Siemens CP343-2 AS-i Master: Slave Addressing and Commissioning

Overview: AS-i Network Architecture with the Siemens CP343-2

AS-Interface (AS-i, Actuator Sensor Interface) is a single-cable fieldbus standardized in IEC 62026-2 that combines 24 V DC power and digital signalling on the same yellow profile cable. A standard AS-i segment supports up to 31 slaves with 4-bit input and 4-bit output each (124 bits per direction); an extended (A/B) segment supports up to 62 logical slaves. The Siemens CP343-2 (6GK7 343-2AH00 / -2AH01 / -2AH11 firmware variants) is the SIMATIC S7-300 AS-i master module that scans this segment and exchanges the data image with the CPU through the S7 backplane, mapping the AS-i bits into peripheral input and output bytes of the PLC.

When you inherit an installation without documentation and must add or replace an AS-i slave, two problems typically surface: how do you determine which slave addresses (1A…31A / 1B…31B) are already occupied, and how do you bring a previously-used module back online without colliding with another physical device? The companion problem that appears once the master has detected the slave is output-side failure — the inputs read correctly but the outputs refuse to change state. This reference covers all three.

Example hardware referenced in the field case: Siemens Sirius 3RK2400-1FQ03-0AA3 (K60 AS-i 4DI / 3DO module, ID/ID2 code = F hex, profile = S-7.A.7). The Sirius 3RK AS-i compact modules sit on the AS-i cable directly and provide IP67 protection for cabinet-free installation.

Prerequisites and Tools

Identifying Existing AS-i Slaves on the Bus

Three independent methods exist for discovering which slave addresses are occupied. Use them together for confidence.

Method 1 — CP343-2 Front-Panel LED Display

The CP343-2 carries a row of dual-colour LEDs that light in pairs per detected slave. Reading these LEDs is the fastest field method when no programming tool is on hand. The standard mapping is documented in the CP343-2 manual (entry ID 5581657):

A green LED = slave present and OK. A red LED = slave configured but missing or faulty. An unlit LED = no slave configured at this address. LEDs 1–15 light on the lower row, 16–31 on the upper row.

Method 2 — STEP 7 / TIA Online Diagnostics

With the online connection open, right-click the CP343-2 in the project tree and select Module Information → Diagnostics. The LDS (List of Detected Slaves) and LPS (List of Permanent Slaves / configured slaves) are returned in hex. Decoding example:

LDS = 0x0200 0040  →  Bit 6 (slave 7A), Bit 30 (slave 31A) active
LPS = 0x0280 00E0  →  Configured active: 6A, 7A, 8A, 14A, 15A

Bit position n corresponds to slave n+1 in the standard profile and (n+1)A / (n+1)B in extended A/B mode. Any bit set in LDS but not in LPS indicates a slave that was auto-detected but not yet permanently projected.

Method 3 — AS-i Power-Supply Diagnostic Display

Modern Siemens AS-i power supplies (e.g. 3RX9 501-0BA00) and IFM AC14xx diagnostic repeaters expose their own slave scan. IFM's AS-i controller / master displays the LDS in the web interface of the unit (default address 192.168.0.10 for AC1414, password ASi). Reference: IFM system integration guide.

Reading the Slave Configuration without Documentation

If you cannot connect to the master (no programming software, blocked IP), the AS-i power supply's ground-fault and overload display can still tell you how many slaves are powered. Counting current draw per slave (~40 mA quiescent for a 4DI module) on the AS-i supply's diagnostic web page gives an independent estimate:

I_total (AS-i segment) ≈ Σ I_slave + I_cable_leakage
For 31 slaves × 40 mA   → I_total ≈ 1.25 A  (before output current draw)

Compare against the AS-i supply's reported output current. A 2.8 A supply with 2.4 A loaded indicates a near-full segment; a 1.1 A load on a 2.8 A supply indicates only ~27 slaves active.

AS-i Slave Profile Versions: V2.0, V2.1, V3.0

AS-i slaves declare their capability through the profile string in their ID / ID2 codes. This is essential when commissioning older installations because the master must support the profile, otherwise it will reject the slave during the SET sequence.

Reference: Schneider FAQ FA275921 for V2.1 / V3.0 timeline; Siemens AS-Interface introduction PDF for mixed-version segment rules.

Mixing slave profiles on a single AS-i segment is permitted; the master cycles through each profile at the corresponding scan rate. The 3RK2400-1FQ03-0AA3 used in the field case is profile S-7.A.7 (V2.1, extended addressing capable), giving it 4 input bits and 3 output bits in A/B mode.

