S7-400 Profibus: Removing DP Slaves Without HWConfig Download

1. Overview

The question of whether a full HWConfig download is mandatory when an interface module and its downstream DP slaves are physically removed from the last segment of a Siemens SIMATIC S7-400 PROFIBUS network is one of the most common configuration-management dilemmas on brownfield plants. Two practical engineering paths exist:

1. Path A — Modify and re-download HWConfig: the canonical, fully documented Siemens procedure. Always used when the topology changes are permanent and the system is in a maintenance window.
2. Path B — Software deactivation via SFC 12 (D_ACT_DP): allows the CPU to logically mark a DP slave as "deactivated" at runtime, suppressing diagnostic interrupts and access errors, without modifying the hardware project. The physical module can be removed and the rest of the network will keep running.

This reference covers both methods in full, with the parameter set, OB requirements, restrictions, edge cases, and a verification matrix. The procedures are valid for S7-400 CPUs (CPU 412/414/416/417) with integrated DP interface or external CP 443-5 as of STEP 7 V5.5 / S7-GRAPH / S7-SCL, and remain valid in TIA Portal projects with S7-400 stations running legacy mode.

2. Prerequisites

3. Method A — Modify and Re-Download HWConfig

3.1 When to use it

• The removed module is permanent and the new project must reflect the as-installed state.
• Documentation and audit requirements mandate an updated CAx data set.
• You intend to add new DP slaves to the freed bus address in the same maintenance window.

3.2 Procedure

1. Open the STEP 7 project that contains the S7-400 station.
2. Launch HWConfig from the SIMATIC Manager toolbar or the TIA Portal "Devices & Networks" editor.
3. Navigate the PROFIBUS subnet. Right-click the DP master (CPU or CP) and choose PROFIBUS > Subnet Properties; verify the bus profile (DP, Universal, User-defined) and transmission rate (typically 1.5 Mbps or 12 Mbps).
4. Identify the slave at the last segment. Right-click the slave station, then select Delete. Confirm that the connected I/O module(s) of that slave are also removed from the table view.
5. If the slave sits behind a repeater or coupler that stays in the network, the repeater/coupler must remain in the configuration.
6. Save and compile. The Compile HWConfig action regenerates the system data blocks (SDBs) — SDB 0, SDB 1, SDB 2, and the DP-specific SDBs (SDB 1000+).
7. Switch the CPU to STOP, or schedule a hot restart per project setting (CPU 417H / 414H only with synchronization). For S7-400 standard CPUs, full download requires STOP mode; partial download for connected / non-connected configuration changes requires an S7-400 CPU with Configuration in RUN (CiR) capability, which is supported on CPU 41x FW ≥ V5.1.
8. Download the new HWConfig to the target station. Monitor the diagnostic buffer; look for Parameter assignment of module OK and the absence of Station failure events for the surviving slaves.
9. Switch the CPU to RUN. Verify that the process image updates as expected.

4. Method B — Software Deactivation with SFC 12 (D_ACT_DP)

4.1 The D_ACT_DP mechanism

SFC 12 is the standard CPU system function used to deactivate and activate configured DP slaves. When the CPU receives the deactivate command, it:

1. Logs the slave out of the cyclic PROFIBUS frame.
2. Stops I/O access and substitutes configured substitute values (or the previous value, depending on the parameter assignment).
3. Suppresses the station failure diagnostic (OB 86 is not triggered) and suppresses process image updates for that slave.
4. Stops module I/O error (OB 85) on the diagnostic address.

4.2 SFC 12 interface

4.3 Critical restrictions on SFC 12

• Not allowed in startup OBs: SFC 12 cannot be called from OB 100 (warm restart), OB 101 (hot restart), or OB 102 (cold restart). Reason: during startup the PROFIBUS master is in the process of parameterizing the slaves; deactivation would race the system data download.
• Not allowed in a programmed loop: the call must be edge-triggered. Calling SFC 12 every cycle is technically legal but undesirable — it issues a new job every cycle.
• OB 1 may be used, but is discouraged. Siemens explicitly recommends wrapping the call inside an FC or FB, then calling that block from OB 1. This keeps OB 1 lean and matches the "OB = block container" philosophy of well-structured STEP 7 code.
• Not allowed for PROFINET IO devices on S7-400 PN/DP CPUs in all firmware versions. SFC 12 targets DP (PROFIBUS) only. For PROFINET IO deactivation, use SFC 12-equivalent (PNIO functions in STEP 7 ≥ V5.5 SP2) or an IO device port disable.
• Cannot be used on the DP master itself. Use MODE = 0 (status check) to read the current state without side effects.

4.4 Standard error codes (RET_VAL)

4.5 Sample code (Structured Text / SCL)

FUNCTION_BLOCK FB_DeactivateDpSlave
VAR
    bDeactivate : BOOL := FALSE;     // Trigger input
    bBusy       : BOOL;
    bDone       : BOOL;
    bError      : BOOL;
    wRetVal     : INT;
    bEdgeHold   : BOOL;              // Edge memory
END_VAR

BEGIN
    // Rising-edge detection
    IF bDeactivate AND NOT bEdgeHold THEN
        // Issue a single SFC 12 call (MODE = 1 = deactivate)
        RET_VAL := D_ACT_DP(
            REQ   := TRUE,
            MODE  := 1,
            LADDR := W#16#0FA0          // Diagnostic address of the slave
        );
        IF RET_VAL = 0 THEN
            bDone  := TRUE;
            bError := FALSE;
        ELSIF (RET_VAL >= 16#80A0) AND (RET_VAL <= 16#80C3) THEN
            bError := TRUE;
        ELSE
            bBusy := TRUE;             // Job in progress, poll next cycle
        END_IF;
    END_IF;

    bEdgeHold := bDeactivate;
END_FUNCTION_BLOCK

Call this FB from OB 1 only. Do not place the call in OB 100/101/102 or in a cyclic timer that re-issues REQ every cycle. After successful deactivation, the slave is no longer polled on PROFIBUS and OB 86 is suppressed. The physical module can be powered down and removed.

