Siemens LOGO! 8: Implementing NC Output Behavior with NOT Blocks

Overview

The Siemens LOGO! 8 is a compact logic module with 8 digital inputs, 4 outputs, an Ethernet interface, and an expansion interface, designed for fast implementation of small automation tasks. In burner-control applications such as industrial ovens, the controller must often hold a permissive contact closed (energized) until a timed or operator event opens it. Because every LOGO! base-module output is wired through a normally open (NO) relay or transistor channel, engineers who need "normally closed" (NC) behavior must program it logically using a NOT function block (in FBD) or an inverted output (in LAD).

This tutorial walks through the hardware, the two programming methods, a complete oven pre-purge / cool-down example, and the fail-safe caveats that force an external changeover relay whenever the controlled load must remain in its safe state during a LOGO! power loss.

Prerequisites

• LOGO! 8 base module (12/24 RCE, 230 RCE, or 230 RCEO variant). For a 230 V AC installation with relay outputs, the 230 RCE / 230 RCEO is the correct SKU; the "O" suffix denotes no onboard display.
• LOGO!Soft Comfort V8.x or later programming software (free download from the Siemens support portal).
• Ethernet patch cable (for direct PC-to-LOGO! programming) or micro-SD card for offline program transfer.
• Personal computer running Windows 10 / 11 with .NET Framework 4.7.2 or higher.
• Familiarity with the difference between FBD (Function Block Diagram) and LAD (Ladder Diagram) editor modes in LOGO!Soft Comfort.
• For burner-control applications, the local fuel-gas code authority (e.g., EN 298, NFPA 54, BS 6173) must be reviewed because most jurisdictions require a hardwired, fail-safe shut-off that cannot be implemented in software alone.

Understanding the LOGO! 8 Hardware

Per the LOGO! Basic Modules specification page, every LOGO! 8 base module ships with the same I/O count regardless of the variant:

The 230 RCE and 230 RCEO variants include four relay outputs, each rated 10 A at 250 V AC resistive, 3 A inductive. The 12/24 RCE and 12/24 RCEO variants ship with four transistor outputs rated 24 V DC / 0.3 A. The starter kits described in the LOGO! Starter Kits product page ship with the 12/24 RCE (relay output, not transistor) plus a 24 V / 1.3 A LOGO! Power supply.

For an oven timer application using 230 V AC contactors, the 230 RCE base is the conventional choice. For low-voltage DC logic interfacing to a contactor coil or interposing relay, the 12/24 RCE base is sufficient.

The Normally Closed Output Challenge

LOGO! outputs are physically NO. The contact closes only when the program evaluates the coil (Q1-Q4) true. There is no parameter in LOGO!Soft Comfort that flips an output channel to NC at the hardware level. Engineers who require NC behavior must implement it in logic.

Two editor-native methods exist:

Both methods produce logically identical behavior: the physical NO contact mirrors the negation of the input condition, so the downstream load sees the relay as "closed at rest." Neither method changes the fact that the contact opens if the LOGO! module loses power.

Step-by-Step: Implementing an Inverted Output in FBD (NOT Block)

1. Open LOGO!Soft Comfort and create a new project. Select File > New, choose the FBD editor view from the toolbar.
2. From the Co (Coil) list in the right-hand palette, place a normal coil block labelled Q1 on the diagram canvas.
3. From the GF (General Functions) list, drag a NOT block onto the line feeding the coil. The block is a single-input inverter with one input pin (X) and one output pin.
4. Wire the upstream condition (e.g., the output of a timing relay, a comparator, or a digital input) to the NOT input.
5. Wire the NOT output to the coil Q1.
6. Assign a meaningful block number, e.g., B001 for the NOT block. Comments are stored in the program and visible in the LOGO! on-screen menu.
7. Download to the LOGO! via Tools > Transfer > PC <-> LOGO!. Select Ethernet (or use micro-SD) and confirm.
8. Force the input to FALSE in the simulation view: the Q1 contact should close. Force the input to TRUE: the Q1 contact should open. This validates the inversion.

FBD Boolean Truth Table

Step-by-Step: Implementing an Inverted Output in LAD (Inverted Coil)

1. Switch the editor view to LAD: View > LAD.
2. Drag an input contact, e.g., I1, onto the left rail.
3. Drag an output coil onto the right rail. The default symbol is a normal coil "-""-(\-)""\-. To invert, click the coil once and use the toolbar button "Toggle negated" or change the symbol to "-""-(\/)""- (inverted coil).
4. The Boolean expression now reads: Q1 = NOT I1. The ladder rung shows the input contact as normally open driving a normally-closed coil, which is the LOGO!Soft Comfort notation for an inverted output.
5. Compile and download as in the FBD procedure.

Application Example: Industrial Oven Pre-Purge and Cool-Down Timers

A typical oven control sequence is:

1. Pre-purge (30 s): Exhaust fan must run for at least 30 s before the burner can fire. Implemented as an on-delay (logo!Soft Comfort block B002, function "On-Delay") with PT = 30 s. The exhaust fan is driven from Q1; the burner enable interlock reads Q1 in series.
2. Burner run: Energized via Q3 once the pre-purge timer has elapsed and the start permissive is high.
3. Cool-down (10 min): When the operator presses Stop (a momentary contact on I2), the burner must drop out immediately and the exhaust fan must continue to run for 10 minutes. The fan is driven from Q2.

