Troubleshooting Siemens LOGO! 8 IP Address Changes from DHCP

Problem Description

On a Siemens LOGO! 8.FS4 (firmware revision 4) network of three logic modules connected through a five-port unmanaged switch, a controller whose project was set to a static IPv4 address of 192.168.1.250 came up after every restart with the address 192.168.1.220. The MAC address reported by the switch or by an ARP scan (E0-DC-A0-17-CC-8E) belonged to the same physical LOGO! 8 module that had been configured to .250, confirming that the device was not being replaced or swapped, but that the LOGO! 8 itself was accepting a different address than the one stored in the project.

Symptoms of this failure mode are:

• LOGO! 8 is reachable on xxx.220 instead of the configured xxx.250.
• The address changes back to xxx.250 only when the project is downloaded again from LOGO! Soft Comfort, then reverts on the next power cycle.
• Other LOGO! 8 modules on the same subnet are unaffected and retain their static addresses.
• The change occurs immediately at boot, before any user logic is executed, which excludes a program-driven overwrite.

Because the project stores the static IP and the change happens before any LSC (LOGO! Soft Comfort) program runs, the cause is external to the application code. The two remaining candidates are: (1) the LOGO! 8 client accepting a DHCP offer that overrides the static, and (2) an SD card containing an older program with a different IP being read on startup. Both mechanisms exist by design in the LOGO! 8 firmware.

Affected Hardware and Firmware

The behavior is specific to the LOGO! 8 platform with the on-board Ethernet interface (the BM variant and the CPU variants 6ED1052-1xx08-0BA1 family). Modules without Ethernet (LOGO! 8 with part suffix -0BA0 FS03 or earlier) cannot exhibit this fault. The IP override mechanisms are documented in the LOGO! 8 system manual, available on the Siemens Industry Online Support portal. The same chapters cover both LOGO! 8.FS4 (ES2 generation with Ethernet) and the LOGO! 8.3 (FS5) modules released in 2023. The remediation steps below apply to both firmware branches because the boot-time behavior was not changed between FS4 and FS5.

Relevant LOGO! 8 catalog numbers with on-board Ethernet:

For all of the modules above, the same Network Settings dialog in LOGO! Soft Comfort V8.x or V9.x governs IP address, subnet mask, gateway, and DHCP behavior.

Root Cause Analysis: DHCP Override

The single most common cause of a LOGO! 8 acquiring a different IP from a DHCP pool at restart is a DHCP server on the same physical or logical network. A typical installation places the five-port switch next to the LOGO! 8 modules and uplinks that switch to the office router. If the office router runs a DHCP server with a scope that covers the entire 192.168.1.0/24 subnet, the LOGO! 8 will receive a DHCP offer before its statically configured address is honored, provided that the LOGO!'s DHCP client is enabled in the project.

LOGO! Soft Comfort exposes the Network Settings dialog (Tools → Ethernet Connections → Ethernet → Properties in older versions; Project → Network Settings in V9.x). Two checkboxes govern the boot-time behavior:

• Obtain IP address automatically (DHCP): when checked, the LOGO! 8 sends a DHCPDISCOVER on every restart and uses the first DHCPOFFER it accepts. The IP entered in the dialog is then only a fallback if DHCP fails within the timeout window.
• Static IP address: when checked, the LOGO! 8 uses the IP, subnet, and gateway fields verbatim. The DHCP client is not started at boot.

If the project was created with the static IP dialog filled in and the Obtain IP address automatically box left enabled, the LOGO! 8 will prefer DHCP. This is the default project template in older versions of LOGO! Soft Comfort, so it is an easy mistake to make when starting a new program.

A secondary effect is the DHCP lease behavior. If the LOGO! 8 received a lease for .220 at any earlier time, the DHCP server may continue to map that lease to the LOGO!'s MAC (E0-DC-A0-17-CC-8E) in its lease table. Each time the LOGO! 8 boots, the server offers the same address because the lease is still valid, and the client accepts it. The result is a deterministic but unwanted .220 address, exactly matching the symptom reported.

Root Cause Analysis: SD Card Program Override

The second mechanism that overrides the project IP is the micro SD card slot on the front of every LOGO! 8 module. The slot is rated for SD cards up to 32 GB formatted as FAT or FAT32, but the file the LOGO! 8 actually reads from it is a single BIN file written by LOGO! Soft Comfort. If the SD card was used to clone or transfer a previous program and that program contained a different static IP, the LOGO! 8 will use the SD-card program on the next restart regardless of what is stored in internal flash.

