AS-i Slave Automatic Addressing with S7-300 and DP/AS-i Link 20E

AS-i Slave Automatic Addressing with S7-300 and DP/AS-i Link 20E Gateways

This technical reference documents the end-to-end procedure for adding new AS-i slaves to an existing PROFIBUS network built around a SIMATIC S7-300 controller and Siemens DP/AS-i LINK 20E gateways. The focus is assigning addresses to factory-default AS-i slaves (address = 0) without a handheld addressing unit, using the built-in automatic-addressing behavior of the gateway triggered by a STEP 7 hardware download, with a documented fallback to a handheld addressing tool when the auto-addressing preconditions cannot be satisfied. The same procedure applies to non-safety and safety slaves in the SIRIUS 3RK1 family such as the 3RK1205-0BQ00-0AA3, but safety slaves require an additional safety-code verification step after the AS-i address is written.

1. System Overview and Network Topology

AS-Interface (AS-i) is a single-cable, two-wire, mechanically-keyed fieldbus standardized in IEC 62026-2. A yellow profile cable carries both 24 V power and the AS-i signal simultaneously; slaves are pierced onto the cable and the master gateway exchanges up to 4-bit process data with up to 31 standard slaves (or 62 slaves with A/B extended addressing) per segment. Each AS-i master gateway exposes its slaves as a process image to the higher-level PLC, where they appear as ordinary I/O.

The Siemens DP/AS-i LINK 20E (catalog number 6GK1415-2BA10) is a PROFIBUS DP slave that operates as an AS-i V3.0 master on its AS-i side. It exposes the AS-i segment as a compact PROFIBUS DP slave with a fixed or configurable I/O footprint, so the S7-300 CPU 31x can read AS-i inputs and write AS-i outputs without an additional CPU on the AS-i side. The reference configuration treated in this article is:

• One SIMATIC S7-300 station (CPU 315-2 DP / CPU 317-2 DP / CPU 319-3 PN/DP family) acting as PROFIBUS DP master.
• Three DP/AS-i LINK 20E gateways already configured and operational on PROFIBUS.
• Two additional DP/AS-i LINK 20E gateways to be brought into service (the procedure below covers initial commissioning of these two gateways).
• AS-i safety slaves, type 3RK1205-0BQ00-0AA3, from the SIRIUS 3RK1 family, each delivered from the factory with AS-i address = 0.

The discussion also applies when migrating from the older DP/AS-i LINK (non-20E) to the 20E series, and when extending segments on the existing gateways with additional slaves beyond their current fill.

2. AS-i Address Theory: LDS, LAS, LPS

Every AS-i slave can be assigned any address in the range 1 to 31 (single address per segment) or 1A..31A / 1B..31B (extended addressing for 62 slaves per segment). The factory-default address is 0, which is reserved for "unaddressed": a slave at address 0 reports itself to the master but does not participate in cyclic data exchange. The first step of any AS-i commissioning exercise is therefore to move every slave from address 0 into the configured address range.

The AS-i master maintains three data structures per segment that drive both commissioning and runtime diagnostics:

A healthy segment has LDS equal to LAS, an empty LPS, and APF / CFG / PF all clear. A factory-default slave appears in the LDS but not in the LAS. The transition from "detected but unaddressed" to "activated and addressed" is what the commissioning procedure below achieves.

3. Hardware Components in the Reference Configuration

For unambiguous documentation, the components referred to in the procedure below are listed with their Siemens catalog numbers. Confirm the latest revision in Siemens Industry Online Support before commissioning.

4. Automatic Addressing: Mechanism and Preconditions

The DP/AS-i LINK 20E implements automatic addressing on each segment under a strict set of preconditions documented in the gateway manual and in the Siemens AS-i master application notes. Automatic addressing is initiated by the gateway when it detects that exactly one address is missing in the LAS and that the missing address corresponds to a slave currently present in the LDS but reporting an address of 0. The gateway then writes the configured target address to the single address-0 slave during the next configuration cycle, and the slave moves from "LDS only" into the full LAS.

The complete preconditions are:

1. The desired target address exists in the LAS downloaded to the gateway (i.e., STEP 7 hardware config includes that address with a valid slave ID/ID2 code).
2. Exactly one slave with address 0 is connected on the segment.
3. No other configuration drift exists between the LAS and the LDS beyond the single address-0 slave (no extra slaves, no missing slaves other than the one being addressed).
4. The AS-i power supply is healthy and the segment is not in APF state.

When these conditions are satisfied, automatic addressing is invisible: the engineer connects the new slave, the gateway acknowledges it, and the slave appears at the configured address. No handheld tool is required.

