Siemens LOGO! 8 CMK2000 KNX Module: 50 Communication Objects Explained

The Siemens LOGO! 8 CMK2000 communication module is the gateway between the LOGO! 8 logic relay family and a KNX (TP1) twisted-pair building automation bus. It exposes LOGO! internal flags, inputs, outputs, analog values, and network variables as KNX communication objects, and converts incoming KNX group telegrams back into LOGO! memory areas. The module is documented in the Siemens application note LOGO! 8 Gate Circuit for KNX (PDF) and is intended for non-safety building services such as lighting, blinds, HVAC, and metering.

1. CMK2000 Hardware Identity and Slot Rules

The CMK2000 is a slim expansion module that snaps onto the right-hand side of a LOGO! 8 base unit (LOGO! 8.FS4 and later 6ED1052-* and 6ED1052-*-8A firmware families). It occupies one expansion slot in the LOGO! backplane and is addressed internally like any other digital or analog expansion.

2. What "50 Communication Objects" Actually Means

A KNX communication object is the smallest addressable data element a KNX device can send or receive. Each object has:

• A direction — transmit (the device publishes on the bus), receive (the device subscribes), or both (read/write flags such as C, R, W, T, U in the ETS property dialog).
• A data point type (DPT) — e.g. DPT 1.001 (switch), DPT 5.001 (percent 0–100 %), DPT 9.001 (temperature °C), DPT 12.001 (counter pulses).
• A group address binding — one or more KNX group addresses that fan the object out to other devices on the bus.
• A length — 1 bit, 1 byte, 2 bytes, 4 bytes, or 14 bytes (14-byte COs are not used by the CMK2000).

The CMK2000 ships with a fixed set of 50 object slots in its ETS database. The integrator chooses, slot by slot, which KNX data point to expose, the DPT, the direction flags, and which group address binds to it. The 50-slot budget is therefore the total of both transmit-side (LOGO! → KNX) and receive-side (KNX → LOGO!) objects.

2.1 Counting Examples

3. CMK2000 is Not 50 KNX Devices

A single KNX actuator (for example a 4-channel dimmer or an 8-output switching actuator) can subscribe to hundreds of group addresses and respond to any of them; the CMK2000 is not aware of how many physical actuators, sensors, or wall panels exist on the bus. The bus itself is the shared medium; every device listens to every telegram and acts on the group addresses it has been bound to in ETS. The CMK2000 simply contributes up to 50 unique data points to that shared namespace.

Similarly, one wall-mounted KNX thermostat can supply and consume far more data points than a single LOGO! CO. The number of physical KNX end devices on a TP1 line is governed by the KNX standard's bus power budget (typically 64 devices per line, with a maximum of 256 group addresses per device and several thousand per line) — none of those limits are imposed by the CMK2000's 50-object ceiling.

4. Topology: One CMK2000 per LOGO! 8

The official Siemens documentation states that a LOGO! 8 base unit supports exactly one CMK2000 module. There is no daisy-chaining, no IP backplane bridging of multiple CMK2000 units, and no support for stacking two CMK2000 modules to gain 100 communication objects. If the application genuinely needs more than 50 KNX data points at one logic location, the project must either:

1. Aggregate objects on the KNX side (one CMK2000 CO can carry a 4-byte value, packing multiple logical signals in one DPT 13.xxx or 14.xxx telegram — though 14-byte objects are not used by CMK2000, 4-byte IEEE float and 32-bit unsigned are).
2. Use a second LOGO! 8 with its own CMK2000 and a peer-to-peer KNX coupling between the two projects.
3. Replace the CMK2000 with a Siemens KNX/IP router plus a LOGO! CMR (LOGO! Communication Module for Ethernet) and handle the protocol translation in an external controller.

4.1 Backplane Stack Example

[LOGO! 8 base] — [DM8 230R] — [DM8 230R] — [AM2] — [CMK2000]
   CPU slot        I/O slot 1   I/O slot 2   Analog  KNX interface
   (Slot 0)        (Slot 1)     (Slot 2)    (Slot 3)  (Slot 4 — last)

4.2 Network Topology SVG

5. Communication Object Types Supported by the CMK2000

The CMK2000 ETS database predefines a curated set of object types. Available DPTs include:

6. Mapping Between LOGO! Tags and KNX Communication Objects

The CMK2000 uses a fixed mapping table between LOGO! memory elements and KNX CO slots. Inside LOGO! Soft Comfort the user does not assign objects arbitrarily — the CMK2000 plug-in generates a lookup of available source/sink symbols and the integrator chooses, in ETS, which CO slot is bound to which group address.

