LOGO! Output Exhaustion: Solving with Markers and Expansion

1. Problem Overview: Premature Saturation of LOGO! Digital Outputs

The Siemens LOGO! logic module is a compact relay-replacement controller, but its small footprint creates a sharp ceiling on physical I/O. A common failure mode on first-time LOGO! designs is running out of physical outputs (Q1, Q2, Q3 …) before the control logic is finished, leaving the wiring diagram or function-block program physically unrealizable. The user's design illustrates a textbook example: a single LOGO! Basic (no display) plus four digital expansion modules, with sixteen declared output functions — interlocks, status indications, ACB commands, SCR firing permits, phase-failure trips, overtemperature trips, and emergency-stop indication — all mapped to physical Qi terminals.

Per Siemens Support entry 23670550, the maximum number of inputs and outputs is limited to the specifications of the basic device, and from device series 0BA3 onwards a LOGO! basic device can be extended with digital modules. The user's stack physically cannot support sixteen dedicated hard-wired outputs, so the engineering response is to (a) compress redundant status logic into internal markers and (b) re-evaluate which functions genuinely require a physical contact closure versus a soft indicator.

2. LOGO! Hardware I/O Capacity Reference

Before remediating the output exhaust, the engineer must know the absolute ceiling of the configured stack. The following table consolidates the published I/O limits for LOGO! 8 (series 0BA8) and earlier generations that are still encountered in installed bases.

Per Siemens KB 23670550, the absolute maximum addressable digital I/O on a single LOGO! 8 base with expansion is 24 digital inputs and 20 digital outputs, with up to 8 analog inputs and 8 analog outputs. The maximum number of expansion modules is constrained by the LOGO! backplane (typically 10 modules of mixed type) and by the internal address map, which is why stretching the count beyond 20 DO requires architectural re-thinking, not just adding slots.

3. Root Cause Analysis: Why the Output Map Filled Up

Decomposing the user's function list against the published ceiling makes the failure mode obvious. The sixteen Q-assignments break down as follows:

Of sixteen allocated outputs, only six (Q1, Q2, Q4, Q5, Q9, Q14) genuinely close external contacts to a coil or thyristor gate unit. The remaining ten are either interlocks (consumed by other blocks in the same program) or pure status indicators (consumed by lamps or by SCADA/LOGO! TD display). The first-pass design treated every Q reference in the function block diagram as if it had to land on a real terminal — the engineering equivalent of writing assembly code and assuming every temporary must be a register. That assumption is false: LOGO! provides internal markers (M1, M2, M3 … M27 or more, depending on firmware version) that perform the same boolean role without consuming a Qi terminal.

4. Solution 1: Reclassify Soft States as Internal Markers (Mi)

Internal markers are the LOGO!-equivalent of a scratch flag: a single-bit storage element that participates in the function block diagram exactly like an output coil does, but never breaks out of the program. They are zero-cost in the address budget.

Marker reference (LOGO! 8 / 0BA8)

Reclassification of the user's Q-map

Applying marker reclassification to the sixteen Q-assignments shrinks the physical-output requirement from sixteen to six, fitting comfortably inside a single LOGO! 8 Basic (Q1–Q4) plus one DM8 (Q5–Q8) — a stack of two expansion modules instead of four.

Result: 16 Q → 6 Q. The 4-DM8 that previously hosted Q5–Q8/Q9–Q12/etc. is freed entirely, dropping the expansion stack from four modules to one. The user's request for "two more Q" for S3/S4 flicking is also satisfied — S3 and S4 become M12 and M13.

5. Solution 2: DM8 / DM16 Expansion Modules for Genuine Output Growth

When marker consolidation is insufficient because the application truly requires more than six or eight physical contacts, the LOGO! 8 system supports a stacked expansion architecture. Per Siemens KB 23670550, up to 24 DI and 20 DO are addressable in a fully-loaded stack, with expansion allowed on series 0BA3 and later.

Stacked I/O count with marker consolidation

For the user's motor-control + SCR + ACB + safety application, the recommended end-state stack is:

• LOGO! 8 Basic (relay outputs, with display) — MLFB 6ED1052-1MD08-0BA1 (or the equivalent Ethernet variant 6ED1052-2MD08-0BA1 for LOGO! 8.3 / 0BA8 communication). Provides 8 DI, 4 relay DO (Q1–Q4).
• 1 × DM8 relay — MLFB 6ED1055-1CB00-0BA2. Provides 4 DI, 4 relay DO (Q5–Q8). Handles the bypass, stop, fault summary, and shunt trip outputs.

This two-module stack supplies 12 DI / 8 DO, more than enough for the six physically-driven outputs and the user's hard-wired interlocks.

