Siemens LOGO! 8 I/O Limits: Achieving 32 Inputs and 20 Outputs

Overview

The Siemens LOGO! 8 mini-PLC has hard architectural limits that constrain its use in larger installations. This reference clarifies the maximum digital I/O count per single LOGO! base, evaluates the feasibility of a 32 DI / 20 DO project, documents the KNX/EIB integration path, and quantifies when an upgrade to the SIMATIC S7-1200 is the correct engineering decision.

Engineers evaluating a LOGO!-based installation should treat this as a structured feasibility study, not a feature catalogue. The same project requirements can be addressed by either a single controller with enough onboard I/O or a networked cluster; the wrong choice multiplies cost, panel space, and software complexity. The reference closes with a verification checklist for commissioning and a decision matrix for selecting between LOGO! 8 and S7-1200.

LOGO! 8 Hardware Architecture

LOGO! 8 (the 0BA8 generation in the LOGO! family) is the current logic module line. Each LOGO! system consists of:

• A base module with onboard CPU, power supply, digital inputs, digital outputs, and (on RCE and CE variants) an Ethernet interface.
• Up to 24 expansion modules daisy-chained to the right of the base on the integrated bus connector. The chain order is fixed: power supply side, base, then expansion modules.
• Optional communication modules (CMK, CMR, CSM families) inserted between the base and the expansion chain or attached on the left side of the base.

Base Module Variants

For a 32 DI / 20 DO project with KNX integration, the 12/24 RCE is typically the most flexible starting point because its onboard analog inputs (AI1-AI4) cover the most common sensor types and its relay outputs drive mains-voltage loads without external interposing relays.

Maximum I/O Capacity Limits

Regardless of which base module is selected, a single LOGO! 8 system - one base plus its expansion chain - has the following hard limits imposed by the firmware's process image size:

These figures come from the firmware process image. Exceeding them is not a matter of buying larger modules - the LOGO! 8 does not have the tag memory to map more points. The 24 DI cap is reached by combining the 8 onboard DI with the expansion chain (for example, base 8 + DM16 8 + DM8 4 = 20, or base 8 + DM16 8 + DM16 8 = 24). The 16 DO cap is reached by combining the 4 onboard DO with one DM16 (4 + 8 = 12), two DM16 (4 + 8 + 4 = 16), or one DM16 + one DM8 (4 + 8 + 4 = 16).

Expansion Module Catalogue

The 32 DI / 20 DO Project Requirement Analysis

Feasibility study for a single LOGO! 8 base addressing the target of 32 digital inputs and 20 digital outputs:

A single LOGO! 8 system cannot meet the stated I/O requirement. The shortfall is not addressable by purchasing more expansion modules; the firmware simply will not map the surplus points. Three engineering paths remain:

1. Network two or more LOGO! 8 systems, with each LOGO! controlling a subset of the I/O and exchanging data over Ethernet (MODBUS TCP or S7 PUT/GET).
2. Step up to the SIMATIC S7-1200, whose onboard plus signal-module architecture easily covers 32 DI / 20 DO with one CPU.
3. Hybrid: one LOGO! master plus a remote ET 200SP or LOGO! expansion over PROFINET. Note that LOGO! 8 does not act as a PROFINET IO controller, so this requires S7-1200 or higher as the master.

Communication Modules: KNX/EIB and Alternatives

The original KNX/EIB communication module for LOGO! was the LOGO! CM EIB/KNX, which mapped LOGO! inputs and outputs onto KNX group addresses and let the LOGO! participate in a KNX installation as a peripheral device. This module is no longer in the active Siemens price list for new LOGO! 8 projects, although stock and refurbished units exist in the field.

