Resolving LOGO! 8 FS4 Network Input VB Block Read Failures

Field engineers frequently report that a second Siemens LOGO! 8 FS4 controller cannot read variable byte (VB) values from a peer device's network input (NI) area, even when the Ethernet link is healthy, the Q/NI pair works, and IP addressing is correct. The most common cause is duplicate signal routing created in LOGO! Soft Comfort when both a client and a server connection are configured for the same VM payload. This reference documents the root cause, the corrected project flow, and the diagnostic checks that resolve the failure on the 6ED1052-xxx08-0BA2 (LOGO! 8.4 FS4) platform and earlier variants of the LOGO! 8 generation.

1. Problem Description

A typical installation consists of two LOGO! 8 FS4 controllers interconnected on a 192.168.1.0/24 subnet:

• Device 1 (Master/Client+Server): IP 192.168.1.5, mask 255.255.255.0, gateway 192.168.1.1
• Device 2 (Peer): IP 192.168.1.6, mask 255.255.255.0, gateway 192.168.1.1

Network Inputs NI4 (read as Q/NI4 in the partner) and NI5 (read as I/NI5 in the partner) return correct values on Device 2. However, NI1, NI2, and NI3 - which are mapped to variable byte blocks V1.0, V1.1, and V1.2 of Device 1 - are stuck at 0, intermittent, or contain stale data. The S7 connection is active in the diagnostics view; the underlying Ethernet link is up; ARP resolves both endpoints.

2. Affected Hardware and Firmware

The behavior was observed on the LOGO! 8.4 FS4 generation, part numbers in the 6ED1052-2xx08-0BA2 family:

• 6ED1052-1MD08-0BA2 - LOGO! 8.4 FS4 with display
• 6ED1052-1CC08-0BA2 - LOGO! 8.4 FS4 basic variant
• 6ED1052-2MD08-0BA2 - LOGO! 8.4 FS4 with Ethernet and display

It also reproduces on the LOGO! 8.3 (6ED1052-xxx08-0BA1) and earlier releases. All variants share the S7 communication model used by LOGO! Soft Comfort's network editor, so the resolution procedure is identical across the platform. The product family and firmware matrix is documented in the LOGO! 8 system manual entry 109751728.

3. Root Cause: Dual Client/Server Binding of the Same VM Payload

The LOGO! 8 peer model is an S7-protocol gateway. Each device can act as a client (it polls the partner's V/Q/M area and writes the result into its own NI registers) and as a server (it serves its own V/Q/M area to a partner that requests it). The two roles are independent, but they consume the same VM payload if a single V register is bound to an NQ twice.

In the failed project, the user created two parallel NQ connections for V1.0, V1.1, and V1.2:

1. Connection A - LOGO! 1 configured as client, reads V1.0/V1.1/V1.2 from its own NQ table and writes them into LOGO! 2's NI1/NI2/NI3.
2. Connection B - LOGO! 1 configured as server, the same V1.0/V1.1/V1.2 registers are again published to LOGO! 2 as NI1/NI2/NI3.

The compiler in LOGO! Soft Comfort does not flag this as a configuration error because both binding entries are syntactically valid. The runtime on the partner interprets the second binding as a write that overwrites the first, producing one of the following outcomes depending on cycle phase and timing:

• NI registers remain at the last successful server-side value while the client request is dropped on write conflict
• NI registers flicker between two transfer results that arrive in different scan cycles
• The connection is torn down by the S7 partner if a parameter validation error is returned

The instruction from the resolved thread is unambiguous: remove the unnecessary transfers and keep only one binding per VM-to-NI path. The easiest implementation is drag-and-drop from the network project tree.

3.1 Logical Topology of the Failure

4. Solution Procedure

The corrected project flow is executed inside LOGO! Soft Comfort. Open the same project used to program Device 1.