Readdressing a Used AS-i Slave Module

A factory-fresh AS-i slave ships with address 0, which is reserved and never used in normal operation. Address 0 exists exclusively so that when a defective slave is replaced, the new unit is automatically re-assigned the old address by the master. Once a slave has been addressed for the first time, its number persists in internal EEPROM and is retained for the slave's lifetime. You cannot overwrite it without an addressing unit.

Procedure with Siemens 3RK1901-1HA00 Addressing Unit

1. Disconnect the AS-i cable from the segment (or remove the slave from the segment entirely). The addressing unit powers the slave directly through its own AS-i socket.
2. Set the rotary switch on the handheld to ADR.
3. Plug the slave's yellow AS-i cable into the handheld.
4. After the handheld displays the current address (e.g. 25A), use ▲/▼ to select the target address (1–31 for standard, add A or B suffix for extended).
5. Press PROG. The handheld displays OK when the EEPROM write has succeeded (~2 s typical).
6. Disconnect and reconnect the slave to the AS-i trunk. Do not power down between write and verify; the new address is non-volatile.

Procedure with IFM AC1404 / Pepperl+Fuch VBP-HH1

The IFM and P+F handheld units use the same AS-i wire protocol and write the same EEPROM area. Operating sequence is identical: select Programming → Slave → New Address → Write. They differ only in connector geometry (M12 vs. piercing terminals) and in their ability to read diagnostic data from V3.0 slaves.

Connecting a New Slave to the CP343-2 Master

Once the slave has been re-addressed to a free address (verified by reviewing the LDS bitmap), it must be added to the master configuration.

Step 1 — Physical Installation

• Mount the 3RK2400 module on the AS-i yellow cable using the insulation-piercing contacts (IP67 K60 housing).
• Route the black 24 V DC auxiliary cable to the same module; this is a separate power source, not the AS-i supply.
• Verify yellow cable pinout: pin 1 = AS-i +, pin 3 = AS-i −, pin 2/4 spare or shield.
• Verify black cable pinout: pin 1 = 24 V DC +, pin 3 = 24 V DC −.

Step 2 — Switch the CP343-2 into Configuration Mode

1. Bring the CPU to STOP (or verify it is already in STOP). The CP343-2 SET button is only effective in STOP on most firmware revisions.
2. Press and hold the SET button on the CP343-2 front panel for approximately 2 seconds until the CM (Configuration Mode) LED lights steady orange.
3. Wait for the master to perform a full scan (~5–10 s for 31 slaves). The slave LEDs 1–31 on the master will display the detected LDS.
4. The master writes the LDS into the LPS automatically; the new slave is now in the projected configuration.
5. Release the CM mode by pressing SET again. The CM LED goes out, the master returns to protected mode, and the CPU can be returned to RUN.

Step 3 — CPU Restart and Scan Verification

Return the CPU to RUN. The CP343-2 starts the cyclic scan within 1 s. Confirm the slave LED on the CP343-2 corresponding to the new address is green and steady. Any red flashing on that LED indicates the slave dropped out of the scan cycle (typically: cable length > 100 m, repeater needed).

I/O Address Mapping in the SIMATIC S7-300

The CP343-2 places the AS-i segment's input and output data into peripheral bytes of the S7-300 I/O area. With the CP343-2 inserted in slot 4 of an S7-300 rack and the default STEP 7 address assignment, the mapping is:

With 31 slaves, the entire AS-i image occupies 16 input bytes (E 80–E 95) and 16 output bytes (A 80–A 95), exactly as reported in the source field case. Each slave consumes 4 bits (½ byte) of the image. In extended A/B mode, the second half (1B–31B) maps to bytes 96–111 in each direction.

Programmatic Read/Write

No special function calls are required; the CP343-2 exchanges the AS-i image transparently with the CPU. Reading inputs in ladder logic:

A   "ASi_Slave_25A_Input_0"  // symbolic, mapped to E 92.0
=   "Cabinet_Door_Closed"

Writing outputs:

A   "Start_Motor_Request"
=   A 92.0                    // raw address for output bit 0 of slave 25A