The challenge: the cool-down fan must run while the timer is counting and stop exactly when the timer reaches 10 minutes. The desired behavior is therefore: Q2 = NOT (cool-down timer done), which is exactly the inverted-output pattern.

Wiring Diagram (ASCII representation)

  I1 (Start) ----+----------[ On-Delay 30s ]-----( Q1 Exhaust Fan )
                 |
                 +----------[ AND ]----[ On-Delay 10m ]---( Q3 Burner )
                                   |
  I2 (Stop, NC) ------------------+
  I3 (Burner Prove) --------------+
  I4 (Cool-Down Done, from NOT)  -+---( Q2 Exhaust Fan Hold-On )

In FBD:

• B001 - NOT block: input is "Cool-Down Timer Done" boolean; output drives Q2.
• B002 - On-Delay, 30 s, drives Q1 (pre-purge exhaust).
• B003 - On-Delay, 10 min, holds its output TRUE for the duration of the cool-down, and the boolean "Done" output toggles at 10 minutes.
• B004 - RS flip-flop, set by I1 (Start), reset by I2 (Stop), output drives Q3 (burner).

Sample FBD Program Skeleton (Pseudo-code for clarity)

  // Pre-purge
  Q1 := OnDelay(I1, T = 30s)

  // Burner latch
  B004.Set := I1
  B004.Reset := I2
  Q3 := B004.Q

  // Cool-down timer runs while burner is off AND I2 is high
  B003.IN := (NOT Q3) AND I2
  B003.T := 600s
  Q2 := NOT B004.Q  // alternate formulation: hold exhaust ON until burner stops
  Q2 := Q2 AND NOT B003.Done  // drop out when 10-minute timer completes

Using the NOT-block pattern, the simplest cool-down circuit is:

  Q2 := NOT B003.Q   // Q2 is ON for 10 minutes, then OFF

Where B003 is an on-delay that turns ON the instant Stop is pressed and turns OFF 10 minutes later. The NOT inversion makes the relay "closed at rest, open at end of cycle," which is exactly the NC behavior required by the operator sequence.

Fail-Safe Considerations and the External Changeover Relay

Per the Siemens LOGO! 8 system manual, the relay outputs are Form A (SPST-NO) and have no NC contact. A software-driven NC behavior is therefore not fail-safe: if the LOGO! loses 24 V power, if the program crashes, or if the Ethernet cable is disconnected during a remote download, the inverted Q2 output will de-energize and the contactor or fan will drop out.

For applications where the controlled load must default on (e.g., a gas valve that must stay open during normal operation and only close on a confirmed stop), insert a changeover relay downstream of the LOGO! output:

With this arrangement, loss of LOGO! power, a broken program, or a watchdog trip all cause the interposing relay to de-energize, the NC contact to close, and the load to enter its safe state. This satisfies the typical gas-code requirement that the safety shut-off device default to a closed (energy-supplying) state during controller failure - though every specific installation must be reviewed against the local authority having jurisdiction.

Verification and Commissioning

After downloading the program, perform the following checks before energizing the oven:

1. Simulation: In LOGO!Soft Comfort, run the simulation and toggle I1 and I2. Confirm that Q1 closes after 30 s, Q3 follows Start, and Q2 stays on for 10 minutes after Stop, then drops out.
2. Online Monitor: Connect via Ethernet, switch to online mode, and watch the block states in real time. The NOT block output should be the inverse of the upstream input.
3. I/O Test on the Bench: With the LOGO! powered but disconnected from the field, use a multimeter to measure continuity across Q1, Q2, Q3, Q4 in the OFF state. All should read open circuit. Manually force each output ON in the LOGO! on-screen menu (Start > Program > Set Output on a unit with display) and verify continuity. This step is also part of the standard LOGO! 8 TDE commissioning flow described in the LOGO! 8 TDE commissioning guide.
4. Power-Cycle Test: With the load connected, start the sequence, then remove power from the LOGO!. The load driven by an inverted Q2 should drop out; the load driven through an external changeover NC contact should energize. Document both results.
5. Watchdog Trip Test: Force a program-loop freeze (e.g., unplug Ethernet during a download) and confirm that the LOGO! goes to STOP. All outputs drop out; the interposing relay drops out; NC contacts close. This is the desired fail-safe response.

Troubleshooting Matrix

Migrating from LOGO! 8 to LOGO! 9

Siemens notes on the LOGO! product family page that LOGO! 8 digital and analog expansion modules are not hardware-compatible with LOGO! 9. When upgrading, the base module can be replaced and the program reused, but every expansion module (DM8, AM2, CMR, TDE) must be physically swapped. The programming technique covered in this article (NOT block in FBD, inverted coil in LAD) is identical between the two generations, so the application logic transfers directly.

Field-Proven Caveats

• The maximum relay switching rate on the LOGO! outputs is approximately 10 Hz mechanical, 2 Hz under inductive load at rated current. Do not drive a contactor coil directly from a NOT-inverted Q2 if the upstream logic toggles faster than this; use a seal-in auxiliary contactor.
• LOGO! 8 outputs are not short-circuit protected on the relay SKUs. Provide a UL-listed branch-circuit fuse or circuit breaker per NEC 409 if the controller is installed in a panel.
• Surge suppressors (RC snubbers for AC coils, flyback diodes for DC coils) should be installed across any inductive load - including the external changeover relay - to extend the life of the LOGO! relay contacts.
• Online program edits can briefly stop the scan and drop all outputs. Use the LOGO!Soft Comfort Run/Stop switch in the menu bar to verify the operating mode before any field work.

FAQ