Configuring S7-300 to S7-200 PROFIBUS Communication via EM 277

Overview

The S7-200 CPU family does not expose a native PROFIBUS interface. To exchange cyclic process data with a PROFIBUS DP master such as an S7-300 CPU 317-2DP, the S7-200 must be expanded with the EM 277 PROFIBUS-DP slave module. The EM 277 attaches to the right side of the S7-200 CPU and behaves as a DP-V0 slave, advertising its I/O through the standard Siemens GSD file SIEM081D.GSD. The S7-300 master maps the slave's inputs and outputs into its own process image or peripheral area, and the S7-200 user program reads/writes those locations in the V-memory area using a configuration data block generated by the STEP 7 Micro/WIN PROFIBUS wizard.

This article walks through the full commissioning path:

1. EM 277 mechanical installation and rotary address selection.
2. GSD file import into STEP 7 (classic or TIA Portal).
3. Master-side hardware configuration of the EM 277 DP slave.
4. Micro/WIN PROFIBUS wizard configuration on the S7-200 side.
5. Byte-aligned I/O mapping rules and bit extraction in user code.
6. Resolution of the "Invalid parameter was specified in configuration information" error reported by the Micro/WIN wizard.
7. Online verification with status tables, watch tables, and LED diagnostics.

Hardware Prerequisites

Reference manuals for the components above are published on the Siemens Industry Online Support portal:

• SIMATIC S7-200 EM 277 PROFIBUS-DP Module Manual
• SIMATIC S7-200 Programmable Controller System Manual
• SIMATIC S7-300 CPU 317-2DP Device Manual
• Siemens FAQ 2615830 - Communicating data between S7-200 and S7-300 via PROFIBUS DP

EM 277 Mechanical and Address Setup

The EM 277 mounts on the right side of the CPU 226 and locks in place with the captive thumb screw on top. It draws 24 V DC from the S7-200 base unit's expansion bus; no separate power supply is required for the module itself, although the PROFIBUS connector shell must be tied to a low-impedance ground through the D-sub backshell to maintain shield continuity.

Rotary Address Switches

Two rotary switches on the front of the EM 277 set the PROFIBUS node address. The high-order switch is the tens digit (x10) and the low-order switch is the units digit (x1). Valid range is 1 through 99; address 0 is reserved and addresses > 99 are not supported by the rotary inputs. The address must:

• Be unique on the PROFIBUS segment.
• Match the address assigned to the EM 277 in STEP 7 hardware configuration.
• Match the address declared in the Micro/WIN PROFIBUS wizard.

Address changes take effect only after a power cycle of the S7-200 + EM 277 stack. A change made online will not be picked up by the master until the slave re-initializes.

PROFIBUS Connector and Termination

Use a 9-pin D-sub PROFIBUS connector with integrated termination resistor (e.g., 6ES7972-0BB12-0XA0). The EM 277 does not supply terminator power; if the segment ends at the EM 277, the on-connector termination switch must be set to ON. If the segment continues, leave termination OFF. Always leave terminator power ON at the CPU 317-2DP end of the segment as well so both ends of the segment are terminated.

Baud Rate

The EM 277 auto-detects baud rate during DP parameterization. Supported rates range from 9.6 kbps to 12 Mbps. For a single-slave segment between the CPU 317-2DP and one CPU 226 + EM 277, a 1.5 Mbps profile provides ample bandwidth and tolerates a 200 m segment. Maximum segment lengths per EN 50170:

GSD File Import in STEP 7

The EM 277 is integrated into STEP 7 through its GSD file, SIEM081D.GSD, plus the bitmap file SIEM081D.BMP. Both files ship with STEP 7 Micro/WIN under C:\Program Files\Siemens\Automation\Microwin\GSD or can be obtained from the Siemens Industry Online Support page for the EM 277. A direct summary of how the data block is laid out and what parameters are exposed to the master is given in Siemens FAQ 2615830 - "How can you communicate data between the S7-200 and the S7-300 via PROFIBUS DP?".

To install in STEP 7 V5.x:

1. Launch HW Config.
2. Open Options > Install GSD File....
3. Browse to the directory containing SIEM081D.GSD and confirm.
4. Restart HW Config if the EM 277 does not appear under PROFIBUS DP > Additional Field Devices > I/O > SIMATIC > EM 277 PROFIBUS-DP Slave.

For TIA Portal V13 and later, the same GSD file is supported via Options > Manage general station description files (GSD). The EM 277 then appears in the hardware catalog under Other field devices > PROFIBUS DP > I/O > SIMATIC > EM 277.