4.6 Ladder logic equivalent

Network 1: rising-edge SFC 12 call
    "Deactivate_Slave_Start"   "Deactivate_Edge"
        |P|----[Call SFC 12 (D_ACT_DP)]----( )--|
        |   |   REQ := "Deactivate_Edge"        |
        |   |   MODE := 1                       |
        |   |   LADDR := W#16#0FA0              |
        |   |   RET_VAL=> "DB_Diag_Deact".RET  |
        |   |-----------------------------------|
        |                                        |

5. Fault OB Requirements

Whenever a DP slave disappears — even if the disappearance is intentional — the CPU will normally raise the following asynchronous events:

Even when using SFC 12, keep OB 85 and OB 86 loaded as empty blocks. Although SFC 12 suppresses the station-failure interrupt, any peripheral access inside the user program that targets a removed input or output word will still trigger OB 85 unless OB 121/122 handles it. With both fault OBs in place, the rest of the network continues operation unaffected.

6. Step-by-Step: SFC 12 Deactivation Procedure

1. Open the existing STEP 7 project; do not delete the slave in HWConfig yet.
2. In HWConfig, note the diagnostic address of the slave to be removed (the first I/O row of the slave, slot 0). This is the value that goes into SFC 12's LADDR.
3. Create a new FB (e.g., FB_Deactivate_DpSlave) in the S7 program and implement the SFC 12 call (see §4.5).
4. Add an instance DB and call the FB from OB 1, gated by a flag (e.g., Deactivate_Slave_X) that is set via HMI or PG during the maintenance window.
5. Compile the S7 program and download only the changed blocks (no full HWConfig download).
6. Verify with STEP 7 > Online > Monitor/Modify that the SFC 12 call returns 0. The slave's diagnostic address will change from "OK" to "deactivated" in the online view.
7. Check the CPU diagnostic buffer for the entry "DP slave deactivated by user" (event ID 0xE4C4 or similar depending on firmware).
8. Switch off power to the slave (and its downstream I/O on the last segment). Disconnect the PROFIBUS connector if the segment is being physically dismantled.
9. Run the surviving plant for a verification period; confirm no OB 85 / OB 86 entries appear.
10. Optionally, after a stable period (typically 30–60 days), re-open HWConfig and remove the slave for documentation consistency. This second step requires a planned download.

7. Verification Matrix

8. Edge Cases and Field-Proven Caveats

• PROFIsafe slaves: SFC 12 returns error 80B2h if you try to deactivate a slave that participates in a PROFIsafe group. The safety layer relies on a continuous response time; removing the slave will disarm the safety function. Modify the F-CPU's PROFIsafe configuration first.
• Y-link / DALI / IE/PB-Link behind the slave: if the slave is a DP/PA coupler (e.g., IM 157-0AA82) that bridges to PROFIBUS PA, the entire PA segment downstream is unreachable. SFC 12 with a single call disconnects the entire coupler. Recompute T_tr_max and Number of call attempts in HWConfig for the new (shorter) topology.
• DP-V1 and DP-V2 slaves: SFC 12 is fully supported for DP-V0/V1/V2 slaves in the data area. The iSlave_LEN parameter is irrelevant for SFC 12 (the system function uses the diagnostic address only).
• Repeater in a chain: the last segment's terminator may be on a Siemens RS 485 Repeater instead of the slave itself. Leave the repeater powered and the termination switch on; the slave is downstream of the repeater.
• Multiple slaves with the same diagnostic address: SFC 12 will fail with 80C2h. Diagnostic addresses are CPU-side and must be unique. Re-assign via HWConfig and download before retrying.
• Hot Swap of the S7-400 interface module: if the IF module is the DP master (e.g., a CP 443-5 Extended in a separate slot) and is itself the one being removed, SFC 12 cannot help. The CP must be physically present for the bus to operate. Removing the master is a separate change with project redesign.
• CPU 410 with mixed S7-400 / S7-1500 functionality: SFC 12 behavior is the same; the firmware ≥ V8.x is required.
• ET 200M with hot-swap modules: the station is not removed; SFC 12 is not appropriate. Use SFC 59 / SFB 52 for diagnostic reads of the individual ET 200M slots.

9. Troubleshooting Matrix

10. Field Commissioning Notes

Two practical habits separate a successful SFC 12 deactivation from a CPU stop:

1. Always load OB 85 and OB 86. A 6-byte empty block in the program prevents the most common STOP cause during planned maintenance.
2. Document the LADDR ↔ PROFIBUS address mapping in a separate watch DB, not inside the FB instance. The HMI or maintenance engineer may need to type in the address when a PG is not available. A simple WinCC text field bound to a status DB is sufficient.

For plants governed by FDA 21 CFR Part 11 or similar audit regimes, the SFC 12 path is acceptable provided the audit log captures the deactivation event with the user ID. STEP 7's audit log on the PG side records the operator's user account; the CPU's diagnostic buffer records the deactivation with a timestamp. Pair the two for traceability.