Connecting Multiple LOGO! PLCs Without KNX: Wired Methods

Problem Context: Multi-LOGO! Coordination for Building Automation

A recurring automation requirement in multi-floor residential and small commercial buildings is to coordinate several Siemens LOGO! controllers — typically one per floor — to drive a shared function such as shutter, blind, or awning control. The reference application in the field typically involves four or five LOGO! units, each running its own local program (local pushbutton station, end-limit evaluation, runtime monitoring, thermal cut-out) but needing to respond to a central command such as "raise all blinds" or "lower all blinds" without investing in a full KNX/Instabus infrastructure.

Engineers evaluating this architecture quickly find that the LOGO! platform historically does not provide an onboard networking layer for the classic 0BA5/0BA6/0BA7 base units. The base unit exposes only local digital inputs, local digital outputs, optional analog inputs, and the LOGO! TD text-display port. There is no native CAN, RS-485, or Ethernet bus between LOGO! peers on those generations. As a result, multi-unit coordination must be solved through one of four engineering paths:

• Wired point-to-point I/O handshake between units (DO/DI bus).
• Parallel wiring of shared sensors and central commands to every unit simultaneously.
• Pulse-frequency signaling between units using transistor outputs and threshold triggers.
• A retrofittable communication module (CM EIB/KNX, or the onboard Ethernet of a newer LOGO! 8 generation).

This reference documents the wired and signaling methods that work on the classic LOGO! generations, and compares their cost, scalability, and commissioning effort against the KNX/Instabus alternative.

Native LOGO! Networking Constraints

For the LOGO! 0BA5 and 0BA6 base modules, the hardware does not provide any inter-controller communication path. Programming and online monitoring are performed over the LOGO! PC cable (RS-232 to the LOGO! port), but no protocol exists to exchange data between two base units over that port. The 0BA7 generation adds a 10/100 Ethernet jack on the "...E" variants, but the Ethernet port is restricted to LOGO! Soft Comfort programming and SD-card web-page download — it is not a peer-to-peer bus.

KNX/Instabus Cost Reference: Why the Path Is Often Rejected

KNX/Instabus is the architecturally clean answer for distributed building automation and the supported Siemens path for LOGO!-to-building-bus integration. The CM EIB/KNX communication module plugs into the left side of any LOGO! base unit and exposes a subset of LOGO! variables as KNX group objects. However, the total system cost is driven by items that are external to the LOGO! itself:

• One CM EIB/KNX module per LOGO! — historically in the €150 retail range each at single-unit list.
• KNX-certified twisted-pair cable (typically a green-jacketed EIB/YcYM 2x2x0.8 mm) run in a bus topology from device to device, with bus terminations.
• A KNX bus power supply — e.g. SIEMENS N 125/22 (or N 125/13) — supplying 29 V DC SELV with integrated choke.
• A KNX/USB or KNX/IP interface for engineering — e.g. SIEMENS N 148/11 (USB) or N 146 (IP router).
• Licensed ETS (Engineering Tool Software) — at the time of the original field discussion, ETS3; current releases are ETS6 — sold as Professional or Light edition.
• KNX integrator training and certification, mandatory for some ETS licences and for certified project handover.

For a 4-LOGO! project that exists to roll up four sets of blinds in the evening, the KNX path is structurally correct but financially disproportionate — the field consensus is that it represents roughly a ten-times cost factor over a hard-wired solution. The next sections document the engineering alternatives.

Method 1: Bidirectional Wired DI/DO Network

The classic field technique to exchange control information between two or more LOGO! units is to wire relay or transistor outputs from one unit directly into the digital inputs of the other units. The bus is the wire, and the protocol is the contact pattern on that wire.

Two-LOGO! Configuration (4 states per direction)

With two digital outputs from LOGO! 1 wired to two digital inputs of LOGO! 2, and two outputs from LOGO! 2 wired to two inputs of LOGO! 1, each unit can command four mutually-exclusive states to its peer:

The reverse wiring allows LOGO! 2 to send its own four-state command set back to LOGO! 1. With two pairs of wires you therefore have a fully bidirectional 4-by-4 state exchange — 16 unique combinations if both sides drive simultaneously, of which only the meaningful subsets are used in practice.