Boot priority is, in order:

1. SD card program, if present and valid (LOGO! 8 always tries SD card first).
2. Internal flash program.
3. No program (LOGO! 8 idles with no I/O and no network).

This priority cannot be reversed. The only way to force the internal program to be authoritative is to remove the SD card or to rewrite it from a fresh download so that the SD card program matches the internal program. Both must be re-downloaded after the IP is corrected, otherwise the SD card will revert the IP on the next restart.

The factory setting of the card-write behavior is controlled by a separate Card to LOGO! option in the LOGO! Soft Comfort transfer dialog. Setting Card to LOGO! = Off prevents the LOGO! 8 from copying internal flash to the SD card on power-up. Setting it On forces an overwrite. In either case, the IP of the program being copied is the IP that will boot on the next restart, so a stale card with a stale IP will keep causing the problem until the card is either re-flashed or removed.

Root Cause Analysis: Router Address Mapping

A third, less common contributor is the office router or the PC itself acting as a DHCP server. Many small office routers default to 192.168.1.0/24 with a pool starting at 192.168.1.100. If the LOGO! 8 was previously configured to 192.168.1.250 and the user disabled the DHCP client on the LOGO!, but the network still has a DHCP server, the router may still be in a state where the .250 address is mapped to a different MAC because that other device (laptop, printer, IP camera) previously held the lease. In that case, the LOGO! 8 with MAC E0-DC-A0-17-CC-8E will not be able to claim .250 via ARP until the router refreshes the table. The temporary workaround is to use an address that has never been issued, e.g. 192.168.1.230, while keeping the DHCP client disabled on the LOGO! 8.

For deeper analysis of why a static address can be unreachable, Microsoft's IP address change guidance explains that a static address should never change unless a higher-priority service such as DHCP overrides it. This is the same principle that applies in the LOGO! 8 case: either a DHCP server is providing the address, or another program is providing it. Disabling both fixes the issue.

Diagnostic Procedure

Follow the steps in order. Each step produces a yes/no answer that points to the correct remediation branch.

1. Open LOGO! Soft Comfort V8.4 or later, connect via Ethernet, and read the project from the LOGO! 8. Note the IP address, subnet, and gateway currently shown in Network Settings. Also note the Obtain IP address automatically checkbox state.
2. Power-cycle the LOGO! 8 and within 5 seconds, ping the .250 address. If ping succeeds, the project IP was retained and the original report is wrong; if it fails, the LOGO! 8 has indeed overridden it.
3. Open the LOGO! 8's IP scan utility (Tools → Ethernet → IP Scan). The scan will show the actual current IP of every LOGO! 8 on the broadcast domain. Confirm whether the affected module is at .250 or .220 after a fresh boot.
4. From a PC on the same subnet, run arp -a and look for the entry matching MAC E0-DC-A0-17-CC-8E. The IP next to that MAC is the IP the network believes the device is using. If it is .220, the network is in agreement with the LOGO!'s boot behavior, which means the LOGO! chose .220 for a reason.
5. From the PC, run ipconfig /all and identify the default gateway. Telnet or HTTP to that gateway and look at its DHCP lease table. Search for MAC E0-DC-A0-17-CC-8E. If it appears, record the IP, the lease expiry, and whether the lease is reserved. This is the most direct evidence of a DHCP server being involved.
6. Remove any SD card from the LOGO! 8. Power-cycle. If the IP now sticks at .250, the SD card was the source of the override. Reinsert the card only after the corrective procedure in the next section has been completed.
7. With the SD card still removed, ping the .220 address. If it still answers the LOGO! 8's MAC, the issue is DHCP and not the SD card. Continue to the remediation section.

Remediation: Force Static IP and Disable DHCP

1. Open the LOGO! 8 project in LOGO! Soft Comfort.
2. Navigate to Tools → Network Settings (V8.x) or Project → Network → Ethernet (V9.x).
3. Uncheck Obtain IP address automatically (DHCP).
4. Enter the desired static IP (for example 192.168.1.230 to stay outside the typical .100–.199 DHCP pool of a SOHO router), the subnet mask (255.255.255.0), and the router's default gateway if Internet access is required.
5. Click OK and save the project.
6. Compile and download to the LOGO! 8 over Ethernet.
7. On the LOGO! 8's front panel, navigate to Network → IP Address and verify the value matches. If the LOGO! 8 has a display, the menu path is Menu → Network → IP Address. With no display, use LOGO! Soft Comfort's Online → Identify function.

