                MODE   := mode_set,
                LADDR  := i_LADDR,
                RET_VAL:= sfc12_RET,
                BUSY   := sfc12_BUSY
            );
            o_RetVal := sfc12_RET;
            o_Busy   := sfc12_BUSY;
            o_Status := BYTE#0;  // pending confirmation

            IF sfc12_RET <> 0 AND NOT sfc12_BUSY THEN
                o_Error := TRUE;
            END_IF;
        END_IF;
    ELSE
        // Status query path (MODE=0)
        D_ACT_DP(
            REQ    := FALSE,
            MODE   := B#16#00,
            LADDR  := i_LADDR,
            RET_VAL:= sfc12_RET,
            BUSY   := sfc12_BUSY
        );
        o_RetVal := sfc12_RET;
        o_Busy   := sfc12_BUSY;
        IF sfc12_RET = 0 AND NOT sfc12_BUSY THEN
            o_Status := BYTE#1;  // active
        END_IF;
    END_IF;
END_FUNCTION_BLOCK

7. Ladder Logic Equivalent

For engineers who prefer ladder:

Network 1: Rising edge of i_Req
 A   i_Req
 FP  M 10.0     // local edge memory
 =   #tempEdge

Network 2: Mode selection
 AN  i_Enable
 L   B#16#01
 T   #mode

Network 3: Activate path
 A   i_Enable
 L   B#16#02
 T   #mode

Network 4: SFC12 call (must be a separate network)
 A   #tempEdge
 AN  sfc12_busy
 CALL D_ACT_DP
   REQ    := #tempEdge
   MODE   := #mode
   LADDR  := i_LADDR
   RET_VAL:= sfc12_ret
   BUSY   := sfc12_busy

8. SFC12 Return Value Reference

9. PROFIsafe Interaction

On a CPU 6ES7 315-2FH10-0AB0 with firmware V2.3.3, SFC12 was used to deactivate a PROFIBUS DP slave that contained F-I/O. The following behavior was observed and matched against the F-CPU documentation:

• No SF or BF LED on the F-CPU was lit while SFC12 executed.
• All F-I/O on the target slave was automatically passivated by the F-library.
• The passivation is normal and required: removing the slave from cyclic exchange trips the F-host watchdog and the F-library sets the affected channels to safe state.

10. PROFINET Behavior on F-CPU

On the same 6ES7 315-2FH10-0AB0 V2.3.3 F-CPU, SFC12 did not affect a PROFINET IO device with F-I/O. No change in the BF LED or the device state was observed. Firmware V2.3.4 release notes did not list a fix, indicating the limitation was architectural in the V2.3.x line.

The replacement CPU 6ES7 315-2FH13-0AB0 with firmware V2.5.1 includes updated SFC12 PROFINET support. The firmware download is documented in Siemens entry 23877553. Before deploying PROFINET deactivation, verify on the target CPU that:

1. The PROFINET device is configured with a valid device name and IP.
2. The device's diagnostic address is assigned and not zero.
3. The PROFINET interface is not shared with an IRT configuration that locks devices.

11. Bus Termination Pitfall (End-of-Line)

Disabling the last physical slave on a PROFIBUS segment removes the segment's bus terminator. Observed symptoms:

• BF LED on the CPU/IM turns on immediately after deactivation.
• Diagnostic buffer records "DP master: bus fault, end of line lost."
• Reactivating the slave clears the BF.

Solutions, in order of preference:

1. Install an active PROFIBUS terminator (for example 6ES7 972-0DA00-0AA0) on a free DP connector. Active terminators are powered from the bus and remain effective regardless of station state.
2. Reorder the topology so that the last physical node is a slave that is always active.
3. Use a DP repeater (6ES7 972-0AA02-0XA0) with its own terminating resistor at the segment end.

12. Integrating with Report System Error

Report System Error (RSE) is enabled in HW Config on the CPU. RSE uses OB82, OB83, OB86, OB122 to populate the WinCC alarm log. The combination of SFC12 + RSE produces the following desired behavior:

To prevent the initial OB86 from generating a flood, call SFC12 in OB100 (warm restart) or in the first scan of OB1 with a known HMI-confirmed list of "intentionally absent" slaves.

13. Verification Procedure

1. Build a small test rack with one configured PROFIBUS DP slave whose diagnostic address is W#16#03E8 (1000 decimal).
2. Disconnect the slave and bring the CPU to RUN.
3. Confirm the BF LED on the CPU is lit and OB86 has run.
4. Trigger the FB1000 instance with i_LADDR := W#16#03E8 and i_Enable := FALSE.
5. Read the return value via VAT: DB1000.DBX0.0 = 0 (no error) and o_Busy = 0 within 5 seconds.
6. Open the diagnostic buffer (online > diagnostic buffer). Confirm an entry of the form "DP slave: station deactivated."
7. Reconnect the slave and call FB1000 with i_Enable := TRUE. Confirm OB83 "Module reinserted" is logged and process I/O updates resume.

14. Troubleshooting Matrix

15. Memory and Scan-Time Budget

SFC12 is a synchronous system call. It blocks OB1 execution for the duration of the activation/deactivation handshake with the DP master. The time scales with the number of configured slaves on the segment. Typical figures from a 6ES7 315-2FH10-0AB0 at V2.3.3:

Call SFC12 only on operator request, never in a high-priority cyclic OB (OB35) without time-budget review. For projects that change slave states at high frequency, use OB35 at 100 ms and gate at most one SFC12 call per cycle.

16. Alternative: HW Config Group Suppression

If the slave list is fixed at commissioning time, the simpler approach is to use STEP 7's "Station not present" option in HW Config on a per-slave basis (DP slave properties > Parameters > "Station not present in configuration"). This avoids SFC12 entirely but is not HMI-driven.

17. Frequently Asked Questions