Sizing the Siemens CM AS-i Master ST 3RK7137-6SA00-0BC1 for AS-i Networks Above 4 A

The CM AS-i Master ST (Siemens part number 3RK7137-6SA00-0BC1) is the standard AS-i master module for the SIMATIC ET 200SP distributed I/O system. It integrates an AS-Interface v3 master into the backplane bus of an ET 200SP station and exposes the standard and safety slaves to the higher-level S7-1500 CPU through PROFINET. The module is rated for up to 62 AS-i slaves per master and a maximum of 8 masters per ET 200SP station, but the integrated power path that feeds the AS-i cable from the master is constrained to a hard 4 A limit through the busbar. When the cumulative slave current exceeds 4 A, the network must be split into multiple AS-i segments. This reference explains the electrical limits, the load calculation, the topology options, the TIA Portal configuration, and the verification procedure for a representative 42-slave Alfa Laval Think Top network totalling 4.4 A.

1. Problem Definition

The reference design consists of:

• 1 × S7-1500 CPU (e.g. CPU 1511-1 PN or CPU 1511F-1 PN for safety applications).
• 1 × ET 200SP IM 155-6 PN ST interface module (or HF variant for extended diagnostics).
• 1 × CM AS-i Master ST, 3RK7137-6SA00-0BC1.
• 42 × Alfa Laval Think Top AS-i valve positioners, mix of:
• 35 units with 90 mA nominal current consumption.
• 7 units with 180 mA nominal current consumption.
• 1 × AS-i power supply, 30 V DC, 8 A (e.g. 3RX9501-0BA00 or 6EP1334-3BA10).

Total continuous AS-i current from the slaves:

I_total = (35 × 0.090 A) + (7 × 0.180 A) = 3.150 A + 1.260 A = 4.410 A

This is 0.41 A above the 4 A conductor limit on the master's power busbar. A direct connection to a single CM AS-i Master ST is therefore not permissible — the module's integrated current path cannot carry the load, and the derated thermal envelope (see Section 3) will trip long before the cable insulation fails. The slave set has to be re-distributed across two AS-i segments.

2. AS-i Power Architecture Fundamentals

AS-Interface (actuator-sensor interface) is a single-cable bus that combines 30 V DC power and digital I/O signalling on the same yellow profile cable. The relevant electrical and protocol parameters for v3 masters are:

3. CM AS-i Master ST 3RK7137-6SA00-0BC1 Electrical Limits

The CM AS-i Master ST is a single-width ET 200SP base unit (BU type A0) that carries the AS-i voltage from the AS-i power supply onto the yellow bus cable through the integrated power busbar. The terminal layout is:

• 1L — AS-i + (connected to power busbar P1)
• 1N — AS-i − (connected to power busbar P2)

Conductor limits (per the manual page 27 cited in the project discussion):

For a complete pinout and wiring diagram refer to the TIA Portal Fail-Safe Modules Manual Collection and the Siemens AS-i Master brochure (DFCP-B10157-00).

4. Load Calculation for the Think Top Network

Alfa Laval Think Top valve positioners (Think Top D30, Think Top Basic, Think Top V50, Think Top V70) report nominal current consumptions between 60 mA and 200 mA depending on variant, valve feedback, and output configuration. The project uses two populations:

The 8 A AS-i power supply selected for this project is correctly sized for the slave current, but it cannot be routed through a single CM AS-i Master ST because the master's 4 A feed-through is the bottleneck.

5. Topology Options for >4 A Networks

Three architectural options exist for AS-i networks whose slave current exceeds the master's 4 A limit:

5.1 Option A — Two CM AS-i Master ST modules in one ET 200SP station (recommended)

Insert a second 3RK7137-6SA00-0BC1 module next to the first in the same ET 200SP station. Up to 8 CM AS-i Master ST modules can be plugged into a single station. Each master becomes the gateway for its own AS-i segment. The two segments are electrically isolated and can be fed from a single 8 A AS-i power supply through two independent feed terminals, or from two parallel 4 A supplies.

Advantages: clean separation, each segment ≤ 4 A, no extra hardware beyond a second base unit and a second master module, both masters are visible in the S7-1500 as a single PROFINET device with two AS-i sub-systems.

5.2 Option B — Two independent ET 200SP stations on the same PROFINET

Use two separate IM 155-6 PN stations, each with one CM AS-i Master ST. Both are PROFINET devices of the same S7-1500 CPU. This is functionally identical to Option A but spreads the wiring across two cabinets. Use it when physical segmentation is required (e.g. one cabinet per process cell).

5.3 Option C — One CM AS-i Master ST + an AS-i repeater

An AS-i repeater (e.g. 3RK1901-1MX01) extends the cable and re-isolates the segments, but it does not increase slave count or split the power source. Both sides of the repeater are powered from the same 30 V supply through the same master, so the 4 A limit on the master still applies to the entire combined segment. A repeater alone does not solve a >4 A load problem. It is the right tool to extend cable length beyond 100 m, not to scale current.

5.4 Recommended topology for the 4.4 A Think Top network

Use Option A: split the 42 slaves into two groups of 21. For an even load distribution, group the seven 180 mA units and 14 of the 90 mA units on Segment 1, and the remaining 21 × 90 mA units on Segment 2:

Each segment is well below 4 A. Future expansion of up to ~7 additional 90 mA units per segment is possible before the 4 A limit is reached.

