LOGO! Network Project Ladder Logic: FBD-Only Workaround Guide

Overview: The Network Project Ladder Logic Constraint

LOGO! Soft Comfort V8.x exposes two project topologies: the classic Single Project (one program, one LOGO!) and the Network Project (up to 16 networked LOGO! 8 base modules on a shared Ethernet segment). When you switch into Network Project, the editor forces the entire workspace to Function Block Diagram (FBD); the Ladder Logic (LAD / LLD) editor is greyed out and the menu View → Ladder Diagram disappears. This is documented in Siemens LOGO! 8 (0BA8) System Manual, Edition 06/2021 and reproduced in the LOGO! manual collection.

The constraint applies even on the most recent hardware, the LOGO! 8.3 base modules (MLFB suffix 0BA2, firmware V1.8.x / FS4) and the LOGO! Soft Comfort V8.4 editor. The reason is internal: the Network Project view stores each LOGO! as a child circuit under a shared Ethernet topology descriptor, and the FBD-only editor was chosen because FBD scales cleanly across the multi-master bitmap. LAD is preserved in Single Project, but is intentionally blocked in Network Project to keep block ordering deterministic when the compiler distributes code to multiple slaves.

Why Network Project Is FBD-Only

Three technical drivers force the FBD-only behavior in Network Project mode:

1. Block address resolution across multiple slaves. The compiler pre-computes a contiguous block table that spans all LOGO! nodes in the topology. FBD stores every block as a referenced function call (no implicit rung ordering), so the address rewriter does not need to know the physical position of a coil. LAD, by contrast, is order-sensitive because contact/coil references are positional in the rung table.
2. Cross-node Variable Memory (VM) sharing. Network I/O (NI1…NI64, NQ1…NQ64) is mapped through the VM bitmap. FBD exposes these directly as block pins; LAD would require duplicate coil/contact conventions per node, which LOGO! Soft Comfort does not implement in the Network Project editor.
3. Simulator fidelity. The integrated network simulator reproduces cycle-time jitter between client and server LOGO!. FBD is deterministic enough to simulate the full 16-node mesh in real time on a single PC; LAD would require a multi-threaded rung evaluator that LOGO! Soft Comfort does not ship.

The takeaway: the Ladder Logic editor is fully functional on a Single Project that you push to a single LOGO!, but you cannot keep LAD if you want the Network Project GUI to draw the topology and the simulator.

Prerequisites

Hardware Catalog Numbers (MLFB) for Reference

Block Memory Budget on LOGO! 0BA8

The 0BA8 family (and the 0BA8.FS4 hardware running LOGO! Soft Comfort V8.3 / V8.4) provides a fixed block budget of 400 function blocks per program. This ceiling is independent of the editor language:

• Each FBD instance, each LAD contact, each LAD coil, each function (timers, counters, math, UDF) consumes exactly one block slot.
• Virtual flag markers, shift registers, analog multiplexers, and PI controllers also consume blocks.
• The VM bitmap is 64 KiB regardless of block count, so a small program with many NI/NQ bindings is just as viable as a dense block-rich program.
• Program upload/download over Ethernet does not change the 400-block ceiling.

Workaround Procedure — Ladder Logic with Network Communication

To retain Ladder Logic while still networking multiple LOGO! 8 base modules, abandon the Network Project topology and build each node as an independent Single Project. Then configure Ethernet master/slave bindings manually inside each project.

Step 1 — Create a Single Project per LOGO!

1. Open LOGO! Soft Comfort → File → New → Single Project.
2. Set the editor to LAD via View → Ladder Diagram (or press Ctrl+2).
3. Right-click the LOGO! icon → Properties → Hardware Type and select your 0BA8 part number.
4. Save the project as master.lsc for the active node and slave1.lsc, slave2.lsc, … for each passive node.

Step 2 — Assign Static IP Addresses

Static addressing removes the DHCP probe jitter that occasionally corrupts the first 30 seconds of a master poll cycle.

On each physical LOGO!: Setup → Network → IP Address. On the editor side, open Tools → Ethernet Connections and tick Set IP via project.

Step 3 — Enable Network Coils and Contacts in LAD

In a Single Project set to LAD, the network binding is achieved through Network Output (NQ) coils and Network Input (NI) contacts. Drag these from the toolbox (F5 → category Special):

• NQ1…NQ64: a coil that broadcasts the result of the rung to all listeners. Each NQ is bit-addressed for digital signals, with byte/word overlays for analog use.
• NI1…NI64: a contact that is driven by the most recent value published by a remote master LOGO!.

Step 4 — Build the Master Rung

Master ladder example (read two digital inputs on the master, publish them, read one analog from a slave, drive one output on the master):

|     I1       I2         |
|------| |------| |-------|
|                          |
|        NQ1     NQ2       |   -- two rungs are shown compacted
|        ( )     ( )       |

This publishes the state of I1 to NQ1 and I2 to NQ2 on every cycle of the LOGO! scan (~85 ms typical at 0BA8).