Remediation: Clean SD Card

1. Power down the LOGO! 8.
2. Press the SD card gently until the click-release mechanism ejects it.
3. Insert the card into a PC. If the card shows a single BIN file (typically LOGO0001.BIN or similar), it contains a program image. Re-format the card as FAT32 or just delete the file.
4. Insert the card back into the LOGO! 8 only if you specifically need SD-based program cloning. If you do, open the project in LOGO! Soft Comfort, choose Transfer → to Card, and confirm that the IP in the project at the time of the transfer is the correct static address.

For long-term reliability, leave the SD card out of the LOGO! 8 after commissioning. The internal flash is non-volatile and survives power cycles. The SD card is a convenience for firmware updates and program cloning, not a permanent storage requirement.

Remediation: Router Lease Table Cleanup

If a DHCP reservation for MAC E0-DC-A0-17-CC-8E exists in the router pointing at .220:

1. Open the router's admin interface (usually 192.168.1.1 or 192.168.1.254).
2. Locate DHCP → Static Leases or Address Reservation.
3. Delete the entry for MAC E0-DC-A0-17-CC-8E.
4. Save and reboot the router.

After the router is back up, the LOGO! 8 with DHCP client enabled will receive a new address from the pool. If the LOGO! 8 has been switched to static, no further action is needed. If the LOGO! 8 was given a new static address outside the pool, reboot the LOGO! 8 to confirm that the static value is retained.

Network Architecture Best Practices for LOGO! 8

Several design choices eliminate the entire class of failure described above.

• Isolate the automation subnet. Put the five-port switch and the LOGO! 8 modules behind a dedicated router or L3 device. The automation subnet should not be the same broadcast domain as office workstations, printers, or Wi-Fi access points. This breaks the link between the office DHCP server and the LOGO! 8.
• Use a /29 or /28 mask. With a 255.255.255.240 mask you have 14 usable addresses, enough for one or two LOGO! 8 modules, a single HMI, and a maintenance laptop. The smaller broadcast domain makes IP conflicts obvious within seconds of connecting a new device.
• Disable DHCP client on the LOGO! 8 by default. Static addressing is the operational norm in industrial control. Reserve DHCP for commissioning laptops only.
• Reserve a sub-range for static devices. If the automation subnet must coexist with DHCP, allocate .1 for the router, .2–.15 for static devices, and .16–.254 for the DHCP pool. This eliminates the overlap and removes any ambiguity about which device is at which address.
• Document MAC-to-IP mapping. Print the MAC address of every LOGO! 8 module (visible on the housing or through LOGO! Soft Comfort's IP scan) next to its IP in the project documentation. This makes future troubleshooting a single-lookup task.

Verification

After applying the remediation steps, run the following checks to confirm that the IP change is fully resolved.

1. Power-cycle the LOGO! 8 three times. After each cycle, the IP reported by the LOGO! 8 front panel (or by IP scan) must match the static address configured in the project.
2. From a PC, run ping -t 192.168.1.250 (or whatever the static address is) for two minutes while power-cycling the LOGO! 8. The ping must not lose more than 3 packets during the boot window, and the IP must remain correct when the LOGO! 8 returns to RUN.
3. From the PC, run arp -d * followed by arp -a. The MAC E0-DC-A0-17-CC-8E must appear next to the static IP, not next to .220.
4. From the PC, log into the LOGO! 8 via the Web Configurator (open the static IP in a browser; default credentials LOGO / LOGO if password protection is still at factory default). The Web Configurator shows the current IP, subnet, MAC, and DHCP state. The DHCP state must read Off.
5. If the SD card was removed, insert it and power-cycle again. The IP must remain at the static value. If it does not, the SD card still has a stale program — repeat the SD card cleanup procedure.
6. From the router, confirm that the DHCP lease table no longer contains an entry for MAC E0-DC-A0-17-CC-8E. If a reservation exists, the address is being assigned by the router regardless of what the LOGO! 8 is configured for, and the original symptom will recur.

Troubleshooting Matrix

Related Considerations: Web Configurator and Security

The LOGO! 8's Web Configurator is reachable on TCP/80 (HTTP) on the device's current IP. Once the IP is stable, set a strong password in System → Security → Change Password to prevent an attacker on the same network from rewriting the IP. The default factory password is LOGO, and the module is shipped with Web Configurator access enabled. Any device on the same broadcast domain can otherwise view the current network configuration and modify it.

LOGO! Soft Comfort V9.x and later can be used to push a configuration that sets the Web Configurator password, disables FTP, and disables the LOGO! 8's Web server entirely if the network is isolated. The same project carries the IP settings, so the Web Configurator password and the IP address are downloaded together. The procedure is documented in the LOGO! 8 system manual, chapter Network Security.

Frequently Asked Questions