Current options for KNX connectivity on a LOGO!-based system:

• LOGO! 8 base Ethernet can exchange data with a third-party KNX/IP gateway (Weinzierl BAOS, MDT SCN-IP100.01, Gira X1, or ABB IPR/S) via MODBUS TCP or UDP raw socket. The gateway handles the KNX telegram conversion.
• LOGO! CMR2020 / CMR2040 cellular routers add remote access and IP routing. They are not KNX interfaces, but they enable the LOGO! to reach a cloud-based KNX visualization or be programmed remotely over the cellular network.
• LOGO! CSM unmanaged switch modules provide additional Ethernet ports when several KNX/IP gateways or HMI panels share the LOGO! subnet.

MODBUS TCP on LOGO! 8

LOGO! 8 RCE/CE bases ship with MODBUS TCP slave support enabled by default on the Ethernet interface. The default port is 502. Each LOGO! exposes its digital and analog process image as holding/input registers, allowing any MODBUS master (including a S7-1200, a SCADA server, or a KNX/IP gateway) to read inputs and write outputs.

Register map (selected):

This is the cleanest way to integrate multiple LOGO! 8 units into a single coordinated control scheme. Program the master poll cycle at 100-200 ms to keep the slave LOGO! responsive without saturating its single Ethernet port.

Networked Multi-LOGO! Configurations

When a single LOGO! cannot meet the I/O count, multiple LOGO! 8 bases can be networked. The most common topology is a master-slave architecture where one LOGO! acts as the supervisory controller and the others as remote I/O concentrators.

The practical constraint: every networked LOGO! reserves some of its digital I/O for inter-controller handshaking. Field experience shows that each LOGO! slave typically loses 4 DI and 4 DO to MODBUS communication, even though the slave LOGO! does not "consume" those signals locally. This is the figure that drops the effective I/O count noticeably when scaling the cluster.

For a 32 DI / 20 DO target, the 2-LOGO! configuration is the smallest cluster that works and leaves headroom. The downsides: two separate programs in LOGO! Soft Comfort, two IP addresses to maintain, two LWE dashboard files to version-control, and additional panel space.

S7-1200 Alternative: Capacity and Expansion

The SIMATIC S7-1200 is the natural step up. Compared with a clustered LOGO! solution, a single S7-1200 CPU with one or two signal boards and signal modules typically:

• Reduces hardware cost by 30-50% at the same effective I/O count.
• Reduces panel space to a single CPU row.
• Provides a single TIA Portal project instead of multiple LOGO! Soft Comfort projects.
• Provides native PROFINET for distributed I/O (ET 200SP) instead of MODBUS TCP handshaking.
• Supports the full IEC 61131-3 language set including SCL and GRAPH.

Relevant S7-1200 CPUs

For 32 DI / 20 DO, the CPU 1214C plus one SM 1223 DI16/DQ16 (16 DI + 16 DO) reaches 30 DI / 26 DO without needing additional rack space. An alternative is CPU 1214C plus SM 1221 DI16 plus SM 1222 DQ16, which exceeds the 32 DI / 20 DO requirement cleanly. For mains-voltage loads, replace the SM 1222 DQ16 with the SM 1222 DQ16 RLY variant.

KNX Integration with S7-1200

S7-1200 does not have a native KNX interface. Integration options:

• Third-party KNX/IP gateways (Weinzierl BAOS 771, MDT SCN-IP100.01, ABB IPR/S 3.1.1, or Gira X1) with a MODBUS TCP or KNXnet/IP interface to the CPU.
• CP 1243-1 communication processor for secure IP communication to the gateway or to a SCADA.
• CM 1241 communication module for RS485 MODBUS RTU to legacy KNX serial bridges.

Implementation is more involved than the legacy LOGO! CM EIB/KNX module because the gateway exchange is a custom MODBUS mapping rather than transparent KNX telegrams. However, the resulting system is more flexible, scales to thousands of group addresses, and is supported long-term.