Step 5 — Build the Slave Rung

Slave ladder example (consume NI1 from master, drive local relay):

|     NI1                |
|------| |----------------|
|                          |
|        Q1               |
|        ( )              |

Conversely, on the master, the analog value published by the slave appears at NI17 (word) and can be wired into an analog flag or directly to a math block.

Step 6 — Configure the Connection Table

1. On the master project only, open Tools → Ethernet Connections.
2. Click Add and fill in:

   Field	Value (master → slave A)
   Connection name	Master_to_SlaveA
   Type	LOGO! ↔ LOGO!
   Local IP	192.168.0.10
   Remote IP	192.168.0.11
   Master NI start	NI1
   Master NQ start	NQ17
   Slave NI start	NI1
   Slave NQ start	NQ17
   Polling cycle	200 ms
   Timeout / retry	3000 ms / 3 retries
   Watchdog (sec)	10

3. Repeat for each slave. The 0BA8 family supports up to 8 concurrent Ethernet connections per master; the 0BA8.FS4 raises this to 16.
4. Click Verify → Compile. The compiler should report zero cross-reference errors and zero block-budget overruns.

Step 7 — Download and Verify

1. Connect the master LOGO! over USB or Ethernet.
2. Tools → Transfer → PC → LOGO! for the master, then to each slave.
3. After download, switch the editor to Online → Run Mode to verify NI/NQ states in real time.

Network Topology (Inline SVG)

Alternative: Staying in FBD with Network Project

If retaining LAD is not strictly required, the Network Project editor delivers a much faster commissioning path for any topology above two nodes:

• Auto-discovery: Tools → Ethernet Connections → Discover populates the topology from the broadcast packets the LOGO! 8 ships every two seconds.
• Cross-node block linking: drag a pin from one node's program onto another node; the compiler wires it through NI/NQ transparently.
• Integrated network simulator: the on-screen green/yellow/red link indicators reproduce cable faults, IP conflicts, and timeout behavior without flashing real hardware.

This is why the Siemens documentation recommends Network Project for any topology beyond a single LOGO! even though the editor locks to FBD.

Protocol Details for Cross-Reference

Even when the user is on Ladder Logic, the underlying S7 and Modbus servers remain active and listen on the same ports; the choice of editor does not change the wire format.

Verification Checklist

Run through this list before going to site. It is the same checklist Siemens support uses when a customer opens a ticket with the symptom “LAD Network Project will not compile”.

1. Open the offending project. Confirm the top bar reads Single Project, not Network Project.
2. Confirm View → Ladder Diagram is checked. If the menu entry is greyed, the project is still in Network Project mode — convert via File → Save As → Single Project.
3. Verify IP addresses on every node with a ping from the PC.
4. Open Tools → Ethernet Connections; check that the Active checkbox is on for every master link and off for every slave link.
5. Compile (F7) and confirm zero warnings under the Cross Reference tab.
6. Run a block count check: Tools → Statistics. The Used column must be ≤ 400.
7. In Online → Run Mode, toggle an input on the master and verify the matching NI on the slave updates within two polling cycles (default 200 ms each direction).
8. Trigger a network fault (disconnect the cable) and confirm the master's VM0 status flag toggles to error within the configured watchdog window.

Troubleshooting Matrix

Field-Engineering Tips

• Build the network skeleton first. Lay down all NI/NQ coils on every node before writing logic. This catches address collisions at compile time, not at commissioning time.
• Reserve one NQ per node for the heartbeat. A 1-Hz blinking flag makes a stuck slave trivially identifiable on the master's display.
• Document every NI/NQ binding in the rung comment. Right-click the rung → Comment. The comment travels with the project into the LOGO! and shows on the TDE.
• Pin cycle time to 100 ms minimum. Faster polling than 100 ms can starve the slave's own scan loop on 0BA8 hardware below firmware V1.08.x.
• Use the SD card for cloning. Insert an SD card, navigate Card → Clone to LOGO!, and replicate a verified master project to many slaves in seconds.
• Validate against the official LOGO! application examples. Siemens publishes a library of vetted network templates under LOGO! Application Examples that are reference-quality.

Safety Caveats

Migration Path from Older Generations

Older 0BA6 (LOGO! 6) and 0BA7 (LOGO! 7) hardware used a different network protocol with no direct upgrade path. To migrate:

1. Read out the old program via LOGO! Soft Comfort V7.x and save the .lsc archive.
2. Open LOGO! Soft Comfort V8.x and import via File → Import → Older Version.
3. Manually rebuild the cross-node bindings using NI/NQ. The 0BA6/0BA7 “Network Project” mapped to the same NI/NQ semantic, so the bindings translate 1:1.
4. Convert all unused blocks to UDFs to free up the 400-block budget.

Related Resources

• LOGO! 8 (0BA8) System Manual
• LOGO! Soft Comfort V8.4 Download and Help
• LOGO! 8 Manual Collection (manuals, function guides)
• LOGO! Product Family Overview
• LOGO! Application Examples Library

Frequently Asked Questions