6.1 Typical Mapping (Transmit Direction — LOGO! → KNX)

6.2 Typical Mapping (Receive Direction — KNX → LOGO!)

7. Step-by-Step ETS5 Configuration Procedure

The following procedure assumes a Windows PC with ETS5 or later installed and a Siemens or third-party KNX USB / IP interface connected to the bus.

7.1 Prerequisites

1. LOGO! 8 base unit (firmware FS04 or higher) programmed with LOGO! Soft Comfort V8.3 or later, with the CMK2000 explicitly added to the hardware configuration.
2. CMK2000 ETS product database installed. Import the .knxprod / .vd5 file supplied by Siemens Support into ETS.
3. Free ETS licences equal to the number of physical devices commissioned. The CMK2000 counts as one device.
4. One free physical KNX address on the line. The CMK2000 will receive its individual address (e.g. 1.1.45) during commissioning.
5. KNX programming interface (USB, IP, or RS-232) connected and bus voltage present (typically 29 V DC from the KNX power supply).

7.2 Procedure

1. Add the CMK2000 to the ETS project. In the ETS project tree, navigate to Buildings → Line 1.x, right-click and Insert Devices. Select the LOGO! 8 CMK2000 product.
2. Assign a physical address. Right-click the device, choose Properties and set the individual address (e.g. 1.1.45). Make sure the address is unique on the line.
3. Inspect the 50 communication objects. Open the device dialog. The Communication Objects tab lists all 50 CO slots with their current DPT assignment and direction flags. Unused COs remain in the database but cost no group address resources.
4. Create the group address structure. Define a 2- or 3-level group address tree (Main/Middle/Sub, e.g. 1/Office/1 Light On). Two-level is also legal but harder to maintain.
5. Bind COs to group addresses. Drag a group address from the tree onto a CO slot, or use the Group Objects tab in the CO property dialog. Set the Transmit (T) and Update (U) flags as needed:
   • T enabled → CMK2000 may send the value on the bus when the LOGO! tag changes.
   • U enabled → CO can be read with a GroupValueRead telegram.
6. Download the application to the CMK2000. Click Download → Application, choose the CMK2000 in the device list, and confirm. ETS transmits the application over the bus; the CMK2000 stores it in non-volatile memory.
7. Assign the physical address. Click Download → Physical Address and confirm. The CMK2000's programming LED lights up; press the physical programming button on the module to confirm the address write.
8. Verify in ETS Diagnostics. Open Diagnostics → Bus Monitor and force a read on a transmit CO; you should see a GroupValueRead followed by a GroupValueResponse from the CMK2000 carrying the current LOGO! tag value.
9. Cross-check with LOGO! Soft Comfort online mode. Connect LOGO! Soft Comfort to the base, go online, and toggle a digital input. The corresponding CO should publish on the bus (visible in the bus monitor) and any subscribed device (e.g. switching actuator) should react.

8. LOGO! Soft Comfort Integration Steps

In LOGO! Soft Comfort V8.3 and later, the CMK2000 is configured as a "Communication Module" in the device catalog. The relevant steps are:

1. Open the project, double-click Hardware in the project tree.
2. Drag the LOGO! CMK2000 from the catalog onto the rightmost expansion slot.
3. In the module properties, set the LOGO! tag to be exposed for each transmit CO and the LOGO! tag destination for each receive CO. Soft Comfort writes these to a special mapping block in the LOGO! program memory.
4. Compile and download the program to the base. The base firmware loads the mapping table into the CMK2000 during startup.
5. Verify with the Soft Comfort online test: every CO whose source tag changes in the LOGO! program must be visible in ETS Bus Monitor within ~50 ms (typical cycle time of the CMK2000 send task).

9. Bidirectional Signal Flow Example

Consider a small office with one window:

• LOGO! input I1 wired to a local override switch (potential-free contact, 24 V).
• LOGO! output Q1 wired to the relay coil of a 230 V blind motor contactor (realised with an external contactor; the LOGO! output is rated for the contactor coil only).
• LOGO! flag M10 = blind-up command, M11 = blind-down command, M12 = blind-stop command.

