S5-95U Profibus Master ET200S and ET200M Compatibility and Limits

Overview

The Simatic S5-95U compact PLC exposes a Profibus DP master port on the bottom edge of the CPU module. The port is electrically and protocol-compatible with the EN 50170 / IEC 61158 Profibus DP standard, which means any DP-V0 slave that complies with that standard can, in principle, be operated downstream of it. In practice, however, the S5-95U's DP master firmware contains a hard limitation on the maximum length of a diagnostic telegram that the CPU is willing to accept from a slave. This limitation is the single most important selection criterion when replacing legacy WAGO 750-x remote I/O stations with newer Siemens ET 200 distributed I/O.

This reference covers the compatibility matrix for the S5-95U Profibus master against the ET 200S (IM 151-1), ET 200M (IM 153-1) and ET 200L product families. It also documents the 32-byte diagnostic ceiling, the role of the Max_Diag_Data_Len GSD parameter, the behaviour of COM PROFIBUS V5.x versus older releases, and the field-proven workarounds used to keep the CPU out of STOP when the I/O count is in the 90 DI / 90 DO range per machine.

S5-95U Profibus Master Specifications

The S5-95U DP master port implements DP-V0 only. Key parameters that govern slave selection:

The relevant device manual is the "ET 200 Distributed I/O System with IM 308-C and S5-95U" manual, which lists the I/O modules, max stations and diagnostic conventions for the S5-95U master. The companion reference "S5/505 ET 200 Distributed I/O System" (Siemens support PDF 505-8206-2) provides the reaction table when the S5-95U transitions from STOP to RUN for the first time and contains the officially supported IM 151 / IM 153 / IM 308-C combinations.

The 32-Byte Diagnostic Data Constraint

Every Profibus DP slave has a maximum diagnostic message length declared in its GSD file under the keyword Max_Diag_Data_Len. For DP-V0 slaves this is the length of the standardized diagnostic buffer (6 standard bytes + extended diagnostic entries) that the slave can deliver during the cyclic parameterization / diagnosis handshake.

Behaviour summary by GSD value:

The diagnostic buffer is built up by the slave firmware whenever an event occurs: wire break on an AI, overload on a DO, channel fault on a PM, hot-swap event on an active backplane, etc. The 32-byte cap is therefore not a static property of the device - it is a runtime event trigger. A slave that ships with Max_Diag_Data_Len = 8 can still emit a 64-byte diagnostic message in the field if a multi-channel fault is reported, which is why the GSD value is a worst-case declaration.

ET200S (IM 151-1) Compatibility

The ET 200S with interface module IM 151-1 (6ES7 151-1AA02-0AB0 and higher) is the most common modular Profibus slave. Its default GSD file declares Max_Diag_Data_Len in the 60 - 80 byte range because the ET 200S reports module-level diagnostics (channel status, hot-swap state, module missing, parameter errors) for every slot in the station.

Implications for the S5-95U:

• COM PROFIBUS V5.1+SP2 will refuse the IM 151-1 configuration for an AG95 master and emit a non-compatibility warning.
• COM PROFIBUS V3.3 / V4.0 will accept the configuration because the diagnostic-length check is absent.
• At runtime, the first channel fault on the ET 200S will produce an extended diagnostic telegram exceeding 32 bytes and force the S5-95U into STOP.

The exception noted in the field is the ET 200S with integrated CPU (IM 151-7 CPU or IM 151-8 PN/DP CPU), used as an intelligent slave. The COM PROFIBUS V5.x import does not flag the CPU variant as incompatible, because its GSD is registered as a "DPM-Slave" device whose diagnostic buffer is the host-CPU controlled. The CPU variant is, however, considerably more expensive and is normally reserved for true pre-processing architectures, not for a direct 1:1 replacement of a WAGO 750-x.

ET200M (IM 153-1) Compatibility

The ET 200M with IM 153-1 (6ES7 153-1AA02-0AB0 and higher, including the -2AB0 and -2BA20 follow-on modules) is widely deployed with S7-300 stations. Its GSD also declares a Max_Diag_Data_Len well above 32 bytes because the IM 153-1 reports diagnostic data for every S7-300 I/O module in the rack (SM 321 DI, SM 322 DO, SM 331 AI, SM 332 AO, and F-modules such as SM 326F).

Compatibility matrix for ET 200M on the AG95:

Siemens' own FAQ "DP slaves on AG95 DP master" explicitly states that the IM 153-1 is supported, but field experience shows that this support is conditional on the station never producing a diagnostic message longer than 32 bytes - which is impossible to guarantee once more than 4-5 I/O modules are populated. For machines that are required to run 24/7, treat the IM 153-1 as not supported on the AG95.

ET200L and Lower-Complexity Alternatives

The ET 200L family is the most pragmatic 1:1 replacement for the WAGO 750-303 block I/O when the AG95 must remain as the master. The ET 200L is available in two forms:

Because the ET 200L is a fixed block (no hot-swap, no module-level diagnostics, no firmware revisions), its Max_Diag_Data_Len stays below 32 bytes for the life of the device. For a 90 DI / 90 DO retrofit, six ET 200L-SC blocks (three 16DI, three 16DO) per WAGO station is a clean and supported topology.

