Replacing Siemens ET200B with ET200S on S5 PLC via IM 308C

The SIMATIC ET 200B distributed I/O family was officially phased out in 2005. Sites that still run SIMATIC S5 controllers with an IM 308B or IM 308C Profibus DP master can in fact migrate to the modern ET 200S, ET 200S Compact, or ET 200SP distributed I/O systems without changing the S5 CPU. This reference covers the full migration path: confirming the bus type, choosing the correct replacement hardware, integrating the GSD file, configuring the project in COM Profibus, wiring the new station, and verifying cyclic data exchange on the existing S5 program.

1. ET 200B Phase-Out and Migration Background

The ET 200B was a compact, block-style Profibus DP slave with a fixed channel count and a fixed I/O map. It was delivered in 8-, 16-, 24-, and 32-channel variants for digital I/O and in analog variants for standard signals and RTD/TC. Because the device is a sealed block (no replaceable electronics), a faulty module is replaced as a whole. From 2005 onward, Siemens stopped accepting new orders for ET 200B, and the spare-parts horizon has been shrinking each year.

When an ET 200B fails or a new installation is needed on the same S5 line, the supported successor is the ET 200S family (and the long-term successor is ET 200SP). The migration is a Profibus DP reconfiguration task, not a CPU swap. The S5 program does not need to be rewritten as long as the new slave is mapped to the same I/O addresses in the S5 process image.

2. Hardware Candidates for the Replacement

There are three realistic candidates for the replacement slot. The choice depends on physical envelope, channel granularity, and lifecycle horizon.

For a like-for-like block replacement on an existing S5 line, the ET 200S Compact (6ES7 138-4xxxxx series) is the most direct drop-in because the housing footprint and the Profibus DP address slot are similar to ET 200B. For a full mechanical redesign where channels are re-arranged, the modular ET 200S with a Profibus DP head module (IM 151-1, e.g. 6ES7 151-1AA04-0AB0) is the standard choice. For long-term availability (10+ years), the ET 200SP family with IM 155-6 DP HF (6ES7 155-6BU00-0CN0) head module is the safer option.

3. Profibus DP Master on the S5 Side: IM 308B vs IM 308C

Only one S5 interface module supports the modern Profibus DP GSD-based slaves such as the ET 200S: the IM 308C. The IM 308B is a SINEC L2-DP master and is not compatible with the standard GSD-based ET 200S family.

The IM 308C occupies an S5 I/O slot (P or Q bus address range) and exposes the Profibus DP cyclic data to the S5 as plain input/output words. The S5 user program reads process input bytes from the IM 308C address area and writes process output bytes to the same area. When the ET 200S is mapped to the same address bytes, the S5 program continues to operate unchanged.

The IM 308C must be configured for the S5 environment. Key parameters to record before migration:

• Base address of the IM 308C in the S5 I/O area (e.g. P area starting at PE 128 / PA 128)
• Profibus DP address of the ET 200B being replaced (typically 3 to 49, default 3)
• Number of input bytes and output bytes the existing ET 200B was mapped to (e.g. 4 DI = 4 bytes in, 4 DO = 4 bytes out)

4. GSD File Selection: DPV0 vs DPV1

Every Profibus DP slave is described to the master by a General Station Description (GSD) file. The IM 308C only supports DPV0 services, so the GSD file used for an ET 200S on an IM 308C must also be a DPV0 GSD file.

The official GSD download index for ET 200S and other Siemens slaves is published in Siemens support entry ID 652154. Always download the GSD that matches the firmware of the IM 151-1 head module in your station. Cross-reference the GSD against the Siemens Industry Online Support catalog for the head module order number.

5. COM Profibus Configuration Procedure

COM Profibus (current release COM Profibus V5.1 + SPx) is the configuration tool for the IM 308C. The tool runs on Windows XP / Windows 7 (32-bit) and accepts the imported DPV0 GSD as a slave description. The migration is performed in the following sequence.

