ET 200S 1SI Modbus Module: Configuring 8 vs 32-Byte Process Image

ET 200S 1SI Modbus/USS Module Overview

The ET 200S 1SI serial interface module (order number 6ES7 138-4DF11-0AB0) is a point-to-point communication module designed for the ET 200S distributed I/O system. It provides one RS-232C / RS-485 / RS-422 channel per module and is used to integrate Modbus RTU masters/slaves, USS drives, and 3964(R) / ASCII protocols into an S7-300 / S7-400 / ET 200S station. The module is mechanically identical to any other ET 200S electronic module and snaps onto a TM-E terminal module.

When the 1SI is loaded into HW Config (STEP 7 V5.x) or the device configuration of TIA Portal, the catalog entry exposes two process image variants: an 8-byte I/O width and a 32-byte I/O width. Both variants point to the same physical module; the choice only defines how many bytes of consistent user data are exchanged per cycle between the backplane bus (PROFINET or PROFIBUS) and the module's internal buffer.

Functionally, the module always processes Modbus RTU / ASCII frames of up to 255 bytes per transaction internally; the "32 byte" setting does not change the frame size. It only changes how many process-image bytes are reserved in the slot, which directly affects how many user-data words the S7 CPU can read or write per backplane cycle.

Module Identification and Order Numbers

The following table summarizes the catalog numbers relevant when sizing an ET 200S station that hosts a 1SI module:

Process Image Width: 8-Byte vs 32-Byte Selection

During hardware configuration, right-click the 1SI slot and select Object Properties → I/O Addresses. The dialog exposes the input and output address ranges. The HW Config catalog entry you drag into the slot determines the address length:

Why does the same physical module appear twice in the catalog? The HW Config catalog tree is grouped by the head module. Some ET 200S interface modules historically shipped with a catalog that only listed the 8-byte variant; the 32-byte variant was added in later Service Packs of STEP 7 (typically ≥ SP2 for STEP 7 V5.4) and is found under the "ET 200S" node rather than the older "ET 200S / Profibus" subnode. Both entries write the same module to the slot — the difference is the address length that HW Config reserves.

Why 32 Bytes Appear Only Under Certain Interface Adapters

The catalog in STEP 7 is filtered by the currently selected interface module. Older HSP (Hardware Support Package) revisions of STEP 7 V5.3 limited the 1SI to 8 bytes on PROFIBUS-DP heads to keep the DP slave's total I/O footprint within the typical PROFIBUS slave limit of 244 bytes total. The 32-byte option was added to the catalog once the head modules had:

1. Sufficient slot memory to address 32 input + 32 output bytes for the module.
2. A consistent backplane transfer mechanism for 32-byte blocks (PROFINET or PROFIBUS DP-V1 with 32-byte consistency).

On the IM 151-8 PN/DP CPU (6ES7 151-8AB00-0AB0), both PROFINET and PROFIBUS sides support 32-byte consistency, so the 32-byte catalog entry is available. On older IM 151-1 heads (6ES7 151-1AA04-0AB0 and earlier), only the 8-byte variant is listed because the DP-V0 only guaranteed 4-byte consistency.

Decentralized vs Centralized Byte Budget Constraints

On a PROFIBUS DP slave, the total I/O footprint of all modules in the station must fit in the DP slave's max data length. The limits are:

If the 1SI is part of a PROFIBUS DP slave station with several other modules, choosing 32 bytes may push the slave over the 244-byte budget. In that case you must either drop the 1SI to 8 bytes, split the station, or move the 1SI to a different DP slave. In a PROFINET IO station or in a centralized configuration (1SI plugged into the IM 151-8 PN/DP CPU backplane along with the CPU function), this is rarely a concern.

Throughput Impact: Cycle Time per Data Exchange

Each backplane cycle between the CPU and the 1SI module transfers exactly the configured process image. If your Modbus telegram requires N bytes of user data (registers, holding, input, coil words), the number of backplane cycles needed to move that data is:

cycles = ceil(N / process_image_width)

For a typical Modbus function code 03 (Read Holding Registers) requesting 32 registers (64 bytes payload):

With the IM 151-8 PN/DP CPU the backplane cycle is sub-millisecond, but the principle still applies: the 32-byte variant is up to 4× faster for register-heavy transactions. For slow slaves (typical Modbus RTU at 9 600 bit/s to 115 200 bit/s), the serial line is the bottleneck, so the cycle advantage is usually invisible. For time-critical Modbus traffic on a fast link (≥ 38 400 bit/s) the 32-byte setting is recommended.