4.1 Prerequisites

• LOGO! Soft Comfort 8.4 or later, build 8.4.1 or newer recommended for FS4 project templates
• Ethernet connectivity from the PC to both LOGO! 8 devices
• Read/write access to the .lsc project file (Administrator rights on Windows if the project is stored in Program Files)
• Knowledge of the VM-to-NI mapping currently in service (Q-NI4 and I-NI5 are the reference pair that should continue to work)
• Backup of the original .lsc file before any structural change

4.2 Step-by-Step Correction

1. In the project tree, expand Network > Connections and list every connection that references V1.0, V1.1, or V1.2.
2. For each duplicate pair, identify whether the binding is declared as a client read or a server read on the partner. Keep the binding whose direction matches the intended master role of Device 1.
3. Right-click the redundant entry and select Delete Connection. Confirm the deletion in the dialog.
4. Open the Tools > Connection Status view and verify that exactly one entry per VM byte remains.
5. Rebuild the project. From File > Properties > Memory Usage, confirm that the VM area still allocates the expected number of bytes and that no warning is emitted for the NI area.
6. Download the corrected program to Device 1 with PC > LOGO! (Download). Wait for the Transfer complete message before powering Device 2.
7. Cycle power on both LOGO! 8 devices in this order: Device 1 first, then Device 2. The boot order is important because the client expects the server to be online before its first poll.
8. Re-validate from the on-device display of Device 2 (see Section 5.1).

4.3 Correct Mapping After the Fix

No client connection is required for the read direction. Device 2 reads from Device 1's server; Device 1 does not pull values from Device 2 unless an explicit client connection is added for a separate VM byte.

5. Verification

After the corrected program is loaded, perform the following four checks before declaring the fault closed.

5.1 On-Device Display Check

On the LOGO! 8 FS4 display of Device 2, navigate to Network Inputs. The values for NI1, NI2, NI3 must update synchronously with the source VM bytes on Device 1. Toggle V1.0 from the LOGO! 8 display of Device 1 (use the cursor keys in Status mode) and confirm that NI1 follows within one scan cycle (typically 20-100 ms depending on program size).

5.2 LOGO! Soft Comfort Online View

Select Tools > Online > Connect and target Device 2 at 192.168.1.6. In the online view, drag the NI1, NI2, NI3 blocks onto the watch window. The values should be stable and equal to the V1.0, V1.1, V1.2 values observed on Device 1's online view.

5.3 Connection Status

From Network > Connection Status, confirm that the S7 connection from Device 2 to Device 1 reports Established with zero Error Count. If the error count increments, revisit the duplicate-binding check described in Section 4.

5.4 Wireshark Validation

Run Wireshark on the management port of the subnet, capture on TCP port 102, and apply the display filter ip.addr == 192.168.1.5 && ip.addr == 192.168.1.6 && s7comm. The S7 communication should consist of a single PUT/GET pair per scan; a second PUT/GET on the same TSAP indicates a residual duplicate binding.

6. LOGO! 8 FS4 Communication Architecture Reference

The LOGO! 8 family uses the Siemens S7 communication protocol on top of ISO-on-TCP (RFC 1006) on port 102. Each device is identified by its IP address; the local TSAP is fixed (01.01) and the partner TSAP is configured in the connection table. The FS4 firmware revision expanded the VM area to 850 bytes and the NI/NQ table to 64 entries each. Background material is available on the Siemens LOGO! product page and in the LOGO! 8 system manual.

The increase in server connections on FS4 is the underlying reason the duplicate-binding bug becomes visible: the runtime can now hold both a client and a server session open against the same VM byte, and the resulting race condition is no longer masked by the previous single-server limit. The earlier firmware tore down the second connection outright, which produced a clearer log entry but still left the user confused.

7. NI/NQ Memory Mapping Reference

Network Inputs (NI) on the receiving LOGO! 8 are populated by the S7 communication stack and exposed to the FBD program as standard markers. The mapping is one-to-one with the V area of the partner when the connection is of type Read from server. The table below covers the offset calculations most frequently used in peer configurations.

The "Block Type" selector in the LOGO! Soft Comfort block dialog is critical: when the user reported being unable to find NI or NQ in the block type list, the correct path is to open the network project's Insert > Network Input dialog, not the FBD block library. NI/NQ are network-only symbols; they are not visible in the FBD editor's standard block list.

8. Common Configuration Pitfalls

9. Advanced Configuration: Replacing the Drag-and-Drop Workflow

When the project has been authored programmatically, the equivalent of the drag-and-drop binding is the NI and NQ tag references in the LOGO! Soft Comfort network XML. The relevant fields are:

<connection name="V_to_NI1" type="S7" direction="read">
  <partner ip="192.168.1.5" tsap="01.01"/>
  <source area="V" byte="1" bit="0"/>
  <target area="NI" byte="1" bit="0"/>
</connection>

Validate the file with Tools > Check Project before any download. A connection of identical source area, source byte, and source bit appearing twice is the programmatic equivalent of the duplicate-binding fault. The same warning applies to source area="M" bindings that target marker memory rather than VM memory.