Remote Access Architecture

LOGO! 8 RCE and CE variants include an Ethernet interface. Out of the box, the LOGO! supports:

• LOGO! Soft Comfort programming over Ethernet on port 8080.
• LOGO! Web Editor custom dashboards served on port 80 (HTTP only).
• MODBUS TCP server on port 502.
• S7 communication (PUT/GET) for peer-to-peer LOGO!-to-LOGO! or LOGO!-to-S7 data exchange.

The Web Editor project (LWE file) is stored in the LOGO! internal flash and survives power cycles. It is created offline in LOGO! Soft Comfort and downloaded alongside the program.

For remote programming over the Internet, the recommended patterns are:

1. Corporate VPN: place the LOGO! behind a router that terminates an IPsec or OpenVPN tunnel from the corporate firewall. The engineer then connects to the LOGO! as if it were on the LAN.
2. LOGO! CMR2020 / CMR2040 cellular router: gives the LOGO! its own 4G connection with a fixed SIM. Combined with Siemens' remote management service, the engineer can reach the LOGO! through an outbound tunnel from the corporate firewall, with no incoming firewall rules.
3. SINEMA Remote Connect: Siemens' VPN server for remote maintenance. Compatible with SCALANCE M routers and the LOGO! CMR.

Remote access to LOGO! 7 (the older 0BA7 generation) is technically possible but the older firmware does not support the LOGO! Web Editor and exposes fewer remote diagnostics. Plan for LOGO! 8 if remote visualization is a hard requirement.

Cost and Engineering Trade-offs

The crossover between LOGO! clusters and S7-1200 occurs around 30-40 effective DI/DO. Below that, LOGO! is cheaper per point; above that, S7-1200 wins on hardware and software lifecycle. Field reports from conveyor and small-machine installations consistently show a S7-1200 CPU 1214C delivering equivalent I/O capacity at roughly two-thirds the cost of two clustered LOGO! bases plus DM16 expansion modules.

For a project of exactly 32 DI / 20 DO, the S7-1200 CPU 1214C plus one SM 1223 DI16/DQ16 (16 DI / 16 DO) is the cleanest single-CPU solution. The S7-1200 reaches 30 DI / 26 DO in this configuration, which exactly covers the 32 DI / 20 DO target once the SM 1221 is replaced with an SM 1223 DI16/DQ16 (and a portion of the 16 DO bank is left spare for growth).

Decision Matrix: LOGO! vs S7-1200

Use the following decision matrix to select the platform. Each criterion maps to a recommendation.

The 32 DI / 20 DO plus KNX project described in the original inquiry falls in the second and fourth rows: borderline feasible with a 2-LOGO! cluster, but cleaner and more expandable on a S7-1200 with a third-party KNX/IP gateway.

Verification Checklist and Commissioning

Whichever platform is chosen, perform the following checks before site delivery:

1. Confirm the I/O count is within the firmware process image: 24 DI / 16 DO per LOGO!; no firm cap per S7-1200 CPU beyond rack space and power budget.
2. Program the inter-controller MODBUS exchanges with a master poll cycle of at least 100 ms to avoid saturating the slave LOGO! server's single connection handler.
3. Verify the EIB/KNX group address mapping against the ETS project file. If using a third-party KNX/IP gateway, validate the MODBUS-to-KNX translation tables against both vendor manuals.
4. Configure the LOGO! or S7-1200 to a static IP address outside the DHCP pool. Document the IP, subnet mask, and gateway in the project binder and on the panel label.
5. Test remote access end-to-end from a client on the corporate network before energizing any field wiring. Confirm that the VPN tunnel terminates inside a dedicated PLC subnet, not on the general office LAN.
6. Back up the LOGO! program and LWE project to the engineering workstation. For S7-1200, archive the TIA Portal project including the device proxy and HMI tags.
7. Validate fail-safe behaviour on loss of the inter-controller link. The slave LOGO! should hold its outputs in their last state, or transition to a defined safe state, depending on the application.
8. Document the network topology, IP plan, and KNX group address list in the project as-built drawing set.