Hardware Configuration in STEP 7 (S7-300/S7-400) and TIA Portal

1. Open SIMATIC Manager (STEP 7 V5.x) or TIA Portal.
2. Open the HW Config of the ET 200S station containing the IM 151-8 PN/DP CPU (6ES7 151-8AB00-0AB0).
3. Locate the 1SI module in the catalog: PROFIBUS-DP > ET 200S > I/O > 1SI 6ES7 138-4DF11-0AB0. If the head supports 32 bytes, two entries are visible (8 bytes and 32 bytes).
4. Drag the desired entry into the slot. STEP 7 reserves 8 or 32 I + 8 or 32 O bytes automatically.
5. Double-click the slot, open Properties → Addresses. Adjust the start address if needed (avoid overlap with other modules).
6. Open Properties → Parameters:
   • Protocol: Modbus Master RTU, Modbus Slave RTU, USS, 3964(R), or ASCII
   • Baud: 300, 600, 1 200, 2 400, 4 800, 9 600, 19 200, 38 400, 57 600, 115 200 bit/s
   • Parity: None, Even, Odd
   • Data bits / Stop bits: 8N1, 8E1, 8O1, 8N2, etc.
   • Flow control: None / XON-XOFF / RTS-CTS (RS-232 only)
7. Save, compile, and download the hardware configuration to the IM 151-8 PN/DP CPU.

The hardware configuration is described in detail in the ET 200S Serial Interface Modbus/USS Module User Manual (Siemens document ID 9127658) — section 2 covers terminal assignment and section 4 covers HW Config parameters.

Modbus Master/Slave Library and Licensing

The 1SI is a "raw" serial interface. To run Modbus RTU, you need the Siemens Modbus Master / Slave library:

The library supplies, among others:

• FB 8 "MB_MASTER" — Modbus master; sends request, reads response
• FB 9 "MB_SLAVE" — Modbus slave; responds to master requests
• UDT 1 / UDT 2 — Control and status data blocks for the FBs
• Example project with conversion DB layout (referenced as the "Conversion DB" in the original 1SI documentation)

The FB instance is parameterized with:

• Logical base address of the 1SI module
• Pointer to the conversion DB (the data area the master reads from / writes to)
• Timeout, retry count, inter-frame delay

Conversion DB Parameter Reference

The conversion DB is a standard global DB (DB_W or DB_R) that holds the user-data area the 1SI Modbus FB exchanges with the application. From the ET 200S 1SI Modbus/USS User Manual and the TIA Portal help for "Modbus master (S7-300/S7-400)":

The size of the conversion DB must fit within the configured process image width. A common cause of "STATUS = 0x80C8" (data area too small) is a 64-register holding register area configured on the master but only 32 bytes (= 16 registers) reserved in the process image — fix by enlarging the DB and the 1SI I/O width.

Wiring and Terminal Assignment

The 1SI plugs into a TM-E terminal module. The standard pin assignment (RS-485 two-wire / four-wire / RS-232) is documented in the user manual and reproduced here for reference:

Verification and Commissioning Steps

1. After download, switch the CPU to RUN. The 1SI's DIAG LED should be green and the LINK LED should flash on each received Modbus frame.
2. Open Monitor / Modify in STEP 7 and watch the input bytes of the 1SI. For a slave responding to FC03, the first two bytes (function code echo) and the byte count should change within 1–2 cycles.
3. Use a third-party Modbus scanner (e.g., "Modbus Poll") on a laptop connected through an RS-485 USB adapter to validate PDU round-trip.
4. Check the diagnostic buffer of the IM 151-8 PN/DP CPU for entries of the form "Station failure" or "Module status". A persistent diagnostic indicates a parameter mismatch (e.g., baud or parity) — not a process-image issue.
5. For master mode, force the START bit in the request DB and watch STATUS change from 0x0000 (idle) to 0xFFFF (in progress) to 0x0000 with DONE=1 (success).

Troubleshooting Matrix