1. Back up the existing project. Open the current COM Profibus project, export the project as a *.cpt file, and archive the binary *.cb file that is currently downloaded to the IM 308C. The binary file is the active configuration on the master; losing it makes the bus go down at power-up.
2. Document the existing slave line-up. For each ET 200B on the bus, note Profibus address, manufacturer ID, GSD revision, I/O length, and the S5 P/Q address it is mapped to.
3. Import the new GSD file. In COM Profibus, choose Options → Install New GSD and select the SIEM0xxx.GSD file for the ET 200S IM 151-1. The new slave appears in the device catalog under PROFIBUS DP → ET 200S.
4. Replace the slave in the project. Delete the ET 200B slave at the same Profibus address and insert the new ET 200S slave at that address. Re-use the same S5 I/O base address (e.g. PE 128 / PA 128) so the S5 user program does not need editing.
5. Configure the slot map. The ET 200S modular head has a fixed slot order. Slot 0 is reserved, slot 1 is the PM-E power module, slots 2..n are the electronic modules in physical order. COM Profibus imports the default slot map from the GSD; verify that the I/O length matches the original ET 200B (e.g. 4 DI = 4 input bytes).
6. Set DP slave diagnostics. On the slave properties dialog, leave the default values for fail-safe behavior: Auto-clear = no, Watchdog = 10 s. The S5 program should see a diagnostic interrupt only on a real fault.
7. Compile and download. Compile the project, export the binary *.cb file, and download it to the IM 308C via the COM Profibus download dialog (RS-232 to the IM 308C, or via the S5 port with a PG cable). Cyclic data exchange starts immediately after the download.
8. Verify on the S5. Use STEP 5 online test to force an output at the mapped address and confirm the LED on the ET 200S DO module toggles. Read the mapped input address and confirm the S5 sees the actual field state.

6. ET 200S Module Selection and Slot Rules

Unlike the ET 200B, the ET 200S is a modular system. The head module (IM 151-1) terminates the Profibus DP line, the PM-E power module feeds 24 V to the electronics and the field side, and the electronic modules (DI, DO, AI, AO, ...) plug in slots 2 to a maximum of 63 modules per station. Each module has a fixed I/O footprint in the Profibus process image.

For a 1:1 swap of an 8-channel ET 200B (1 byte in / 1 byte out), use two 4-channel ET 200S electronic modules. For a 16-channel ET 200B (2 bytes in / 2 bytes out), use four 4-channel modules or two 8-channel modules. The Compact block (16 DI or 16 DO) is the simplest mechanical replacement.

Slot rules to honor:

• Power module (PM-E) is always slot 1
• Electronic modules can be arranged in any order, but the byte-packing in the Profibus process image follows the physical slot order
• A maximum of 32 bytes input + 32 bytes output per slave is allowed on the IM 308C without special handling (the S5 process image wraps at 128 / 256-byte boundaries)
• Always leave at least one free slot for the terminating resistor on the backplane bus; the IM 151-1 has the terminator built into the backplane connector

7. Profibus Address and S5 Address Mapping

The IM 308C occupies a window in the S5 P area (process I/O). Typical mapping on a 115U / 135U / 155U system is PE 128..255 for inputs and PA 128..255 for outputs. The mapping between Profibus slaves and the S5 process image is configured in COM Profibus under DP Slave → Properties → S5 Address.

As long as the S5 input/output bytes are preserved, the S5 program (OB 1, FB, PB, etc.) does not need any changes. If the I/O length differs from the original ET 200B, the program must be re-mapped in the symbol table. This is the most common place where a "drop-in" replacement becomes a code change.

8. Verification and Commissioning Checklist

After the binary configuration is downloaded to the IM 308C, run the following checks before handing the line back to operations.