Three-LOGO! Configuration (addressing expansion)

To add a third LOGO! to the same two-wire command bus, you must assign a unit address. The third wire pair (one extra DO from the sender, one extra DI on every receiver) acts as the address-enable. The standard expansion rule is:

• Sender uses 3 DO bits: [A] [B] [ADDR].
• Receivers each have 3 DI bits wired in parallel to the bus; only the unit whose hard-wired "address" input matches the ADDR pattern will accept the command.

This consumes one I/O bit per additional addressable unit, so four LOGO! units require 4 DO + 4 DI per command channel at the sender, and 4 DI per receiver. For practical building projects with 4–5 floors, this still fits inside the LOGO! 0BA7 I/O budget (24 DI / 16 DO on a fully expanded base plus DM16 expansion modules).

Handshake Discipline

The wiring above is only half the engineering. Without a handshake protocol, two units can drive the bus into a 1-1 collision, and a receiver cannot distinguish "command still active" from "command latched and forgot to clear". The standard discipline is:

1. Sender asserts the 2-bit command word, then a 200–500 ms "strobe" pulse on a third DO line.
2. Each receiver latches the command on the rising edge of the strobe, executes the action, and acknowledges with one DI bit back to the sender.
3. Sender holds the command word until all expected acknowledges are seen, then releases all bus lines to 0.
4. Each receiver clears its latched command when the strobe returns to 0 and the command word is 00 for a configurable debounce time (typically 1 s) to avoid chattering.

Bidirectional DI/DO Topology (4 floors)

Method 2: Parallel Central Control Pushbuttons

The simplest possible multi-LOGO! architecture is the "common command bus": the central pushbutton is wired in parallel to a dedicated input on every floor's LOGO!. All four controllers see the central UP and central DOWN commands simultaneously and execute them locally against their own shutter.

The reference application for this method is a building with:

• One LOGO! per floor, each controlling one or two shutter motors.
• One central pushbutton station in the entrance or stairwell.
• Local UP / DOWN / STOP pushbuttons in each room, connected to the local LOGO!.

In the Siemens reference example for 8-shutter control, the central UP and central DOWN pushbuttons land on inputs I17 and I18 of each LOGO!, leaving the lower I1–I16 for local control. The local program blocks the central command for a configurable lockout time (e.g. 30 s) after a local command, to prevent a user upstairs from immediately over-riding a central "all down" command in mid-stroke.

Wiring Rules for the Parallel Bus

• Use a single 24 V DC rail (or 230 V AC where the LOGO! is the "R" relay variant and the pushbutton station is mains-isolated).
• Twist the central pushbutton pair with the supply return to reject injected noise.
• Place a 100 nF X2-class capacitor across each pushbutton to suppress contact arcing — the LOGO! 24 V DC inputs sample internally and will see switch bounce as multiple presses if not debounced.
• Keep total cable capacitance below 50 nF to ground. For a typical 0.5 mm² twisted pair this is roughly 200 m of run length; above that, use a buffer relay at the far end.

Local Program Fragment (LOGO! Soft Comfort FBD)

I17 (Central UP)  --+                    +--[ AND lockout_ok ]-- B001 -- Q1 (Shutter UP)
                    +--[ OR ]-----------/
I01 (Local UP)    --+                       ^
                                                |
                              Timer T01 = 30 s lockout, reset by I01/I02/I03

Method 3: Pulse-Frequency Signaling

When a single wire must carry more than one or two commands between LOGO! units, the field workaround is pulse signaling. A 24 V DC transistor output on the sender generates a square wave at a defined frequency; each receiver decodes the frequency with a LOGO! Threshold Trigger (or a counter at a known clock rate) and acts only on its assigned frequency.

Frequency Plan (4-channel field example)

Each frequency channel needs a sender pulse generator and a receiver threshold trigger. On a LOGO! 0BA7 the asymmetric clock generator (Asynchronous pulse generator) and the threshold trigger block are sufficient. The receiver must filter for at least 3 consecutive valid periods before accepting the command, which translates to roughly 60–600 ms of detection latency depending on the channel.

Hard Limits

• Only LOGO! variants with 24 V DC transistor outputs can be the sender. Relay outputs cannot reach 20 Hz cleanly and will fail mechanically at that rate.
• All units on the bus must share the same power-supply common (the 0 V rail), otherwise the frequency reference drifts by the ground potential difference.
• Wire length is limited by the rise/fall time of the transistor output driving the bus capacitance. Practical limit is around 50 m of twisted pair for 24 V DC; longer runs need a line driver.

