IM306 DP Slave with IM 308-C Master: S5 PROFIBUS Configuration

Overview

An S5-135U central rack equipped with a CPU 928B and an IM 308-C (order number 6ES5 308-3UB11 or 6ES5 308-3UC11) is a classic PROFIBUS-DP master station. Many control retrofits attempt to attach additional digital I/O in EG 183U expansion racks over PROFIBUS, but a frequent misunderstanding is that the IM 308-C itself can act as a standalone DP slave when placed in the EG 183U without a CPU. The IM 308-C is an interface module, not a free-standing I/O station controller; without an S5 CPU it cannot initialize the S5 backplane and therefore cannot expose the EG 183U slots as a DP slave.

This reference documents the constraint, then provides the field-proven alternative: the IM 306 DP Slave interface module (6ES5 306-1UE00) installed in the EG 183U. The IM 306 makes the expansion rack behave as a true PROFIBUS-DP slave, exchanges I/O data with the IM 308-C master in the ZG 135U, and is fully configurable in COM PROFIBUS using a vendor-supplied GSD file. The configuration described below has been used in service on installed machinery for decades and remains the recommended topology whenever the field-level distributed I/O must ride on PROFIBUS-DP rather than the proprietary S5 backplane bus.

Why the IM 308-C Cannot Stand Alone as a Slave in an EG 183U

The IM 308-C sits in the S5 backplane like any other intelligent module and relies on a CPU to initialize it. According to the Siemens support entry on the IM 308-C master-slave functionality (ID 37814), the IM 308-C only operates as a DP slave in combination with an S5 CPU in a central rack; the CPU is required to drive the S5 backplane protocol that the IM 308-C needs to exchange process data.

When an IM 308-C is plugged into an EG 183U expansion rack, there is no S5 CPU present, no backplane master, and therefore no backplane services. The module powers up, but the S5 backplane remains uninitialized, the DP state machine never leaves its initial state, and no PROFIBUS message is emitted. From the perspective of the master in the ZG 135U, this slot behaves as if nothing is connected - the DP diagnostic block reports "slave not found" indefinitely. The conclusion, confirmed in the field, is unambiguous: the IM 308-C cannot convert an EG 183U into a stand-alone DP slave I/O station.

IM 308-C Master-Slave Functionality with an S5 CPU

The IM 308-C is unusual among DP masters because it can simultaneously act as a DP slave. This combined mode is documented in Siemens Support entry 37814 - The IM 308-C Master-Slave Functionality. In this mode, the CPU that hosts the IM 308-C can both:

• function as a DP master to intelligent and standard DP slaves on the network, and
• be polled as a DP slave by another, higher-level DP master (typically an S7-300/400 or another S5 station) to publish a defined I/O image.

This is the exact mechanism used when an S5 station is exposed to an S7 master. Siemens Support entry 6556876 - Operating an S5 (with IM 308-C) as a DP Slave on PROFIBUS-DP describes the procedure: the IM 308-C is added from the "IM 308-C DP-Slave" catalog branch in COM PROFIBUS, the S5 CPU type is selected (e.g. CPU 928B), and the slave image is mapped to the desired P/PI area.

This mode is irrelevant to the EG 183U question, because it still requires the IM 308-C to be in a rack that contains an S5 CPU. It does, however, confirm that the IM 308-C's role in any S5 distributed-I/O design is "intelligent master" or "intelligent slave with CPU" - never "dumb slave interface."

The IM 306 DP Slave - Correct Solution for the EG 183U

The IM 306 DP Slave (order number 6ES5 306-1UE00) is a single-width S5 module designed to be installed in slot 1 of an EG 183U (or EG 185U) expansion rack. It implements the PROFIBUS-DP slave protocol independently of any S5 CPU, initializes the backplane behind it, and presents the I/O modules mounted in the remaining slots as a contiguous PROFIBUS-DP image.

Capabilities

• Acts as a PROFIBUS-DP slave (DP-V0) at any baud rate up to 12 Mbit/s.
• Supports up to 6 S5 I/O slots behind it, addressable as DP slot 1 through DP slot 6.
• Supports up to 32 bytes of inputs and 32 bytes of outputs total (DP-V0 image limit).
• Configurable DP slave address 1 through 99 via DIL switches on the front panel.
• Provides bus-level diagnostics via front-panel LEDs and DP diagnostics telegrams.
• Compatible with any DP master that conforms to EN 50170 (PROFIBUS-DP), including the IM 308-C.

Diagnostic LEDs

Reference Architecture: S5-135U Master with EG 183U Slave

The intended topology for the scenario described in the question is straightforward. The S5-135U central rack (ZG 135U, 6ES5 135-3UA13) hosts the CPU 928B (6ES5 928-3UB11) and the IM 308-C master. The EG 183U expansion rack (6ES5 183-3AA13) hosts the IM 306 DP Slave in slot 1 and the desired S5 DI/DO modules in slots 2 through 7. The two are connected via a PROFIBUS-DP segment.