10. Diagnostics and Monitoring

For continuous monitoring of the S7 session, expose the following markers in the FBD and map them to LOGO! Display or web server variables:

• VM Byte 0 (status): bit 0 = connection to LOGO! 2 established; bit 1 = last read OK; bit 2 = last write OK
• VM Byte 1 (error code): 0 = no error; 1 = timeout; 2 = invalid TSAP; 3 = duplicate binding write conflict
• VM Byte 2 (scan counter): increments on every successful NI update; the value should grow monotonically at the configured poll rate

The S7 error codes returned by the LOGO! 8 server are documented in the LOGO! 8 system manual. The most common return codes for FS4 deployments are:

11. S7 Protocol Reference for Peer Networks

The S7 protocol used by LOGO! 8 is a streamlined subset of the S7-300/400 communication. The relevant opcodes for a peer VM read are:

The transport is ISO-on-TCP (RFC 1006) on TCP port 102. The TPDU length is fixed at 1024 bytes. The S7 payload uses the standard 12-byte header followed by the parameter and data sections. The polling cycle of the LOGO! 8 client is governed by the user program scan time; there is no independent poll timer.

12. Wireshark Capture Example

A healthy peer session produces the following traffic on the wire (sample Wireshark output, abridged):

No.  Time      Source            Dest              Length  Info
1    0.000000  192.168.1.6       192.168.1.5       76      TPKT  CR TPDU 0
2    0.000142  192.168.1.5       192.168.1.6       76      TPKT  CC TPDU 0
3    0.001204  192.168.1.6       192.168.1.5       91      S7COMM  Setup communication
4    0.001350  192.168.1.5       192.168.1.6       91      S7COMM  Ack Setup
5    0.020112  192.168.1.6       192.168.1.5       104     S7COMM  Read Var [V1.0..V1.2]
6    0.020289  192.168.1.5       192.168.1.6       110     S7COMM  Data Read [V1.0=0x55 V1.1=0xAA V1.2=0x33]

The duplicate-binding fault produces a second, interleaved Read Var on the same TSAP. Filter with s7comm.param.func == 0x04 and count occurrences within a 1-second window: a healthy session shows exactly 1 (or 0 between updates); the faulty session shows 2 or more.

13. Integration with S7-200, S7-1200, and S7-1500

The LOGO! 8 can read from and write to a full S7 CPU by acting as a client. The procedure is identical to peer-to-peer, with the partner IP set to the S7 CPU and the partner TSAP set to 01.01 (the default for S7-200/1200/1500).

Reference the S7 CPU's Siemens Industry Online Support entry for the exact CPU article number and the corresponding PUT/GET enable procedure.

14. Firmware Migration Notes

When upgrading from LOGO! 8.3 (6ED1052-xxx08-0BA1) to LOGO! 8.4 FS4 (6ED1052-xxx08-0BA2), review the following items in the project before downloading:

• NI/NQ index above 32 must be re-validated; pre-FS4 firmware truncates the table silently
• VM offset above 48 must be reassigned; pre-FS4 firmware refuses the download with a memory overflow error
• Web server variable list may need to be pruned to 32 entries for backward compatibility with the older LOGO! TD (text display)
• The web server URL changes from http://<ip>/logo to http://<ip>/fs4 on FS4 - update any saved browser bookmarks

Perform the upgrade on a bench setup first if the FS4 system is the master in a multi-CPU topology. The migration is not hot-swappable: the LOGO! 8 must be powered down for the SD card or Ethernet firmware update to apply.

15. Safety and Commissioning Notes

During commissioning, always keep a hardwired backup (a manual override switch wired to a discrete input of the receiving LOGO!) for the network-driven outputs. The S7 session takes 2-4 scan cycles to establish after power-up; the outputs must be off during this window if the application cannot tolerate the initial zero state.

Verify the S7 connection under worst-case load: run a parallel network stress tool (for example, iperf on TCP port 102) at 50% of the link bandwidth while the peer application is active. The LOGO! 8 client should not disconnect; if it does, the network is oversubscribed and the scan time of the master program is too long.

16. Frequently Asked Questions