5. Prerequisites and Required Tools

Before commencing the procedure, verify the following items are available and within their service life:

• STEP 7 V5.5 or higher (Classic) with the GSD file for the DP/AS-i LINK 20E installed. The GSD file revision should match the gateway firmware revision; obtain the latest revision from Siemens Industry Online Support under product 6GK1415-2BA10.
• DP/AS-i LINK 20E firmware at a current revision (V3.x or higher recommended for safety slaves). Newer firmware adds diagnostic features such as the WEB interface for LDS/LAS inspection and improved error reporting.
• AS-i power supply sized for the segment: 100 m of cable with up to 31 slaves typically draws 2 A to 2.6 A; an end-of-segment voltage above 26.5 V is required for stable operation. Siemens 3RX9501-0BA00 (2.6 A) is sufficient for most cabinets; longer segments require 3RX9501-0BB00 (4 A) or a re-evaluation of topology.
• Yellow AS-i profile cable (mechanically keyed, reverse-polarity safe) with proper piercing connectors.
• One handheld AS-i addressing tool as a fallback. The ifm AC1154 (or any equivalent unit from the ifm AS-i addressing units family) covers addresses 1..31 and 1A..31B and provides read access to ID/ID2 codes.
• For safety slaves (3RK1205-0BQ00-0AA3), an AS-i Safety configuration tool: ASIMON (Bihler+Wiedemann / Siemens partner) or the STEP 7 Safety add-on (S7 Distributed Safety or S7 F-Configuration Pack, depending on the safety CPU). The safety-code table must be re-verified or re-taught after the AS-i address is written.
• Portable AS-i line monitor (optional but recommended): a tool that displays segment voltage, telegram error counters, and per-slave signal quality. Useful for diagnosing marginal segments before the new gateways are activated.

6. Commissioning Procedure

6.1 Add the new DP/AS-i LINK 20E gateways in STEP 7 HW Config

1. Open the S7-300 station in STEP 7 HW Config.
2. Locate the PROFIBUS DP subnet and confirm free PROFIBUS addresses for the new gateways (existing gateways typically use 3, 4, 5; assign 6 and 7 to the new ones).
3. From the catalog, navigate to PROFIBUS DP > Gateway > AS-i > DP/AS-i LINK 20E and drag it onto the PROFIBUS line.
4. Set the PROFIBUS address in the gateway properties; verify the baud rate matches the existing subnet (typically 1.5 Mbps for AS-i gateways).
5. Save and compile, then download to the CPU with PLC > Download to Target Station. The CPU must be in STOP or RUN-P; downloading in RUN is supported when no slots are remapped.

6.2 Configure the AS-i slaves for each new gateway

For each newly added DP/AS-i LINK 20E:

1. Double-click the gateway symbol to open its properties dialog.
2. Open the "AS-i Configuration" tab (STEP 7 Classic) or "AS-i slaves" inspector (TIA Portal).
3. Insert each 3RK1205-0BQ00-0AA3 slave from the AS-i slave catalog using its correct ID/ID2 code. The slave appears in the configuration slot with its process image footprint (typically 4DI/2DO or similar, depending on the slave profile; the safety data occupies the upper 4 bits of the cyclic input word).
4. Assign the desired AS-i address to each slave configuration slot. Recommended practice: reserve logical address blocks per gateway (e.g., gateway 4 = 1..10, gateway 5 = 11..20) and reserve 29..31 as "spares" for future additions without reconfiguration.
5. Save the configuration and download it to the CPU (PLC > Download > Configuration in STEP 7 Classic; Online > Download to device in TIA Portal).

At this point, the gateway holds the expected LAS but no slaves are physically present (or the slaves are still at address 0). The gateway reports a configuration error: CFG flag is set, APF clear (assuming healthy supply). This is expected and is resolved by the next step.

6.3 Connect the slaves one at a time to trigger automatic addressing

This is the critical step. The gateway's automatic addressing requires that exactly one address-0 slave be present while exactly one LAS slot is empty:

1. Confirm the segment is powered: AS-i power supply LED green, segment voltage 29.5 V to 31.6 V measured at the supply output, and at least 26.5 V at the end of the segment (the segment drops voltage under load; this minimum must be observed at the most distant slave).
2. Connect a single 3RK1205-0BQ00-0AA3 slave to the segment at its planned mechanical location. Its factory address is 0.
3. Cycle power to the slave or briefly disconnect and reconnect its M12 termination so the gateway re-scans the LDS.
4. Watch the gateway LEDs and the STEP 7 online diagnostics:
   • CM (Configuration Mode) LED should remain off after auto-addressing completes.
   • The slave appears in the LDS.
   • Within seconds, the gateway assigns the configured address to the slave, and the slave moves from "LDS only" into the LAS.
5. Verify the address via the gateway's WEB interface (if firmware supports it), an AS-i line monitor, or the online diagnostics view in STEP 7.
6. Repeat for each subsequent slave: connect one, wait for auto-addressing, confirm before connecting the next. Never connect two new address-0 slaves simultaneously on a single segment.