Other low-diagnostic slaves that have been used successfully on the AG95:

• WAGO 750-303 / 750-304 (legacy, Max_Diag_Data_Len = 64 declared but historically tolerated).
• Siemens DP/AS-i Link 20E (6GK1 415-2BA00) for AS-i I/O replacement.
• Third-party DP-V0 compact I/O from Phoenix Contact (Inline), Beckhoff (BK1x00) and Turck (BL20) - check each GSD individually.

GSD File Inspection Procedure

Before ordering any replacement I/O, inspect the GSD file for the slave candidate. The procedure takes less than a minute per candidate and is the only reliable pre-check because manufacturer datasheets rarely list Max_Diag_Data_Len.

1. Download the GSD file (Siemens GSD library, WAGO GSD download, manufacturer support portal).
2. Open the GSD in a plain text editor (Notepad, Notepad++, vi).
3. Locate the keyword Max_Diag_Data_Len (case-insensitive, located in the slave header section).
4. Read the integer value. Acceptable: 0 - 32. Reject: > 32.
5. If the keyword is absent, the slave has no extended diagnostics and the configuration is safe.

Example GSD excerpt from a compliant slave:

; GSD file: siem8042.gsd  (ET 200L-SC 16DI)
GSD_Revision          = 1
Vendor_Name           = "Siemens AG"
Model_Name            = "ET 200L-SC 16DI"
Ident_Number          = 0x8042
Max_Diag_Data_Len     = 16          ; <= 32, safe for AG95
...

Example GSD excerpt from a non-compliant slave:

; GSD file: siem8011.gsd  (ET 200S IM 151-1)
GSD_Revision          = 1
Vendor_Name           = "Siemens AG"
Model_Name            = "ET 200S IM 151-1"
Ident_Number          = 0x8011
Max_Diag_Data_Len     = 64          ; > 32, NOT safe for AG95
...

COM PROFIBUS Version Behaviour

COM PROFIBUS is the standard Siemens engineering tool for Profibus networks with S5 and pre-Step7 controllers. The diagnostic-length check was introduced in version 5.0, and is present in V5.0, V5.1 and V5.1+SP2 (the last release supporting the AG95).

For a working AG95 - ET 200M / ET 200S configuration the only engineering tools that allow the import are COM PROFIBUS V3.x / V4.x. These are obsolete; the field-proven approach is to keep a copy of COM PROFIBUS V3.3 on a maintenance laptop for the duration of the migration phase. Do not "upgrade" the engineering laptop once a working configuration has been signed off, because the V5.x import will not reproduce it.

Migration Path to S7

The S5-95U is a Phase-out / spare-part product. Long-term migration to an S7-300 (CPU 315-2 DP, 6ES7 315-2AG10-0AB0) or S7-1500 (CPU 1511-1 PN, 6ES7 511-1AK00-0AB0) removes the 32-byte diagnostic limit entirely. The recommended sequence is:

1. Phase 1 (immediate, this article's scope): Replace failing WAGO 750-x stations with ET 200L-SC blocks or with the same WAGO 750-x hardware (verified on the GSD Max_Diag_Data_Len). Keep the S5-95U as the master. Verify the new stations do not generate extended diagnostics during a one-week soak test.
2. Phase 2 (next planned downtime): Install the S7-300 station in parallel, run a Profibus cable back to the new IM 153-1, transfer the I/O map and the user program. The S5 program can be ported using the S5 to S7 converter in Step 7 (TIA Portal) or Step 7 V5.5.
3. Phase 3 (decommission): Power down the S5-95U, remove the WAGO terminator, repurpose the freed cable as the new S7-300 Profibus segment.

Throughout phase 1 the only Profibus topology that is guaranteed not to STOP the AG95 is one composed of slaves with Max_Diag_Data_Len <= 32. The Phase 1 station list should be audited again before commissioning any new module revision, because vendors occasionally update the diagnostic length in a new firmware or GSD revision.

Risk Assessment and Workarounds

Verification Procedure After a Replacement

Once a new DP slave is installed and configured, perform the following verification before releasing the machine to production:

1. Power up the AG95 cold and confirm the RUN LED stays solid (no blinking, no transition to STOP).
2. Read the diagnostic buffer of the new slave from the Profibus master with COM PROFIBUS V5.x or STEP 7 - it must report only the standard 6-byte DP-V0 diagnostic, not the extended entries.
3. Force a single channel fault on the new I/O (e.g. remove a wire from a DI terminal) and verify that the AG95 does NOT enter STOP. Reset the channel fault and confirm the diagnostic buffer is cleared.
4. Force a multi-channel fault (e.g. break two DI wires simultaneously, then three) and repeat the STOP check.
5. Cycle the AG95 to STOP and back to RUN with the new slave connected. Verify the S5-95U returns to RUN within 2 seconds (per the "Reaction to switching the S5-95U for the first time from STOP to RUN" table in the Siemens support PDF 505-8206-2).
6. Soak test 72 h with the process running; the AG95 must remain in RUN the entire interval.

If the AG95 enters STOP at step 3, the slave's actual diagnostic length exceeds 32 bytes. Remove the slave and substitute a confirmed-compliant device (WAGO 750-x or ET 200L-SC).

FAQ