Commissioning Procedure: Step-by-Step

1. Document the segment. Read the CP343-2 LEDs and the online LDS. Export a slave list from STEP 7 (right-click CP343-2 → Save Slave List).
2. Identify the free address. Mark any bit position in LDS that is 0 — these are vacant slots.
3. Readdress the new slave. With the slave disconnected from the trunk, use the addressing handheld to write the free address (e.g. 25A).
4. Verify the new address. Read the handheld display. Disconnect and reconnect the slave's AS-i connector; the address persists across power cycles (EEPROM retention ≥ 10 years, 100 000 write cycles).
5. Connect physically. Yellow AS-i cable + black 24 V aux cable to the new slave.
6. Bring CPU to STOP and press SET on the CP343-2. Confirm CM LED lights and slave LEDs 1–31 update.
7. Return CPU to RUN. Verify the new slave's LED on the CP343-2 turns green.
8. Test inputs. Force the input with a jumper / sensor. Confirm bits appear in the expected peripheral byte (e.g. E 92.0–E 92.3).
9. Test outputs. Set A 92.0 from the watch table and verify the corresponding output LED on the 3RK2400 lights. Measure 24 V at the output terminal with a multimeter (≤ 0.5 A inductive, ≤ 2 A resistive per output on the K60 module).
10. Document the change. Update the LPS export, the AS-i topology drawing, and the PLC tag database.

Troubleshooting: Outputs Not Activating on the 3RK Module

This is the precise symptom from the source case: inputs read correctly, the CP343-2 recognises the slave, but the outputs cannot be set. Walk through this matrix in order.

Most Likely Root Cause in the Source Field Case

The user reported the AUX Power LED was not lit on the 3RK2400 module while inputs worked correctly. AS-i supplies only the inputs and the slave's logic through the yellow cable; outputs require the separate black 24 V DC cable. Without the auxiliary supply, the input scan operates (slave is powered) but no 24 V is available to source the output transistors — the output reads back as 0 even though A 92.0 is set in the PLC. This is the single most common field error when commissioning a 4DI/3DO K60 module.

Resolution path:

1. Verify a separate 24 V DC power supply is wired to the black cable of the slave (pin 1 +, pin 3 −).
2. Confirm the supply is enabled (no blown fuse, no tripped circuit breaker, no e-stop open in series).
3. Measure the voltage at the slave's black cable terminals with the slave connected — 24 V ± 5 % expected.
4. Power-cycle the slave to clear any latched output fault.
5. Re-exercise output bit A 92.0 from the watch table.

Verification Checklist

• CP343-2 CM LED is OFF (normal mode active).
• CP343-2 APF LED (AS-i Power Fail) is OFF.
• CP343-2 CER LED is OFF (no configuration error).
• CP343-2 slave LED for the new address is steady green.
• All 31 slave LEDs match the documentation (no red, no unexpected black).
• Sirius 3RK2400 PWR LED is green.
• Sirius 3RK2400 AUX LED is green (aux 24 V present).
• IN1–IN4 LEDs respond to field inputs.
• OUT1–OUT3 LEDs respond to A 92.0–A 92.2 from the watch table.
• Watch table Modify writes persist (no PLC overwrite).
• Diagnostic buffer of the CPU is clear of AS-i-related entries.
• LDS bitmap equals LPS bitmap (all detected slaves are projected).

Specifications

Edge Cases and Field-Proven Caveats

Segment cable length. A standard AS-i trunk is limited to 100 m total. With one repeater (e.g. 6GK1 901-1BB00) the segment extends to 200 m. Adding a third slave close to the 100 m limit with insufficient derating can produce intermittent slave dropout, observable as a flashing green LED on the CP343-2.

Ground faults. The AS-i power supply's integrated ground-fault detector trips a single ground fault down to 0 mA (some models 5 mA). After a ground fault, the CP343-2 sets the APF LED. Clear the fault and press SET to refresh the LPS.

A/B address swap. A slave at address 25A and a slave at address 25B are the same physical device; the master treats them as two logical slaves occupying one physical slot. You cannot address both halves of a V2.0 slave — only V2.1+ devices accept the A/B split.

CPU behaviour during SET. Pressing SET while the CPU is in RUN freezes the AS-i output image. On the SIMATIC S7-300 with CP343-2 in slots 4–11, the CPU does NOT enter STOP — only the master enters configuration mode. The CPU continues scanning and may overwrite frozen AS-i outputs once the master returns to protected mode. If your outputs are safety-relevant, place the CPU in STOP first.

Polarity on the black cable. AS-i specifies protection against reversed polarity on the yellow cable (it simply fails to communicate). The black auxiliary cable is not reversed-polarity protected; sustained reverse polarity will damage the output stage of the K60 module. Always meter the cable before applying power.

Slave replacement strategy. Always keep one spare slave pre-programmed with address 0 and one spare pre-programmed to a known occupied address (e.g. 25A). When a slave fails, swap with the 0-addressed spare and let the master auto-reassign the address via SET. This is the only AS-i-native hot-swap workflow.