7. Manual Fallback: Handheld Addressing

When the auto-addressing preconditions cannot be met — multiple new slaves on a populated segment, non-sequential address assignment required, or safety slaves with non-default parameter sets — assign addresses with a handheld AS-i addressing unit before connecting the slaves to the live segment.

Procedure with a handheld tool:

1. Isolate the slave from the live AS-i segment (physically disconnect it).
2. Connect the addressing unit to the slave using the addressing cable. The cable injects AS-i voltage and data into the slave without involving the segment master.
3. Power the addressing unit (or rely on the slave's internal supply if available).
4. Select the target address on the unit (1..31 or 1A..31B as appropriate).
5. Execute "Write address"; the unit confirms with a green LED or a display message.
6. Optionally read back the address and the ID codes to verify.
7. Disconnect the addressing cable and reconnect the slave to the live segment at its planned location.

8. Verification Procedures

After each slave is addressed and connected, run through the following checks before declaring the segment healthy:

9. Troubleshooting Matrix

10. Best Practices and Field-Proven Notes

• Always assign a logical address plan on paper before commissioning. AS-i addresses are not auto-negotiable like IP or PROFIBUS addresses; they are persistent in non-volatile memory and require deliberate re-assignment if changed.
• Document the address-to-location mapping in the plant documentation, on the cabinet door, and in the EPLAN / AutoCAD documentation. Address 17 on gateway 4 in cabinet K-12 means nothing two years later without a label.
• For safety slaves (3RK1205-0BQ00-0AA3), keep the addressing cable and a handheld unit in the spares kit. The safety code is independent of the AS-i address but must be re-taught if the slave is replaced with a new factory-default unit.
• When extending an existing segment, consider whether the additional slaves should go on the same gateway or on a new gateway with its own AS-i power supply. A dedicated 24 V AS-i supply per gateway avoids ground-loop and voltage-drop issues on long segments and improves fault isolation.
• Always reserve the last few addresses (e.g., 29..31) as "spares" in the LAS, even if no slave is currently connected. Future additions do not then require a full reconfiguration.
• Keep the DP/AS-i LINK 20E firmware current. Siemens publishes firmware updates via the Siemens Industry Online Support portal; check the entry for 6GK1415-2BA10 for the latest version compatible with your STEP 7 revision.
• Disable "auto-config" mode in the gateway unless the segment is genuinely intended to be self-configuring. Auto-config silently reassigns addresses if a slave is replaced with a factory-default unit, which can mask commissioning errors.
• Run a 24-hour burn-in with telegram error counters monitored after initial commissioning. Marginal cables, poor piercing connections, and EMC issues manifest as low-rate telegram errors that are easy to miss during a brief commissioning visit.

11. Differences: STEP 7 Classic vs. TIA Portal

The addressing workflow described in Section 6 is essentially identical in STEP 7 Classic (V5.5 / V5.6) and TIA Portal, because the underlying mechanism — automatic addressing of a single address-0 slave when one LAS slot is empty — is implemented in the gateway firmware, not in the engineering tool. The user interface differs in catalog navigation and configuration download:

The GSD file approach (used when integrating the gateway into a third-party PROFIBUS master) exposes the gateway as a PROFIBUS DP slave with a fixed I/O footprint and does not provide the engineering comfort of the embedded AS-i configuration editor. For green-field installations, prefer STEP 7 or TIA Portal with the Siemens GSD or HSP.

12. Segment Power Sizing Worked Example

When adding new slaves to a segment, the power supply must deliver at least 26.5 V at the most distant slave under full load. The standard AS-i supply provides 30 V nominal at the output. Voltage drop on the AS-i cable follows the resistive drop of the two conductors carrying the return current, plus the load current of every slave and the segment terminations.

For a single-phase two-wire analysis with current I in amps and total loop resistance R in ohms, the round-trip drop is V_drop = 2 * I * R. With copper AS-i profile cable of cross-section 1.5 mm² (typical), the loop resistance is approximately 0.025 Ω/m including both conductors. For a 50 m segment with 20 slaves drawing 100 mA each (typical for standard slaves; safety slaves drawing more), the loop resistance is 50 * 0.025 = 1.25 Ω and the total current is 20 * 0.1 = 2 A. V_drop = 2 * 2 * 1.25 = 5 V. With 30 V supply at the output, the end-of-segment voltage is 30 - 5 = 25 V, which is below the 26.5 V minimum.

Countermeasures for marginal segments include reducing the segment length, using a higher cross-section cable (2.5 mm²), splitting the segment into two parallel segments each on its own AS-i power supply, or upgrading the supply to a 4 A unit with tighter regulation. After any change, re-measure the end-of-segment voltage under load before commissioning the additional slaves.

13. FAQ