Resolving Siemens LOGO! 8 Expansion I/O Not Responding

Problem Statement

A Siemens LOGO! 8 logic module configured with a digital expansion module is reported with the following symptoms:

• Base unit: 6ED1052-1FB00-0BA8 (LOGO! 8 basic, 230 V AC supply, 8 DI / 4 DO relay, with display).
• Expansion module: 6ED1055-1FB10-0BA2 (LOGO! 8 digital expansion, 0BA2 device generation).
• The diagnostic page of the base LOGO! lists the additional I/O channels (e.g. I9, I10, I11, I12 and Q5, Q6, Q7, Q8) when the expansion is powered.
• Forcing a digital input high on the expansion in the wiring or assigning an "always high" constant to an expansion output in the user program produces no electrical change on the corresponding terminal.
• No parameter, dialog, or project setting is exposed inside LOGO! Soft Comfort to "add" an expansion module to the project.

This is one of the most common LOGO! field tickets because LOGO! expansions do not have to be registered in the program — the bus is enumerated automatically. When the I/O is visible in the diagnostic menu but does not respond, the fault is hardware, not configuration.

Affected Hardware and Catalog Reference

The 0BA8 base unit is the third hardware generation of LOGO! 8. The 0BA2 expansion designation refers to the device generation of the expansion — it is fully compatible with 0BA8 bases. The base firmware FS:04 corresponds to LOGO! 8.2 system software. The corresponding PC-side tool is LOGO! Soft Comfort V8.2 or later (V8.3 for FS:05+ bases).

How the LOGO! Expansion Bus Works

LOGO! modules are connected by a side-mounted bus link, not by a cable. Every LOGO! 8 base has a male gold-plated edge connector on the right side protected by a sliding gray cover. Each expansion module has a matching female bus contact on its left side. The bus carries:

• Internal +5 V logic power rail for the expansion's microcontroller.
• Serial address/data lines used by the base to enumerate attached modules.
• Ground reference.

The modules are physically clamped together by a sliding gray cam on the front of the expansion. When this slider is moved to the right, two internal hooks pull the modules together and the gold contacts engage. When the slider is left to the left (factory-shipped position) the contact springs are open and no communication is possible — the expansion is electrically invisible to the base.

Because the I/O of the expansion is exposed in the diagnostic menu of the base whenever the bus is functional, the simple fact that the new channels appear means the bus is partially alive. The most common cause of the symptoms described in the source case is therefore a mechanical/contact issue on the I/O terminal side, not a bus or firmware fault.

Common Root Causes of "Expansion I/O Visible but Not Switching"

Diagnostic Procedure (Before Touching the Program)

1. Power off the base and the expansion. Wait 30 s for the internal capacitors to discharge.
2. Remove the white plastic cover on the right side of the base by lifting the small flat tab with a screwdriver.
3. Confirm the gold bus contact on the right of the base is clean, unbent, and free of flux or pinching.
4. Align the expansion with the base so the female contact on the left side of the expansion slides onto the base contact.
5. Move the gray slider on the front of the expansion fully to the right until it locks. You should hear two clicks.
6. Apply power to both the base and the expansion. If the supply families differ, verify the expansion L/N or +/- terminals.
7. Inspect the status LED on the front of the expansion:
   • Solid green — bus is alive, expansion is enumerated.
   • Red — internal fault, replace the module.
   • Off — no supply or reversed polarity.
8. Enter the LOGO! on-board menu: > Diagnostics > I/O Status. The new digital channels from the expansion must appear.
9. Use the cursor keys on the LOGO! to force an expansion output from the diagnostic screen (Q5, Q6, ...) to ON.

If the LED is green and the diagnostic force does not produce voltage on the output terminal, you are looking at a mechanical or contact problem on the I/O side, which is the scenario in the source case.

Mechanical Contact Issue: The "Open Screw = No Contact" Fault

LOGO! expansion modules use cage-clamp style screw terminals. The internal spring clamp is the part that grips the conductor, but in several 0BA2-generation digital expansions the same spring cage doubles as the contact that closes the circuit from the I/O pin to the PCB. When the screw is fully opened, the cage is retracted and the contact is isolated, so:

• Touching a meter probe on the bare terminal metal reads an open circuit, not the I/O pin.
• The diagnostic menu still shows the I/O as a valid channel — the base is correctly enumerating the bus, but the contact is broken at the terminal.

