Resolving AS-Interface Slave Communication Failure on CP 343-2

Problem Overview

When an existing AS-Interface (AS-i) network built around the Siemens CP 343-2 master (6GK7343-2AH00-0XA0) is expanded with additional 2DI/2DO digital modules (3RK1402-0BE00-0AA2), the new slaves may refuse to come online at their assigned addresses even though the addressing operation itself reports success. The pattern observed in the field:

• 15 existing 2DI/2DO slaves continue to operate with green status LED only.
• 5 new 2DI/2DO slaves, when assigned addresses 16..20, show both green and red LEDs illuminated.
• The same physical slave accepts address 5 and starts communicating, but returns to a fault state when re-assigned to the intended address (e.g., 15).
• The CP 343-2 has 16 bytes of I/O configured in STEP 7 HW Config (read/write).

Both LEDs on means the slave is not exchanging process data with the master. The slave is alive on the wire (otherwise no LED at all), but the master does not recognize it as a member of the configured network.

Hardware Identification and Specifications

Identify every component on the network before changing parameters. The reference network contains:

The CP 343-2 with MLFB 6GK7343-2AH00-0XA0 is the earlier AS-i master variant supporting specification V2.1. The functional successor — 6GK7343-2AH11-0XA0 — supports AS-i V3.0 and provides additional on-master configuration and diagnostics. If only the 6GK7343-2AH00-0XA0 is available, the field procedure described in this article restores network operation without firmware change.

Confirm the AS-i specification of every slave before installation. The 3RK1402-0BE00-0AA2 carries a standard slave profile (V2.1). Newer 3RK1402 variants with the same MLFB stem can be V2.1 or V3.0 depending on the production date and the silk-screened "S x.y" index on the housing. Master V2.1 reads the LDS (List of Detected Slaves) from the wire; V3.0 slaves advertise additional parameters that a V2.1 master ignores but may tolerate. Always cross-check the version on the side label before commissioning.

AS-Interface Version Compatibility

AS-Interface has three specification revisions relevant to this hardware set:

The 3RK1402-0BE00-0AA2 is documented as a standard slave (V2.1). If the lot of replacement modules received is in fact a V3.0 profile variant, it will still answer at addresses 1..31 but may transmit additional cyclic parameters the V2.1 master does not evaluate. When this happens, the master cannot build a clean LPS (List of Permanent Slaves) and the slave is rejected, producing the two-LED state. Inspect the side label of the slave: the AS-i profile is printed either as "S-0.A.E" (standard V2.0) or as "S-3.0.x.y" (V3.0). The 3RK1402-0BE00-0AA2 should report a standard profile.

LED Status Decoding on the 3RK1402 Module

Always read the LED combination against the system manual, not against generic documentation. The two-LED error state means communication interrupted, which is distinct from address conflict or peripheral fault:

Reference: System Manual AS-Interface (Entry ID 26250840), Section "Diagnostics" / "Displays and Operating Elements".

Diagnostic Interrupts via OB 82 in STEP 7

The CP 343-2 supports diagnostic interrupts to the S7-300 CPU via OB 82. When a configured AS-i slave disappears or comes back, the master triggers a diagnostic interrupt; OB 82 receives an 8-byte record whose first bytes identify the AS-i slave that changed state. This is the on-line, runtime way to see all used network addresses and to detect an address the master refuses.

Enable the diagnostic interrupt before commissioning:

1. Open the S7 project in STEP 7 (V5.x or compatible).
2. In HW Config, double-click the CP 343-2 slot.
3. Open the "AS-Interface" tab — in the "Operating Mode" group, set the master to "Configuration mode" temporarily to scan, or leave it on "Protected mode" for normal operation.
4. Open the "Diagnostics" tab and tick "Diagnostic interrupt" so OB 82 is triggered on slave presence / absence transitions.
5. Assign OB 82 in the CPU properties (block container) and download to the PLC.

In OB 82, the local data contains the diagnostic record. Practical extract (STL / SCL snippet) for the new CP 343-2 manual section "Diagnostic interrupt":

// OB82 — evaluate the AS-i slave that triggered the interrupt
// OB82_MDL_DEF input byte 0 = event class
// LB 8..LB 19 = diagnostic record (per CP 343-2 manual, § 5.4)
// For CP 343-2 the AS-i slave address is encoded in bits 0..4 of LB 8
// (1..31 single address, 0 = no address)

#iSlaveAddr := LB8 AND 16#1F;
IF (#iSlaveAddr > 0) THEN
    // 0 = slave missing, 1 = slave back / parameter error resolved
    #bEventClass := LB8 SHR 5 AND 16#07;
    // ... store in DB for HMI display
END_IF;

The diagnostic record format and exact byte layout are documented in the CP 343-2 / CP 343-2 P AS-Interface Master manual (C79000-G8976-C149), chapter "Diagnostic Interrupt". The CP 343-2 Delivery Release (Entry ID 5663088) confirms that automatic address programming on slave replacement is also configurable from this menu.

Configuration Mode Recovery Procedure

This is the field-proven sequence that resolved the reported incident. The CP 343-2 (6GK7343-2AH00-0XA0) cannot be re-flashed and provides no web interface, so the only available recovery is to force a fresh LDS scan and rebuild the LPS from it.