STEP 7 Master Hardware Configuration (S7-300 Side)

Insert the EM 277

1. In HW Config, drag the EM 277 from the catalog onto the PROFIBUS DP master system that corresponds to the CPU 317-2DP's DP port (X2).
2. Assign the same PROFIBUS address as set on the EM 277 rotary switches.
3. Open the EM 277 slot configuration (double-click the slot table). The EM 277 exposes one configurable slot with selectable I/O sizes as listed below.

The maximum total throughput of the EM 277 is 244 bytes of inputs and 244 bytes of outputs, but in practice the master's slot configuration must match the wizard configuration byte-for-byte. Most installations use either 8 or 16 bytes in each direction; the 32/64 byte profiles are reserved for high-throughput data concentrators.

Compile and Download

Save the configuration back to the S7-300 station and download the hardware configuration. The CPU 317-2DP will start the DP parameterization of the EM 277 as soon as the master enters RUN. If the EM 277 BF LED begins flashing immediately after download, see the troubleshooting matrix below before assuming a cable fault.

Micro/WIN PROFIBUS Wizard (S7-200 Side)

The Micro/WIN PROFIBUS wizard generates a configuration data block (typically DB1) that the EM 277 reads on power-up. This data block must agree with the STEP 7 master configuration in three fields:

• PROFIBUS node address.
• Total input bytes (sum of all configured input slots).
• Total output bytes (sum of all configured output slots).

To run the wizard:

1. Open the S7-200 project in STEP 7 Micro/WIN.
2. Select Tools > PROFIBUS Wizard.... (In Micro/WIN V4.0 SP9 the wizard is titled simply "PROFIBUS".)
3. Set the EM 277 node address to match the rotary switches and the STEP 7 master project.
4. Select the I/O profile (e.g., 8 bytes in / 8 bytes out) to match the STEP 7 slot configuration.
5. Allow the wizard to generate the configuration subroutine (typically a SBR_x sub called from the main scan cycle).
6. Compile and download the program to the CPU 226 over PPI/MPI/USB-PPI.

Byte Alignment and Bit Packing

PROFIBUS DP exchanges I/O at byte granularity. The minimum transferable unit is one byte (8 bits). It is not possible to map a single bit such as V100.0 or Q0.1 across the bus. A frequent configuration mistake is to assign:

• V100.0 ← I0.0 (master input bit).
• Q0.1 → Q4.5 (master output bit).

The Micro/WIN wizard will reject this with the same "Invalid parameter was specified in configuration information" error if the underlying data block expects bit-level exchange. Always transfer whole bytes and extract bits in user code.

Correct Byte Mapping

Instead of mapping V100.0 ← Q0.1, map:

• Output (master → slave): VB100 ← QB0 (entire byte).
• Input (slave → master): IB0 → VB200 (entire byte).

To bring Q0.1 from the master to a specific bit in the slave program, write:

// Network 1 - unpack bit Q0.1 (master) into V100.1 (slave)
LD       SM0.0
MOVB     VB100, VB300           // copy the master output byte
// User logic can now reference V300.1 as the desired bit

Conversely, to send bit V100.0 to the master:

// Network 1 - pack V100.0..V100.7 into a byte the master can read
LD       SM0.0
// Bit-pack V100.0..V100.7 into VB100 (already byte-aligned)
//
// The EM 277 advertises VBxxx to the master as input byte 0
// depending on the configured slot offset and V-range.

Address Assignment Convention

Resolving "Invalid parameter was specified in configuration information"

This message is raised by the Micro/WIN PROFIBUS wizard's pre-download validator when the configuration data block does not match either the wizard inputs or the master's projected I/O. The systematic debug path is:

1. Confirm the wizard has been run. Reopen Tools > PROFIBUS Wizard; if the fields are blank, the project has no PROFIBUS configuration at all. Run the wizard with the correct address and I/O size.
2. Confirm the CPU family in the wizard. The wizard's first screen asks for the EM 277 model. Selecting a model that does not match the order number on the hardware will produce this error on download.
3. Confirm byte alignment. Any attempt to map a bit (V100.0) or to map fewer than 8 bits across the wizard is rejected. Convert to a byte map and unpack in user code.
4. Confirm total I/O size. The wizard reports total bytes; the STEP 7 master slot table must show exactly the same total. 8 in / 8 out on the master must be 8 in / 8 out on the wizard. If the master is 16 in / 16 out but the wizard is 8 / 8, the EM 277 accepts the wizard's configuration, but the master refuses to enter cyclic exchange and the error surfaces on the next Micro/WIN download as the wizard validates against the cached configuration data.
5. Re-download both sides. After correcting the wizard, perform a full download of the S7-200 project (program blocks, data blocks, system blocks) and a fresh download of the S7-300 hardware configuration.
6. Power-cycle the EM 277. The EM 277 latches the wizard configuration only on power-up. A corrected download without a power-cycle leaves the old parameters in effect.