1. LED status on IM 151-1. Green BF (bus fault) off, green SF (station fault) off, green ON (power) on. Any BF flash or SF on indicates a Profibus wiring problem or a slot-mismatch with the COM Profibus configuration.
2. LED status on IM 308C. Green "DP" LED solid, no red "SF" or "BF" LED.
3. STEP 5 force test. Force an output at the mapped address in STEP 5 (IST). The corresponding LED on the ET 200S DO module must light. Clear the force. The LED must extinguish.
4. STEP 5 status display. Display the mapped input bytes (AB/EB). Field-driven inputs must read 1 in the S5; passive inputs must read 0.
5. Bus timing. Using a Profibus diagnostic tool (e.g. Softing PROFIusb or Siemens BT 200), confirm a uniform bus cycle time and no diagnostic frames other than a single startup frame at the IM 308C. Nominal bus cycle is the sum of the configured DP cycle of every slave; for an ET 200S with 4 bytes I/O it is well under 1 ms on a 1.5 Mbit/s bus.
6. Error counter. On the IM 308C, read the station diagnostic list. All ET 200S slaves should report "data exchange, no diagnostics".
7. Watchdog test. Pull the Profibus connector briefly. The IM 308C must report a station failure for the missing slave. Reconnect within the watchdog window. The slave must return to data exchange automatically.
8. Power cycle. Cycle 24 V power on the ET 200S. The station must re-enter data exchange within the configured watchdog time and the S5 must receive fresh process data.

9. Long-term Migration Path: ET 200SP

ET 200S is mature and still in production, but ET 200SP is the long-term recommendation for any new installation. The migration is identical in concept (DPV0 GSD, IM 308C unchanged, COM Profibus unchanged), but the hardware footprint is smaller and the spare-parts horizon is longer. The IM 155-6 DP HF (6ES7 155-6BU00-0CN0) Profibus DP head module for ET 200SP supports DPV0 and is compatible with the IM 308C.

When migrating to ET 200SP, pay attention to the baseUnit concept. The head module no longer carries the backplane bus in the same way; the baseUnits (BU type A0 to BU type D0) are passive carriers that route the field wiring and the backplane. The slot order in the GSD still maps slot 1 to the first electronic module (no power module slot on ET 200SP). The I/O image footprint is per electronic module, identical in concept to ET 200S.

There is no official cross-reference table from ET 200B to ET 200SP. The recommended path is to send the original Profibus configuration and the existing S5 address map to a Siemens solution partner and request a channel-by-channel replacement recommendation.

10. Troubleshooting Matrix

11. Field-Proven Cautions

• Match the I/O length exactly. A 4-byte ET 200B and a 2-byte ET 200S Compact will not be a drop-in: the S5 process image is offset and the program will see garbage from the next slave.
• Never mix DPV0 and DPV1 GSDs on the same line. The IM 308C will silently drop DPV1 slaves after the first data exchange failure. Use only DPV0 GSDs (filename SIEM prefix).
• Set the address with a screwdriver, not the rotary switches alone. The IM 151-1 head reads the Profibus address from the rotary switches at power-up. A 1-digit mistake puts two slaves on the same address and the bus will oscillate.
• Keep the original bus topology. If the existing bus is a 1.5 Mbit/s trunk with one ET 200B at the end, do not insert spurs for the new ET 200S. Spurs beyond 0.3 m on a 1.5 Mbit/s bus cause reflections that look like intermittent failures.
• Document the S5 mapping. A printed export of the COM Profibus project, stapled to the cabinet door, is the cheapest insurance against a future "which address did the ET 200S use" question.
• Spare parts. Order one PM-E power module and one 4 DI / 4 DO electronic module as cold-spare. These are the two items that fail most often and are the cheapest to stock.

12. Standards and Reference Documents

Verify the migration against the following official documents:

• Siemens Support entry 652154 — GSD file index for Profibus DP slaves
• ET 200S Distributed I/O System Manual
• ET 200SP Distributed I/O System Manual
• IM 308C manual, 6ES5 308-3UCxx series (6ES5 998-3UCxx printed manual)
• COM Profibus V5.x user manual, 6GK1 971-1CB10
• IEC 61158 / EN 50170 Volume 2 — Profibus DP specification (DPV0 and DPV1 services)