1. Stop the CPU. Place the S7-300 in STOP so no user program reads stale I/O during the scan. The CP 343-2 must be in a quiescent state.
2. Switch the CP 343-2 to Configuration Mode. In HW Config, right-click the CP 343-2 → "AS-i Master" → "Set to Configuration Mode", or use the diagnostic display on the master front panel. The mode LED on the master switches from steady green to flashing green.
3. Scan the network. The master cycles the yellow AS-i cable, queries every address 1..31, and rebuilds the LDS. Each detected slave is announced by lighting the corresponding address indicator on the master front panel. Wait until the scan completes (one cycle is < 5 ms with 31 slaves, but the LEDs may take a few seconds to stabilise).
4. Compare LDS vs LPS. From the front panel or from HW Config, read the LDS. Any slave that appears in the configuration (LPS) but not in the LDS is reported as "missing". Any slave that appears in the LDS but not in the LPS is reported as "new".
5. Accept the new configuration. Use "Save LPS" in the master menu so the LPS is overwritten with the just-scanned LDS. The new 3RK1402 modules at addresses 16..20 are now permanent members of the network.
6. Switch the master back to Protected Mode. The address indicators on the master go off and the green mode LED returns to steady. The master now exchanges process data only with the slaves listed in the LPS.
7. Restart the CPU. Cold restart is not required; a warm restart (RUN) is sufficient. The CP 343-2 re-initialises the slaves and the diagnostic data (PIQ/PII) becomes valid.

Addressing Tools and Methods

Two practical methods exist for assigning an AS-i slave address:

Method A — Hand-held Addressing Unit (recommended for commissioning)

The 3RK1904-2AB02 (AS-I ADDRESSING UNIT V3.0) connects to the addressing jack of any 3RK1402 module with the standard 4-pin addressing cable. Procedure:

1. Disconnect the addressing cable from the AS-i master (or leave it connected; the unit tolerates a live cable).
2. Plug the unit into the addressing jack on the front of the 3RK1402.
3. Read the current address — the LCD shows "ADDR: nn".
4. Use <+> / <-> to set the desired address (1..31 for standard; 1A..31A and 1B..31B for V3.x extended).
5. Press <OK> to write. The new address is committed only if the slave is currently in addressing mode (the address of 0 / default-factory).

Method B — CP 343-2 addressing command (STEP 7)

The CP 343-2 exposes FC "ASI_3422" (older naming) / SFB / SFC calls in the Siemens AS-i library that can write a slave address via the master. Use only when no hand-held unit is available, and only on a single isolated slave on the cable. The library function performs a temporary address swap and forces the slave to address 0 before writing the new one — the same operational sequence the hand-held unit performs.

Source for both methods: CP 343-2 / CP 343-2 P AS-Interface Master manual (PDF), chapter "Address Assignment".

STEP 7 Integration and I/O Configuration

The CP 343-2 occupies 16 input bytes and 16 output bytes in the S7-300 I/O area by default. The bit-level mapping follows the AS-i slave address order:

The exact bit assignment (DI 1 / DI 2 / DO 1 / DO 2 of each 2DI/2DO module) is found in the slave data sheet — for 3RK1402-0BE00-0AA2, the first two bits are inputs and the last two are outputs. Verify the mapping with HW Config → CP 343-2 → "I/O Assignment" after downloading the new LPS.

To view all used addresses from STEP 7:

1. Open the CP 343-2 in HW Config.
2. Select "AS-i Master" → "Read Configuration". The dialog lists every address 1..31, the detected profile, the I/O length, and the configured (LPS) vs detected (LDS) flag.
3. For runtime visibility, evaluate OB 82 as shown in Section 6 or expose the LDS to an HMI tag.

Verification and Commissioning Checks

After the Configuration Mode scan and PLC restart, run the following verification matrix before handing the network back to production:

Field-Proven Best Practices

• Document the LPS. After every Configuration Mode scan, export the configuration from HW Config and archive it in the project. A fresh scan that drops a slave cannot distinguish "slave failed" from "slave cable cut" without the prior LPS.
• Replace like with like. When expanding an existing network, keep the slave profile consistent. Mixing V2.1 standard slaves and V3.0 slaves on a V2.1 master is a documented source of two-LED faults and is best avoided unless the master is upgraded to 6GK7343-2AH11-0XA0.
• Plan address range. The 3RK1904-2AB02 supports 1A..31A and 1B..31B for V3.x masters only. On a 6GK7343-2AH00-0XA0 stick to 1..31. Reserving 1..10 for the first cabinet, 11..20 for the second, etc. makes LPS debugging tractable.
• Use the addressing jack, not the AS-i pins, to write. The 3RK1402 has a dedicated 4-pin addressing jack on the front. Wiring the addressing voltage onto the AS-i pins themselves damages the slave over time.
• One power supply per AS-i segment. A single 3RX9307-0AA00 is sized for one segment. If the segment grows beyond ~2 A, add a second power supply at the far end — do not parallel 30 V outputs of two units without the AS-i data-decoupling choke; this corrupts the data signal.
• Diagnostic interrupt in protected mode. Always enable OB 82 on the CP 343-2 in protected mode. The interrupt is the only way the CPU learns of a slave drop in real time without polling the master. CP 343-2 manual, § "Diagnostic interrupt can be selected here for the protected mode".
• Spare slave address zero. AS-i address 0 is the factory default and is reserved for "unaddressed" slaves. Never assign 0 in production. If a slave is found at address 0, the addressing unit is the only safe tool to move it off 0.

Related Manuals and References

• CP 343-2 Delivery Release (Entry ID 5663088) — confirms V2.1 spec and the diagnostic-interrupt / auto-address features of the CP 343-2 family.
• CP 343-2 / CP 343-2 P AS-Interface Master Manual (C79000-G8976-C149, PDF) — diagnostic record layout, configuration mode, automatic address programming.
• System Manual AS-Interface (Entry ID 26250840, Edition 11/2008) with Supplements 09/2010 (Entry ID 44365425) — LED definitions, addressing, network topology.