Programming AS-i Valves on S7-300: CP 343-2 Master with ifm AC1411 Slaves

1. Overview

AS-Interface (Actuator Sensor Interface, AS-International) is a single-cable digital fieldbus standardized in EN 50295 and IEC 62026-2. It carries both 24 VDC power and a Manchester-coded digital signal on the same 2-wire yellow profile cable, making it the lowest-cost way to wire binary valves, sensors, and small actuators back to a Siemens S7-300 CPU.

This guide addresses the canonical field problem:

• CPU: SIMATIC S7-300, CPU 317-2 DP (or any S7-300 with a free rack slot, slots 4–11)
• Master: CP 343-2 (order number 6GK7 343-2AH01-0XA0, AS-i V2.0, 31 slaves) or CP 343-2 P (6GK7 343-2AH11-0XA0, V2.1, 62 slaves)
• Slaves: ifm AC1411 (AirBox 4-valve manifold, AS-i profile S-7.F for valve modules with 4 input bits / 4 output bits)
• Goal: command four on/off solenoid valves from a STEP 7 (Classic) program, with one bit per valve, using the digital I/O mapping of the CP 343-2

2. Prerequisites

3. AS-i System Architecture

The topology is a single trunk with drops. The yellow cable is both power and data; the master injects 30 VDC and demodulates the data, the slaves draw 24 VDC and modulate the data using Alternating Pulse Modulation (APM).

4. AS-i Cable and Power Specification

The AS-i specification (EN 50295) defines:

5. Slave Addressing Theory (4-bit Nibble Map)

The CP 343-2 reserves 16 bytes of I/O area in the S7-300 process image. The structure is fixed by the AS-i specification:

For a single ifm AC1411, the slave's 4 data bits are mapped into one nibble. In digital I/O mode (also called "standard mode" in STEP 7), each slave's 4 bits appear as 4 contiguous input bits and 4 contiguous output bits in the process image of the CPU — exactly like a regular digital I/O module. This is the mode you want for simple on/off valves.

In analog I/O mode (the default after some firmware updates and the ifm default), the slaves are packed into process words (PIW / PQW) and you must shift bits manually. This is intended for slaves that expose 4-bit analog values, not for solenoid control.

6. CP 343-2 Hardware Installation

1. Insert the CP 343-2 into a free S7-300 slot (slots 4–11 only). Mechanical keying on the bus connector prevents incorrect backplane insertion.
2. Connect the yellow AS-i cable to the front terminals AS-i+ and AS-i- (removable screw connector, 4-pole). The polarity of the cable does not matter at the master.
3. Connect the AS-i power supply to the same two terminals AS-i+ and AS-i- of the master; the master has a built-in decoupling circuit so the power supply does not see the data signal.
4. If the CP sits at the end of the bus, leave the terminating jumper open; the CP has no internal termination. Termination is on the power supply.
5. Power up. The CM LED (green) should light within 2 s. The APF LED (red) is normal OFF when the bus is healthy.

7. STEP 7 Configuration

Open SIMATIC Manager, open your S7 project, double-click Hardware to launch HW Config. Drag the CP 343-2 from the catalog (path: SIMATIC 300 / CP-300 / AS-Interface / CP 343-2) onto the rack. The default configuration in the hardware catalog uses digital I/O addressing for AS-i V2.0 — this is the configuration you want for AC1411.

7.1 I/O Addresses (digital mode default)

The default address window for a CP 343-2 in slot 4 is:

• Inputs: IB 0 – IB 15 (16 bytes → 128 bits, 4 bits per slave × 31 slaves + 8 bits diagnostics)
• Outputs: QB 0 – QB 15

You may change the start address if it conflicts. The diagnostics nibble in the low 4 bits of IB 0 contains:

7.2 Mapping a Slave to its Bits

With digital I/O, slave N (1 ≤ N ≤ 31) occupies bits:

Inputs:  I ( (N-1) × 4 + 8 )  ..  I ( (N-1) × 4 + 11 )
Outputs: Q ( (N-1) × 4 + 8 )  ..  Q ( (N-1) × 4 + 11 )

The "+ 8" offset skips the 8 diagnostic/flag bits in IB 0. So:

For the ifm AC1411 (AirBox 4-valve manifold), each of the 4 output bits drives one solenoid: bit 0 → valve 1, bit 1 → valve 2, bit 2 → valve 3, bit 3 → valve 4. The 4 input bits are valve position feedback (or empty, depending on the connected valve heads).

8. ifm AC1411 Wiring and Address Setup

The AC1411 has an M12 connector (port 1) for AS-i + power, and 4 M12 sockets for the valve heads. The ifm-supplied ifm AC141x configuration over STEP 7 manual walks through the wiring. Key steps:

1. Mount the AC1411 on the DIN rail or via the manifold mounting holes.
2. Wire the yellow AS-i cable with an M12 AS-i T-piece (ifm AC1057) or a flat-cable piercing clamp (ifm E73064).
3. Connect the valve heads to ports 1–4 of the manifold.
4. Default slave address is 0 (not yet activated). Set the address to 1, 2, 3, etc. with the ifm AC1154 handheld: navigate to Address → New = 1 → Write.