A simplified network topology is shown below.

Hardware Components and Part Numbers

Use the following part numbers when ordering replacement or new hardware for this topology. All numbers are Siemens 6ES5 series and remain supported through the SIMATIC S5 spare-parts program.

PROFIBUS Topology, Cable, and Termination

PROFIBUS-DP segments must be wired with PROFIBUS cable type A (solid-core, two-wire, shielded) at a characteristic impedance of 135-165 Ω and a loop resistance below 110 Ω/km. Maximum segment length is baud-rate dependent. The figures in the table below are taken directly from IEC 61158 / EN 50170 and match what the IM 308-C and IM 306 will negotiate.

9-pin D-sub connectors with switchable termination (Siemens 6ES7 972-0BA12-0XA0 or equivalent) are the field-standard termination method. Slave addresses are set with the DIL switch on the IM 306 front panel (default is 3) and must match the COM PROFIBUS master project exactly.

COM PROFIBUS Configuration Steps

COM PROFIBUS (versions 5.x and later support the IM 306 GSD) is the master-configuration tool. The procedure below covers a typical commissioning session on the S5-135U.

1. Launch COM PROFIBUS and create a new project. Select the S5-135U/928B platform.
2. Insert the master: Drag "IM 308-C" from the master catalog into the master station. Set the master PROFIBUS address (commonly 1) and the baud rate (1.5 Mbit/s is recommended unless segment length forces a lower rate).
3. Import the IM 306 GSD file (Siemens file SIEM804F.GSD or the equivalent for the specific IM 306 variant). The import is done via the menu Options > Install New GSD.
4. Add the slave: From the catalog tree "PROFIBUS-DP > I/O > Siemens > IM 306", drag the IM 306 to the DP bus at the chosen slave address (commonly 3).
5. Configure slots: Right-click the IM 306 slave and add module entries for each physical module behind the IM 306. Typical mapping: slot 1 = 4 byte input, slot 2 = 4 byte output, slots 3 through 6 either occupied or marked empty (if no module is plugged in, mark empty so the master does not expect data).
6. Map to S5 memory: In the master station, define the I and O areas (e.g. IB 100-IB 131 / QB 100-QB 131, or PEB/PAB extended areas). COM PROFIBUS generates the data records that the CPU 928B handshake expects.
7. Compile and check: Use Compile > Consistency Check. The tool will flag any mismatch between the configured GSD image and the physical modules (the same condition the IM 306 SF LED indicates at runtime).
8. Download to master: Connect the PG to the IM 308-C master via the MPI/AS511 interface and download the configuration.
9. Cycle power on the EG 183U after first commissioning so the IM 306 reads its DIL switch address and presents a clean DP startup.

CPU 928B Handshake with the IM 308-C

The CPU 928B exchanges data with the IM 308-C through a set of handshake markers and byte flags. This is required regardless of whether the IM 308-C is acting as master or combined master/slave. Typical parameters are:

The same handshake marker convention applies to an S7 master polling the S5 station as a slave, as described in Siemens Support entry 6556876.

Commissioning and Verification

After the download, perform the following verification sequence:

1. Confirm the IM 306 ON LED is green. If it stays dark, check the 24 V supply at the IM 306 terminals.
2. Confirm the IM 306 BF LED is off. A lit BF LED indicates no master token or wrong baud rate; cycle the master after a configuration change.
3. Confirm the IM 306 SF LED is off. A lit SF LED means the master is sending a configuration that does not match the physical I/O in slots 2-7. Recompile the GSD image in COM PROFIBUS.
4. From the PG, open Diagnostics > Slave Diagnostics for address 3 and confirm the slave reports "Data Exchange" state.
5. In STEP 5, force a known output (e.g. QB 100 = 0x55) and read back the corresponding input byte on the diagnostic block to confirm the round-trip.
6. Use a PROFIBUS trace tool (e.g. Amprolyzer, Softing PBpro) to confirm token rotation through the slave and verify the cyclic time is within 5 ms at 1.5 Mbit/s.

Diagnostics and Troubleshooting Matrix

When to Consider Modern Alternatives

While the IM 306 + IM 308-C topology remains serviceable and supported by the SIMATIC S5 spare-parts program, plants planning major retrofits should weigh two modern alternatives:

• ET 200S / ET 200SP on PROFINET: An S7-1500 (or migrated S5 via gateway) controlling modern distributed I/O over PROFINET is more cost-effective than maintaining the S5 backplane spare parts inventory. Reference the IM 308-C entry on the PROFIBUS product finder for current product status.
• DP/DP coupler: If two independent DP masters must share data without an S7 in the middle, a Siemens DP/DP coupler (6ES7 158-0AD01) replaces a direct IM 308-C master-to-slave handshake with a black-box data exchange.

For ongoing S5-135U service, however, the IM 306 + IM 308-C + CPU 928B combination is still the correct, documented architecture.

Frequently Asked Questions