Verification Procedures

LED Diagnostics on the EM 277

Master-Side Verification (S7-300)

1. In STEP 7 HW Config, right-click the DP master system and select PROFIBUS > Diagnose to view the live slave list.
2. Open a VAT or watch table and force/observe the peripheral I/O words assigned to the EM 277 (for example PQW 0 for outputs, PIW 0 for inputs).
3. From the S7-300 user program, write a known pattern to PQB 0 and verify the same pattern appears at the S7-200's VB100 (or whichever address the wizard chose).

Slave-Side Verification (S7-200)

1. In Micro/WIN, open Status Chart and add the configured V-memory bytes (input and output).
2. Toggle PQB 0 from the S7-300 watch table; the corresponding VB byte should update within one DP cycle (typically 5-10 ms at 1.5 Mbps).
3. Force a bit in the S7-200 input byte and verify the corresponding PIB on the S7-300 reflects the change.

Troubleshooting Matrix

PROFIBUS Wiring and Shielding Checklist

• Use only violet PROFIBUS cable (6XV1830-0EH10). The shield must be bonded to the D-sub backshell at every node.
• Maintain at least 20 cm separation from VFD motor cables and other 50/60 Hz power runs.
• Terminate the bus at exactly two nodes - the physical ends. With a point-to-point link (CPU 317-2DP ↔ EM 277), terminate at both ends.
• Do not exceed the segment length budget; if needed, use an RS 485 repeater (6GK1500-0EA02) to extend the segment.
• Avoid spur (stub) lines longer than 0.3 m at 12 Mbps. Use PROFIBUS connectors with horizontal cable entry rather than vertical to minimize stubs.

TIA Portal Context and Migration Notes

Newer TIA Portal versions (V13 through V20) accept the same EM 277 GSD file and present the same slot table, but the surrounding engineering environment differs. For comparison, the same S7-300 / EM 277 topology in TIA Portal V20 is configured through Devices & Networks; the master DP port is selected on the CPU's Properties > PROFIBUS interface subnode. Connection-resource allocation rules for non-DP S7 connections (used for S7-300 to S7-400/1500 communication) are described in S7 connection general settings (S7-300, S7-400, S7-1500) in TIA Portal V20. The S7-200 cannot terminate an S7 connection; it must always be a DP slave on PROFIBUS, not an S7 connection peer.

When migrating from STEP 7 V5.x to TIA Portal, preserve the EM 277 GSD in the new project catalog and re-import the slot table. Micro/WIN remains the only engineering tool for the S7-200 side regardless of the master's portal choice.

Functional Limits and Edge Cases

• Maximum slaves per DP master: The CPU 317-2DP supports up to 32 DP slaves per master system at 1.5 Mbps with default parameters. The EM 277 counts as one slave.
• Diagnostic interrupt: The EM 277 supports DP-V0 only; it does not emit DP-V1 alarms. Diagnostic data must be polled rather than pushed.
• Consistency: PROFIBUS guarantees byte-level consistency but not multi-byte consistency unless the master uses the Total length read/write services (SFC14 / SFC15 in STEP 7). For data structures larger than 4 bytes, prefer SFC14/SFC15 with the RECORD variant of the slot to maintain consistency across the cycle.
• Hot-swapping: The EM 277 cannot be inserted or removed under power. Removing it under power will fault the EM 277 and force a station failure on the master.
• Operating mode change behavior: On the CPU 226 STOP → RUN transition, the EM 277 re-enters parameterization. The master briefly shows BF flashing during the transition and resumes cyclic exchange automatically.

Field-Commissioning Caveats

• Always export the STEP 7 project after first successful commissioning and archive it with the Micro/WIN source. The two sides must move together; a configuration change on the master without a matching change in the wizard will surface only as a generic wizard error on the next slave download.
• If the CPU 226 is replaced in the field, the replacement must have the same Micro/WIN project downloaded before it is reconnected to PROFIBUS. An unconfigured CPU 226 + EM 277 stack accepts the parameterization from the master but fails to enter cyclic exchange.
• Record the EM 277 rotary address on the cabinet door label. Field service technicians frequently re-set the address during troubleshooting and forget to update the documentation, leaving the master pointing at the wrong node.
• For installations with long cable runs or high EMC exposure, prefer fiber-optic PROFIBUS via an OLM (Optical Link Module) rather than copper, especially when the S7-200 cabinet is mounted near a VFD or motor contactor panel.
• When the DP segment contains a mixture of 1.5 Mbps and 12 Mbps devices, force the master to the slowest device's baud rate or split the segment with a repeater. Auto-baud on the EM 277 only succeeds if the master's parameterization telegram declares a rate the slave supports.

FAQ