6. Topology Diagram (SVG)

7. AS-i Power Supply Sizing and Wiring

The AS-i power supply must be a data-decoupling supply (often called an AS-i power supply or AS-i power extender) — not a generic 30 V DC PSU. The data-decoupling choke integrates an inductor that blocks the AS-i telegram modulation from the supply input while passing the DC component. Suitable Siemens units:

The 8 A supply can drive both segments from a single source. Wire it as follows:

1. Run the 30 V output of the supply to a two-way AS-i power distribution terminal block.
2. From the first output, run a yellow AS-i cable to the 1L / 1N terminals of Master #1, then continue the yellow cable out to the segment-1 slaves.
3. From the second output, run a separate yellow AS-i cable to the 1L / 1N terminals of Master #2 and out to the segment-2 slaves.
4. Earth the FE terminal of the supply. Earth any shielded portions of the AS-i cable per IEC 62026-2.

8. TIA Portal Configuration

Two masters in one ET 200SP station are configured together. Procedure for TIA Portal V17 or later:

1. Open the S7-1500 project. Drag a CPU 1511-1 PN (or 1511F-1 PN if safety slaves will be added later) into the project tree.
2. Drag the ET 200SP station from the hardware catalog → "ET 200SP" → "Interface modules" → "PROFINET" → "IM 155-6 PN ST" (6ES7155-6AU00-0BN0) or "IM 155-6 PN HF" (6ES7155-6AU01-0BN0).
3. Assign a PROFINET device name and IP to the IM. Connect it to the S7-1500 PROFINET interface.
4. Open the ET 200SP device view. In slot 1, leave the IM 155-6. In slot 2, insert "AS-i Master" → "CM AS-i Master ST" → 3RK7137-6SA00-0BC1 on a "BaseUnit BU15-P16+A0+2D" (6ES7193-6BP00-0BA0) or equivalent.
5. In slot 3, insert a second 3RK7137-6SA00-0BC1 on its own base unit. TIA Portal automatically indexes them as AS-i master 1 and AS-i master 2 within the station.
6. In the master properties, set the AS-i mode to "Single master per ET 200SP" (default) and the addressing mode to "Extended (62 slaves)".
7. Wire the 1L / 1N terminals of each base unit to the AS-i power supply. Do not parallel the terminals.
8. Compile and download the hardware configuration. Online, the "AS-i diagnostics" view in TIA Portal will show the slave count and live current per segment.

For projects that include F-CM AS-i Safety ST (e.g. 3RK7136-6SC00-0BC1), refer to the Siemens application note 109480137 for the F-CPU, F-shield, and PROFIsafe addressing. Up to 8 CM AS-i Master ST and F-CM AS-i Safety ST combined can populate a single station — the manual explicitly states "up to 8 CM AS-i Master ST modules" per station.

9. Commissioning and Verification

After wiring and download, verify the network as follows:

1. Power up the ET 200SP and the AS-i supply. The "AS-i voltage OK" LED on each master must be solid green.
2. In TIA Portal, open "Online & diagnostics" → "AS-i diagnostics". Each master should report "Configuration = Actual" with 21 slaves present.
3. Use the AS-i master "Read current" diagnostic (PROFINET record data). Confirm Segment 1 reads ~2.52 A and Segment 2 reads ~1.89 A at idle (valves de-energised). If the readings are higher than the calculated values by more than 10 %, look for a wiring short or a slave drawing excessive current.
4. Cycle each Think Top valve. Watch for AS-i ground-fault alarms. The CM AS-i Master ST detects ground faults on the AS-i cable and disconnects the affected segment to protect the slaves.
5. Force an AS-i reset on each master and verify the slaves re-address cleanly. This validates that the segment has the right number of free addresses.

10. Fault and Diagnostic Matrix

11. Field-Proven Caveats

• Some Alfa Laval Think Top variants draw an inrush current well above the steady-state 90 mA / 180 mA during valve actuation. If the supply cannot sustain the inrush, the AS-i voltage will dip and slaves will reset. Use a supply with at least 50 % inrush headroom or stage the valve actuations.
• The 4 A limit is a continuous rating. Pulsed loads (solenoid actuation, LED indicators on Think Top) can briefly exceed 4 A without tripping the master's thermal protection, but the average must stay below 4 A.
• Never route both segments through the same AS-i power supply terminal block. The two segments need independent feed points to keep the masters isolated.
• When extending the cable beyond 100 m, use an AS-i repeater (e.g. 3RK1901-1MX01) on the same segment — do not put a repeater between two masters. A repeater passes the same AS-i telegram and same 30 V rail; it does not create a new segment.
• For the safety variant, the F-CM AS-i Safety ST (3RK7136-6SC00-0BC1) replaces the standard master on the slot and shares the same wiring. Safety slaves can coexist with standard slaves on the same segment.

12. Summary

The CM AS-i Master ST 3RK7137-6SA00-0BC1 supports up to 62 AS-i slaves but only 4 A through its integrated power busbar. A 42-slave Think Top network at 4.41 A must be split across two masters. The 8 A AS-i power supply is sufficient for the total load, provided it feeds the two segments through independent output terminals. Use TIA Portal to assign the two masters to a single ET 200SP station, verify the load in the online diagnostic view, and observe the 55 °C / 50 °C derating when each master is at its 4 A continuous limit.