Migrating Legacy S7HMODM3 Library to Standard Modbus RTU Driver Blocks for CP 341 and CP 441-2 in SIMATIC PCS 7

The legacy S7HMODM3 Modbus RTU communication library was an obsolete Step 7 CFC block package utilized in early PCS 7 releases (prior to PCS 7 V6.0) to interface point-to-point serial communication processors (CP 341 and CP 441-2) with Modbus RTU slave devices over RS485. Siemens removed S7HMODM3 from official distribution starting with PCS 7 V6.0, superseding it with the standardized SIMATIC Modbus RTU block packages (MODB341 and MODB441) and the modern PCS 7 Industry Library. Engineers requiring RS485 Modbus RTU communications in modern PCS 7 or Step 7 CFC projects must utilize the official CP 341 Modbus Master driver package (6ES7870-1AA01-0YA0) with a hardware loadable driver dongle (6ES7870-1AA01-0YA0 / 6ES7963-1AA00-0AA0) or migrate to Modbus/TCP via Industrial Ethernet.

Historical Background of the S7HMODM3 Library in PCS 7

In legacy DCS installations (PCS 7 V4.x and V5.x), serial integration with third-party devices—such as energy meters (e.g., Schneider EM6400), variable frequency drives, and gas analyzers—relied on early function block collections like S7HMODM3. This library provided basic Modbus RTU Master telegram dispatching directly within CFC.

When PCS 7 V6.0 introduced the standardized driver generator and unified diagnostic structures, Siemens discontinued and deleted S7HMODM3. Continued reliance on archived copies of S7HMODM3 in newer Step 7 or PCS 7 releases leads to compiler errors, memory address conflicts in high-range DBs, and lack of diagnostic integration with the WinCC Operator Station.

Modern Modbus RTU Architecture with CP 341 and CP 441-2

In current PCS 7 and Step 7 architectures, serial RS485 communication is executed via dedicated communication processors:

CP 341 Subsystem: The CP 341 RS422/485 module (6ES7341-1CH02-0AE0) resides in an ET 200M distributed I/O station or central S7-300 subrack, communicating with the AS controller over the backplane bus.
CP 441-2 Subsystem: The CP 441-2 module (6ES7441-2AA05-0AE0) installs directly into the central or expansion rack of an S7-400 / S7-400H system, equipped with plug-in RS485 transmission submodules.
Telegram Flow: The controller program triggers telegram execution via standard communication blocks (FB8 P_SND_RK and FB7 P_RCV_RK or higher-level Modbus Master wrappers FB80 MODB341). Telegrams are queued within the communication processor's onboard buffer to prevent CPU cycle delays.

Hardware Dongle Prerequisites and Loadable Driver Installation

Deploying Modbus RTU Master functionality on CP 341 or CP 441 requires specific hardware and software licensing:

Loadable Driver Dongle: The Modbus Master protocol is not stored in the base firmware of the CP. It requires an encrypted hardware dongle (SIMATIC Hardware Key) plugged into the rear card face of the CP 341 (6ES7870-1AA01-0YA0) or CP 441 (6ES7870-1AA00-0YA0).
Loadable Driver Installation: On the Engineering Station, install the 'Loadable Drivers for CP 341 / CP 441' setup package. In HW Config, double-click the CP module, select Protocol > Load Driver..., and download the Modbus Master driver into the CP flash memory.
Protocol Parameters: Configure the RS485 transmission parameters (baud rate: 9600 to 115200 bps, parity: Even/Odd/None, 8 data bits, 1 stop bit) in the module parameterization window.

CFC Block Configuration: MODB341 vs Modern PCS 7 Channel Blocks

To implement communication in CFC:

Insert Block MODB341 (FB80): Place FB80 into a cyclic interrupt OB (e.g., OB 32 or OB 35 with a execution interval of 100 ms to 500 ms).
Configure Data Source/Destination: Define a Modbus Send/Receive DB in the symbol table. Map Modbus Function Codes (FC 03 - Read Holding Registers, FC 16 - Write Multiple Registers) to target S7 DB addresses.
Poll Scheduling: Use a state-machine sequencer or the block's START pulse input to cycle through slave node addresses (e.g., Slave 1, Slave 2) sequentially without collision.
Error Handling: Monitor ERROR and STATUS outputs. Code 0x1E0F indicates slave timeout, while 0x1E0D indicates a CRC checksum failure.

Comparison of Serial Modbus Communication Options in S7 / PCS 7

The following table compares the communication methods available for serial Modbus integration in SIMATIC automation projects:

Solution FrameworkHardware PlatformProtocol DriverLicensing RequirementModern DCS SupportModern Modbus RTU (MODB341)CP 341 RS485 (6ES7341-1CH02-0AE0)Loadable Driver (HW Dongle)Required hardware license dongleFully supported in Step 7 / PCS 7S7-400 Modbus RTU (MODB441)CP 441-2 (6ES7441-2AA05-0AE0)Loadable Driver (HW Dongle)Required hardware license dongleFully supported in Step 7 / PCS 7Modbus/TCP GatewayIE/PB LINK or Industrial GatewayEthernet Modbus/TCPOpen Modbus/TCP block licenseRecommended for new installationsLegacy S7HMODM3 LibraryObsolete S7-300 / S7-400 CPsUnofficial software macroDiscontinued / UnsupportedObsolete (Deleted since PCS 7 V6.0)

Commercial Accuracy Disclaimer: Software order numbers (MLFBs), package structures, and hardware capacity prerequisites vary across PCS 7 versions and hardware platforms. System integrators must confirm exact software entitlements and licensing requirements against the official Siemens Catalog CA 01 and obtain an authorized Siemens distributor quotation prior to purchasing.

Verification

Verify serial Modbus RTU communications:
1. Inspect the physical front LEDs of the CP 341: the green TxD and RxD LEDs should blink rhythmically, confirming active transmit and receive telegrams.
2. Open the Modbus receive data block online in Step 7; verify that register values (e.g., voltage, current, power factor from an EM6400 meter) update continuously.
3. Disconnect the RS485 cable; observe that the ERROR output asserts true and STATUS reports communication timeout (0x1E0F). Reconnect the cable and verify automatic communication recovery.
4. Verify in CFC test mode that the CPU cycle time does not spike and that execution time stays well within the configured OB 35 window.

Limitations and Operational Precautions

Never attempt to run multiple simultaneous send requests on a single half-duplex RS485 bus; Modbus RTU requires strict master-slave request-response sequencing. If using an RS485 bus spanning long field distances, ensure terminating resistors (120 Ohm) are active on both physical ends of the line and that bias networks (pull-up/pull-down) are enabled on the master port to prevent floating line noise. The hardware license dongle must remain physically plugged into the back of the CP module at all times during runtime.

Engineer FAQ

Where can I download the legacy S7HMODM3 library?

Siemens does not host or distribute the legacy S7HMODM3 library because it was superseded in 2003. Siemens technical support mandates migrating to the official MODB341 or MODB441 packages to ensure system stability and warranty support.

What happens if the CP 341 hardware dongle is missing?

Without the physical hardware dongle plugged into the rear of the CP 341, the Modbus driver will operate in an unverified demo mode, generating persistent configuration alarms and halting communication after a brief evaluation period.

Can I connect Modbus RTU devices directly to the CPU 410-5H onboard ports?

No. The onboard ports on the CPU 410-5H are dedicated PROFINET IO and PROFIBUS DP interfaces. Serial RS485 communication requires a CP 341 in an ET 200M rack or an external Industrial Ethernet-to-Modbus gateway.