9. PLC Program: Writing the Valves

9.1 Ladder (FBD) example — turn valve 1 of slave 1 ON

            I0.3      M 10.0
   ---| |----------------( S )---
      "bus OK"

      M 10.0        Q 8.0
   ---| |----------------( )---
      "run cmd"     "valve_1_on"

9.2 Structured Text (SCL) example — write 4 valves in one block

FUNCTION_BLOCK FB 100 "ASi_Valve_Control"
VAR_INPUT
  i_Start        : BOOL;    // permissive
  i_BusOK        : BOOL;    // I0.3 = config error, invert
  i_SlaveAddr    : INT;     // 1..31
  i_ValveCmd     : WORD;    // bit 0..3 = valve 1..4
END_VAR
VAR_OUTPUT
  o_ValveStatus  : WORD;    // 4-bit feedback, right-justified
  o_Fault        : BOOL;
END_VAR
VAR_TEMP
  t_Offset       : INT;
  t_Word         : WORD;
END_VAR
BEGIN
  o_Fault := NOT i_BusOK;

  // compute output bit offset: slave N -> bits (N-1)*4 + 8
  t_Offset := (i_SlaveAddr - 1) * 4 + 8;

  // mask out previous writes, then OR in new command
  t_Word := (QW0 AND SHL(WORD#16#000F, t_Offset))
            OR SHL(i_ValveCmd AND WORD#16#000F, t_Offset);
  QW0 := t_Word;

  // read status back: bits (N-1)*4 + 8 .. +11
  o_ValveStatus := SHR(WORD#16#000F AND (IW0 SHR t_Offset), 0);
END_FUNCTION_BLOCK

9.3 If the master is in Analog Mode (re-mapping from PQW256)

Some ifm-supplied STEP 7 sample projects ship the CP 343-2 in analog mapping, where slave 1 occupies 4 bits in PQW 256 and slave 2 in PQW 258. To turn ON valve 1 of slave 1 in that mode:

L  W#16#0001        // valve 1 = bit 0, slave 1 = bits 0..3
T  PQW 256          // output image word for slave 1

To turn ON valve 1 of slave 2 you would write W#16#0100 to PQW 256 (bit 8 = valve 1 of slave 2). The two nibbles of PQW 256 map to slaves 1 (low nibble) and 2 (high nibble). A common debugging pattern is to write a single bit pattern such as W#16#0100 then W#16#0000 with a VAT table and observe which physical valve moves.

10. Diagnostic Library and FCs

Siemens provides a free library for the CP 343-2 / CP 343-2 P on the Siemens Industry Online Support portal. The standard FCs you will use:

The full manual for the library is at SIOS entry 51678777. The diagnostics can be visualized on an HMI via the blocks in SIOS 50897766 (Diagnostic Block and Visualization via HMI or web browser).

11. Verification Procedures

11.1 Bus health check

1. In SIMATIC Manager → Online → Accessible Nodes, browse to the CP 343-2.
2. Right-click → AS-i → Diagnostics. The "List of active slaves" should show your AC1411 at the expected address, profile S-7.F, with no Detected / Projected mismatch.
3. Open the CP 343-2 front panel LEDs: CM green, APF dark, CER dark = healthy bus.

11.2 Bit mapping verification with VAT

1. Open a Variable Table (VAT) in STEP 7.
2. Add the outputs for slave 1 (QB 1 as a byte or individual Q 8.0 … Q 8.3) and the inputs (IB 1 / I 8.0 … I 8.3).
3. Click Monitor / Modify. Force Q 8.0 = 1. You should hear valve 1 of AC1411 #1 click open and the corresponding input feedback bit transition to 1.
4. Force all four outputs in sequence and confirm physical motion + feedback.

11.3 Scope timing check

Connect a scope to AS-i+ and AS-i- at the master. Each slave query appears as a ~150 µs Manchester burst. Total cycle is 5 ms for 31 slaves, 10 ms for 62. Cycle time is the upper bound on your valve response latency.

12. Troubleshooting Matrix

13. Field-Proven Tips

• Always keep one spare address in the bus (e.g. address 31) with no slave attached. It is invaluable when swapping a failed AC1411 in the field — you can hot-swap, then renumber later.
• Use the digital mapping for purely on/off valves. The analog mapping was designed for slaves that expose 4-bit analog values; using it for solenoids forces you to write bit patterns by hand and is error-prone.
• Document the start address of the CP 343-2 in the program header (e.g. // CP343-2 slot 4, IB0..IB15 / QB0..QB15, slave 1 = I8.0..I8.3 / Q8.0..Q8.3). It saves the next commissioning engineer hours of tracing.
• Add the CP 343-2 diagnostics DB (DB 100) and the FC 2 read into OB1. When the bus faults, the DB tells you exactly which slave is missing or which duplicate address exists, instead of a generic SF LED.
• For long cable runs (> 50 m), prefer the yellow cable with the integrated data + power on a single jacket over separate power and signal wires — the AS-i cable's profile and capacitance spec are matched to the master's receiver.

14. Standards and Reference Documents

• SIOS 1171856 — AS-Interface: Introduction and Basic Information
• SIOS 5581657 — Manual CP 343-2 / CP 343-2 P AS-Interface Master (08/2008 + 08/2010 supplements)
• SIOS 51678777 — AS-i interface library for S7 AS-i masters (FC blocks)
• SIOS 50897766 — Diagnostic Block and Visualization via HMI or web browser for AS-Interface
• AS-International official site