The condition is resolved by either:

1. Inserting a conductor (at least a short pigtail) and tightening the screw to the rated torque of 0.6–0.8 N·m, or
2. Driving the screw fully home (bottomed) without a conductor so the cage spring is compressed against the contact pad.

Power Supply, LED, and Firmware Checks

If the base LED is green and the expansion LED is red, the expansion is faulted. Power cycle the expansion alone (disconnect and re-apply the L/N or +/- rail) to clear a latched internal fault. If red returns, replace the module.

Programming Considerations in LOGO! Soft Comfort

Because LOGO! enumerates the bus automatically, the project editor does not require a "register expansion module" step. However, the project target must be set to the correct device generation, otherwise the editor will silently omit the I/O points of the expansion or will mark them as unavailable:

1. Open the project in LOGO! Soft Comfort.
2. Select Tools > Select Hardware > LOGO! ... and choose the base with the correct MLFB (e.g. 6ED1052-1FB00-0BA8).
3. The editor auto-detects compatible expansion modules. If the part number of the expansion is not in the drop-down, the expansion is incompatible with the project — install the latest device pack via Help > Device Library Update.
4. Drag the desired expansion I/O into the program (e.g. I9 into an AND block, Q5 to a relay coil).
5. Compile, transfer, and watch the on-board diagnostic menu confirm the new program revision.

Note: writing the project does not require a separate "download to expansion" step. The expansion shares the same program memory as the base.

Step-by-Step Resolution (the Source-Case Procedure)

1. De-energize the assembly. Lock-out / tag-out the supply.
2. Confirm the base part number and the expansion part number. Both must belong to the same voltage family. In this case both are 230 V AC variants (F block in the MLFB).
3. Remove the white plastic cover on the right side of the base. Discard it; the bus must be exposed.
4. Verify the gold contact on the base and the matching contact on the expansion are clean and not bent. Wipe with a dry, lint-free swab if needed. Do not use solvents.
5. Snap the expansion to the right of the base. The plastic housings should sit flush. The screw mount tabs on top and bottom must line up.
6. Slide the gray cam on the front of the expansion to the right until it locks. The two plastic hooks engage the base and pull the modules together.
7. Tighten the bus-mounting screws (top and bottom of the base) so the assembly does not separate when wire is pushed in.
8. Strip 8 mm of insulation on each conductor going to the expansion terminals. Insert fully, then tighten each screw to 0.6–0.8 N·m. Do not leave any terminal with the screw open and no wire — this is the root cause of the reported symptom.
9. Restore power. Confirm the green LED on the expansion is steady.
10. Open the on-board diagnostic menu and force Q5 to ON. The output relay should close audibly and the contact voltage should be measurable at the terminal.

Verification and Functional Test

1. Cycle the base power to confirm the expansion re-enumerates automatically. The on-board menu should show all expansion I/O again.
2. Run a one-shot test program: connect an input switch to I9 of the expansion, and wire a lamp to Q5 of the expansion. Toggle the switch and confirm the lamp responds.
3. Use the LOGO! on-board > Start menu and cycle through Stop > Run once to confirm the bus is stable.
4. Pull one wire from an expansion terminal with the system powered. The corresponding diagnostic flag should immediately show the input as 0. Re-insert and tighten, the flag should return to 1.
5. Document the final wiring in the panel drawing and mark the expansion module on the layout.

Field-Engineer Troubleshooting Matrix

Notes on Topology and Limits

A single LOGO! 8 base supports a defined I/O budget. The maximum count across the base and all attached expansions is:

• 24 digital inputs (DI)
• 20 digital outputs (DO)
• 8 analog inputs (AI)
• 8 analog outputs (AO)

Adding an expansion does not change the programming tool, the project file size limit, or the number of function blocks available. The base CPU continues to scan all configured blocks at the same cycle time. There is no address "slot" to set — the base assigns addresses in the order of bus position from left to right.

For more detail on supported expansions, see the Siemens LOGO! Expansion Modules product page, and for the logic-module family overview the Siemens LOGO! product overview video on DigiKey. The official LOGO! 8 system manual is published on the Siemens Industry Online Support portal under entry ID 109741041; refer to it for the latest pin assignment, derating, and EMC data.

Frequently Asked Questions