6. Solution 3: Direct Input-to-Coil Routing Without a LOGO! Qi

The community suggestion to "connect some contacts connected to the inputs directly into relays or contactors" addresses a class of problems where the LOGO! is not adding any logic — it is just acting as a pass-through. Examples in the user's design:

• Q6 / Q7 — thyristor AC/DC connection verification: these are status reflections of physical limit switches on the thyristor cubicle. The same limit-switch contact can be wired (a) into a LOGO! input for software interlock and (b) in parallel into the contactor or thyristor gate unit's own permissive circuit. The LOGO! does not need to repeat the signal as a Qi output because the device that cares about the state is already physically wired to the same contact.
• Q13 — E-stop status: safety-rated E-stop loops must, by function-safety convention (e.g., ISO 13849-1 PL d/e, IEC 62061 SIL 2/3), use a hard-wired safety relay or contactor monitoring circuit, not a PLC output. The LOGO! can monitor the loop via an input, but the physical E-stop action must be performed by a Pilz / Schmersal / Sick safety relay whose output contact cuts the contactor coil. Folding the E-stop loop into a LOGO! Qi is both an I/O waste and a functional-safety violation.
• Q3 — O/C setting verification: the over-current setting confirmation switch is a binary input; if the downstream device (ACB closing coil) only needs the confirmation as a parallel hard-wire interlock, the confirmation contact can land on the ACB closing coil's permit terminal directly, freeing the LOGO! input for other use.

7. Step-by-Step Remediation Procedure

1. Inventory the field contacts. For every declared function (Q1, Q2, …), answer: does this close an actual external contactor coil, thyristor gate, shunt trip, or pilot lamp? If yes, keep on Qi. If no, classify as M (interlock-only) or eliminate.
2. Classify into three buckets.
   • Hard outputs (Qi): Q1, Q2 (ACB), Q3 (SCR permit), Q4 (Stop), Q5 (Bypass), Q6 (ACB shunt trip OR M9 + external safety relay).
   • Soft states (Mi): M1 (O/C setting), M2 (AC bus), M3 (DC bus), M4 (busbar), M5 (O/C trip), M6 (phase-fail), M7 (overtemp), M8 (E-stop status), M10 (S1 flicking), M11 (S1 hold), M12 (S3 flicking), M13 (S4 flicking).
   • Eliminate by direct wiring: Q6/Q7 thyristor AC/DC, plus the E-stop physical loop — both routed to the actual field device with the LOGO! input providing the interlock.
3. Re-author the function block diagram in LOGO!Soft Comfort. Replace every Qi block that was reclassified to Mi with a Marker output coil. LOGO!Soft Comfort places marker blocks under the "Marker" or "Special" function library. Right-click the block → change coil type → Marker.
4. Add markers for S3 and S4 flicking. The "flicking" (impulse / push-on-push-off) function is implemented in LOGO!Soft Comfort with the Set/Reset Flip-Flop block or the Current Impulse Relay block. Drive the indicator from the flip-flop's Q output, which can be either a physical Qi or an Mi depending on whether a lamp is wired.
5. Configure the expansion stack in LOGO!Soft Comfort: select Tools → Select Hardware, choose LOGO! 8 Basic + 1 × DM8. LOGO!Soft Comfort will auto-assign Q5–Q8 to the DM8.
6. Rewire the field terminals. Move the Q3/Q4/Q5 wires from the original Q3/Q4/Q5 terminals (on the Basic module) to the new positions in the consolidated map. Verify each terminal screw torque (LOGOSoft! relay outputs: 0.6–0.8 N·m typical) and wire gauge (24–14 AWG / 0.2–2.5 mm²).
7. Compile and download the program to the LOGO! via Ethernet (LOGO! 8 / 0BA8) or the LOGO! USB programming cable (legacy). Verify that LOGO!Soft Comfort shows no "address out of range" errors against the configured hardware.

8. Verification Checklist

Use this checklist before energising the panel:

• Address map audit: Open LOGO!Soft Comfort → Tools → Address Map. Confirm no Qi address is double-booked and no required output has been demoted to an M without intent.
• Online monitor sweep: Connect LOGO!Soft Comfort in online mode (Ethernet or USB). Force each input, verify the corresponding marker or output toggles in the on-screen wiring diagram. Pay special attention to the safety interlocks (M1, M2, M3, M4, M7, M8) — they must transition before any drive output (Q1–Q5) is allowed.
• Hardware I/O test: With the panel de-energised, use a continuity tester on each Qi terminal to confirm the correct contact is wired. With the panel energised, use a clamp meter on each output conductor to confirm the field device (contactor coil, thyristor gate, lamp) is being driven.
• Fault simulation: Force each trip input (overcurrent, phase-fail, overtemperature, E-stop) and verify the fault summary marker and the ACB trip output behave as expected. Verify that the ACB shunt trip is actuated by the dedicated safety device, not by the LOGO!.
• Retentivity check: Markers M1–M13 are non-retentive (volatile) on power-cycle by default. If the application requires a state to survive a power loss, set the marker as retentive in the block properties or use the corresponding VM/VW area configured retentive.
• Backplane test: Power-cycle the LOGO! 3–5 times and confirm the expansion modules are detected each time (LOGO! display: → DM8 1 → DM8 2 → … in the menu tree). A flaky backplane contact is a common cause of intermittent output loss in the field.

9. Best Practices and Anti-Patterns

Anti-patterns to avoid

• One Q per indicator lamp. Every "indication" on the LOGO! HMI display is virtual; the LOGO! TD/built-in display reads any Mi or Qi directly without a physical output. Wiring a lamp to indicate a status burns a Qi for no functional reason.
• One Q per interlock. Interlocks are consumed by other LOGO! blocks within the program. An interlock "output" should always be an Mi, never a Qi.
• Stacking the I/O based on first-pass design. Always count the required physical outputs at the schematic stage and verify against the LOGO! 8 published maximum (20 DO) before ordering hardware. A late-stage expansion swap from 4-DM8 to 4-DM16 is cheaper than redesigning the panel layout.
• Routing safety signals through the LOGO!. E-stop, safety gate, and similar PL d / SIL 2+ functions must use a dedicated safety relay. The LOGO! can monitor, but must not perform the safety action.

Best practices

• Use marker numbers in numerical sequence for interlocking layers. Reserve M1–M9 for top-level safety interlocks, M10–M19 for sequence steps, M20–M29 for HMI-visible states, M30+ for diagnostics. This makes the FBD legible to the next maintainer.
• Expose process state via VM and a LOGO! 8.3 Ethernet HMI rather than dedicated output lamps. One Ethernet-connected HMI replaces ten panel lamps and ten panel wiring points.
• Keep the first three Q outputs free for ACB close, ACB open, and master stop — these are the field-side hardware commands the panel designer will always want wired to a physical terminal.
• Document the address map in the panel drawing's terminal-strip table. A side-by-side table of "Function / LOGO! Address / Field Terminal / Wire Number" is the single most useful maintenance document for a LOGO! panel.

10. When to Step Up to a SIMATIC S7-1200

If the application consistently requires more than 16 hard outputs even after marker consolidation, or if the panel must support 3–4 separate motor starters with full safety integration, it is more cost-effective to step up to a SIMATIC S7-1200 (CPU 1214C / 1215C / 1217C) with signal modules. The S7-1200 has:

• 14–24 integrated DI and 10–16 integrated DO on the CPU body (varies by CPU model).
• Up to 8 signal modules (SM) and 3 communication modules (CM/CP) per CPU.
• Up to 1 MB of load memory, 150 KB of work memory, and bit memory (M) that scales into the kilobytes — not the dozens of bytes available on a LOGO!.
• Full integration with TIA Portal, scalable HMI panels (Basic Panels, Comfort Panels), and PROFINET-based distributed I/O (ET 200SP).

The LOGO! remains the right tool for sub-16-DO discrete control; the S7-1200 becomes the right tool when the discrete count exceeds 20–24 outputs or when the program needs more than 200 function blocks.

11. Diagnostic Path: Is the Output Exhaustion Real or a Toolchain Issue?

Before redesigning the program, rule out a toolchain misread:

1. Confirm the configured hardware in LOGO!Soft Comfort — the program compile error "address out of range" or a red-highlighted Qi block in the on-screen wiring is the first sign the basic-module output count has been exceeded.
2. Confirm the LOGO! device series — read the MLFB label on the side of the module. The 7th character is the series code (e.g., 6ED1052-1CC08-0BA1 → series 0BA8). Series 0BA0/0BA1/0BA2 are non-expandable; if the panel has one of these, no DM8/DM16 can be added.
3. Inspect the LOGO! display (or the LOGO! TD / LOGO!Soft Comfort online view) for the device's "Hardware" menu — this lists all detected expansion modules and their starting addresses. A missing module on the list indicates a backplane / addressing fault, not an output-exhaustion fault.
4. Check for unused outputs that are forced ON in simulation but not wired in the field — LOGO!Soft Comfort will count a Qi block as an address even if the contact is never closed at runtime, so a "used" address can still be available for the next function.

12. FAQ