The user wants a KNX wall push-button on group address 1/1/5 to drive M10/M11/M12, and wants the LOGO! to publish the current blind state (computed in the LOGO! program) to group address 1/1/6 so that a KNX visualisation panel can display it.

The wall push-button sends 1/1/5 = 0 (up) or 1/1/5 = 1 (down). The CMK2000 writes the value into LOGO! network input 1; LOGO! Soft Comfort maps NI1 to flag M10 in the user program, which the program uses together with M11 to drive Q1. The current state of the blind is mirrored on AQ1, which Soft Comfort maps to CO33 and the CMK2000 publishes back on 1/1/6.

10. Verification and Field Diagnostics

After commissioning, run the following checks to confirm that the CMK2000 is operational and that the 50-object budget is correctly populated:

1. Power-up LED test — The CMK2000's RUN LED (green) must be solid within 3 s of bus voltage application. A flashing red LED indicates ETS application download failure.
2. ETS device info — Open Diagnostics → Device Info on the CMK2000 in ETS. The application version, the ETS project version, and the current individual address must be present.
3. Bus monitor sanity check — Force a GroupValueRead on a transmit CO; expect a GroupValueResponse with the current LOGO! tag value. Force a GroupValueWrite on a receive CO; expect the corresponding LOGO! flag to change in Soft Comfort online mode within one cycle.
4. Object budget audit — In ETS, right-click the CMK2000 device, choose Show Communication Objects, and count the COs that are bound to a group address. The total must not exceed 50.
5. Cycle time profiling — Connect the ETS bus monitor with a millisecond timestamp. Toggle a LOGO! input faster than the cycle time (e.g. 10 Hz) and verify that the bus only sees telegrams at the configured cycle rate, not at the input toggle rate.
6. Restart persistence — Power-cycle the LOGO! base. Within 5 s the CMK2000 must resume publishing and subscribing. All CO bindings and the LOGO! tag map must be retained.

11. Limitations, Constraints, and Field-Proven Caveats

• One CMK2000 per LOGO! 8 base. Stacking is not supported; do not attempt to use a second CMK2000 to reach 100 objects.
• 50 objects are shared between directions. Heavy receive-side projects (e.g. many wall sensors) and heavy transmit-side projects (e.g. many published analog values) both consume the same 50-slot budget.
• LOGO! Soft Comfort compatibility. Versions older than V8.3 do not include the CMK2000 plug-in. Update both Soft Comfort and the LOGO! base firmware to FS04 or higher before commissioning.
• 14-byte string COs are not exposed. If a downstream KNX visualisation expects ASCII status strings, route the data through an additional KNX device that supports string DPTs (e.g. Siemens N 152 / IP interface with custom logic) or use a 4-byte float and decode on the receiver side.
• Cycle time vs. debounce. The CMK2000's send-cycle is 100–200 ms; sub-cycle toggles are filtered. Use LOGO! debounce ON-delays / OFF-delays in the program if you need bus-event filtering.
• Power supply sensitivity. The CMK2000 draws its KNX-side power from the bus. A weak or overloaded KNX power supply will manifest as the CMK2000 dropping off the bus under heavy parallel traffic. Verify the bus power supply rating (640 mA standard, 1280 mA for large lines) before troubleshooting the module.
• Topological placement. The CMK2000 must be the rightmost module. Inserting a non-CMK2000 module after it stops LOGO! Soft Comfort from enumerating the stack and bricks the configuration upload.
• No safety functions. The CMK2000 is a building-services gateway and is not suitable for safety-critical KNX applications (EIB/KNX Safety is a different ecosystem). Do not use it to transmit fire-alarm, gas-leak, or emergency-stop signals.
• No native KNX Secure. The CMK2000 does not support KNX IP Secure or KNX Data Secure. Where the application requires secured KNX traffic, deploy a KNX IP Secure router with the unsecured CMK2000 behind a line coupler that filters secure segments, or use a newer Siemens KNX gateway that supports KNX Secure.
• ETS database version drift. Older ETS projects created with the CMK2000 product version 1.x may not be loadable into a base running newer firmware without re-issuing the application. Always re-download after a firmware upgrade.

12. Comparison with Similar Siemens KNX Gateways

The CMK2000 is the right choice when the application is built around a LOGO! 8 program and the integrator simply needs to bridge LOGO! flags to KNX telegrams and vice versa. For larger KNX installations, IP-based visualisations, or KNX Secure requirements, a dedicated IP interface or Secure router is more appropriate.

13. Frequently Asked Questions