Method 4: ASi Bus and Newer-Generation Ethernet

AS-Interface (ASi) is often proposed as a cheaper fieldbus alternative to KNX for binary I/O. The ASi master sits on a higher-level controller and addresses up to 31 slaves over a two-wire unshielded yellow cable that carries both data and 30 V DC power.

Directly integrated ASi master support does not exist on the classic LOGO! 0BA5–0BA7 base units. The path is to fit a CP ASi master module (for example a Siemens DP/AS-i LINK or a third-party gateway) to a host PLC, then connect the LOGO! I/O to the ASi bus through standard ASi slave I/O modules. The pure-LOGO! solution, with no host controller, cannot use ASi. This is a documented field limitation and is the reason ASi is excluded from a 4-LOGO! standalone project.

If the installed hardware is a newer-generation LOGO! 8 (0BA8) or later, the most architecturally clean solution is the onboard Ethernet port. That generation supports S7-compatible connections to other LOGO! peers, configurable in LOGO! Soft Comfort under network connections; a Modbus TCP server (port 502) for read-only access to LOGO! variables; and a built-in web server exposing VM (variable memory) addresses for browser-based HMI. All four floors' LOGO! units connect to a single 100 Mbit Ethernet switch on the building riser; the central pushbutton station is wired to one designated LOGO! (e.g. ground floor) and that unit multicasts the command to the other three over the S7 connections. The methods documented above remain valid as retrofits on existing 0BA5–0BA7 installations where Ethernet is not available.

Reference Application: Multi-Floor Shutter State Machine

The state machine below is the canonical 4-state model used for a single shutter driven by either a local command or a central command, with mutual exclusion. The same FSM runs in every floor's LOGO!.

Sample Program for a Single Floor (LOGO! Soft Comfort, FBD)

The same program is loaded into every floor's LOGO!. The I/O assignment is:

The mutual-exclusion logic prevents both contactors from pulling in simultaneously, which would short the motor phase-to-phase through the two contactors. The lockout timer prevents a central "all down" command from immediately reversing a shutter that a local user just started raising.

Wiring, Cable, and Power Supply Specification

Contact Protection

The shutter motor is an inductive load. Even with a contactor between the LOGO! output and the motor, the contactor coil itself is inductive and will generate a back-EMF of several hundred volts at turn-off. Add an RC snubber (100 Ω + 100 nF, X2 class) across each contactor coil to limit the dv/dt to below 1 kV/μs. Without the snubber, the LOGO! relay contact will pit and weld within a few tens of thousands of operations.

Power Supply Sizing

For a 24 V DC system, each LOGO! 0BA7 base draws roughly 85 mA at 24 V with all inputs OFF and 200 mA with all inputs ON. Four base units plus the central pushbutton station and a contactor coil (typically 50 mA holding) requires:

• 4 x 0.20 A (LOGO! worst case) = 0.80 A
• 4 x 0.05 A (contactors) = 0.20 A
• 0.10 A (pushbutton LED indicators, if fitted)

Total worst-case load = 1.10 A at 24 V DC = 26.4 W. Apply a 1.5 derating factor and select a 40 W PSU (e.g. Siemens LOGO! Power 24 V / 1.3 A or equivalent third-party DIN-rail PSU). Do not run a single PSU across more than 8 m of 24 V bus without a remote-sense or local regulation check, otherwise cable drop can pull the far-end LOGO! below its 20.4 V minimum.

Verification, Commissioning, and Troubleshooting

Commissioning Procedure

1. Power up each LOGO! individually with the bus disconnected. Verify that the local program runs and the local shutter moves on a local pushbutton press.
2. Wire one floor at a time. After each floor is added, press the central UP and central DOWN and verify that only the connected floor responds.
3. Once all four floors are wired, press central UP. All four shutters should begin to rise within 1 s of the strobe edge. Time each shutter to its end limit and confirm 60 ± 5 s on a typical residential roller blind.
4. Press central DOWN immediately after a central UP, before the shutters reach the up limit. Verify that the FSM transitions to Moving DN and that local controls remain disabled for the configured 30 s lockout.
5. Disconnect one LOGO! from the bus (simulate a peer failure). Verify that the other three still respond to central commands. This validates the parallel-bus architecture's fault tolerance.
6. Connect LOGO! Soft Comfort in online mode to each floor in turn and confirm that I17 and I18 toggle in the monitoring view when the central pushbutton is pressed.

Troubleshooting Matrix

FAQ