Configuring USB-HART Modems and COM Port Assignments in SIMATIC PDM

To connect a USB-HART modem (such as a Pepperl+Fuchs or Siemens modem) to field instruments using SIMATIC PDM within STEP 7, the communication must be configured within the SIMATIC PDM Process Device Plant View or Network View—not inside the classical PLC hardware configuration (HW Config). In PDM Network View, insert a 'HART Modem' network object, open its Object Properties, select the communication interface as 'Serial interface', and set the COM port to match the virtual COM port (e.g. COM4) assigned to the USB modem in Windows Device Manager. Once configured, insert the field instrument under the HART modem network to initiate parameterization.

Executive Summary

Point-to-point parameterization and diagnostics of intelligent field instrumentation using HART communication is a standard task for maintenance engineers. When using SIMATIC PDM (Process Device Manager) on a Field PG or laptop connected to non-HART I/O modules or bench testing instruments directly, engineers utilize a portable USB-HART modem. While third-party vendor software readily discovers the modem on its assigned virtual COM port, engineers attempting to use PDM within STEP 7 frequently struggle to locate where COM ports are assigned. Because PDM handles point-to-point field modems outside the PLC station architecture, the configuration resides in the PDM Network View. This article provides a step-by-step setup guide for USB-HART modems in SIMATIC PDM.

Architecture and Fundamental Mechanics

SIMATIC PDM Network Topology, Virtual COM Port Emulation, and HART FSK Protocol Routing:

Architecture of Point-to-Point vs Networked PDM:
In an integrated DCS environment, PDM communicates with field devices through HART routing: the ES routes communication over Industrial Ethernet to the S7-400 CPU, across PROFIBUS DP, into ET 200M/ET 200SP HA interface modules, and through HART-enabled analog input cards.
When I/O cards lack HART capability or when an instrument is tested on a workbench, communication is established via a Direct Point-to-Point HART Modem.
Virtual COM Port Emulation:
USB-HART modems (e.g. Pepperl+Fuchs VIATOR USB-HART, Siemens HART Modem USB, or MacTek VIATOR) utilize FTDI or Silicon Labs USB-to-UART bridge chipsets.
Windows assigns the device a virtual serial communications port (e.g. COM3, COM4) in Windows Device Manager.
The modem modulates serial digital frames into Bell 202 Frequency Shift Keying (FSK) audio tones (1200 Hz for '1', 2200 Hz for '0') superimposed over the 4-20 mA analog loop.
Configuration Boundaries in STEP 7:
HW Config manages physical automation racks and PLC networks; it has no concept of local PC USB serial ports.
Therefore, COM port assignment is managed strictly within SIMATIC PDM Process Device Network View (accessible via SIMATIC Manager View > Process Device Network View).

Step-by-Step Implementation Procedure

Verify Virtual COM Port in Windows Device Manager:
* Plug the USB-HART modem into the Field PG or laptop.
* Press Win + X and open Device Manager.
* Expand Ports (COM & LPT).
* Locate the USB-HART modem entry (e.g. USB Serial Port (COM4)).
* Note the exact COM port number (e.g. COM4). Ensure the port number is between COM1 and COM8 for optimal PDM compatibility.
Open SIMATIC PDM Process Device Network View:
* Launch SIMATIC Manager.
* Create a new standalone project (e.g. HART_Field_Testing).
* Select menu View > Process Device Network View.
Insert and Configure the HART Modem Network Object:
* In the left tree, right-click the project root and select Insert New Object > Network.
* Select network type: HART modem; name the network (e.g. HART_USB_Modem).
* Right-click HART_USB_Modem and select Object Properties.
* In the Connection tab, configure:
- Interface: Serial interface (COM)
- Port: Select the matching port noted in Step 1 (e.g. COM4).
- Baud rate: 1200 baud.
- Parity: Odd (standard for HART).
- Retries: 3.
- Click OK.
Insert Field Instrument and Assign Device Description (EDD):
* Right-click the HART_USB_Modem network object and select Insert New Object > Object.
* Assign device type: browse the PDM Device Catalog and select the exact instrument manufacturer and model (e.g. Endress+Hauser Promag 53 or Siemens SITRANS P).
* Set the HART address (typically address 0 for 4-20 mA point-to-point loops).
Open Device and Execute Parameterization:
* Double-click the field device object in PDM.
* SIMATIC PDM establishes communication through the virtual COM port.
* Upload parameters from the device (Device > Upload to PG/PC) and perform calibration.

Verification

Observe the TX/RX LEDs on the USB-HART modem flashing rapidly during communication.
In SIMATIC PDM, verify that the status bar displays 'Device connected' and online measured values update in real time.
Execute a parameter read; verify that the serial number, tag name, and measuring range match the physical instrument nameplate.

Limitations and Operational Precautions

Ensure the 4-20 mA current loop has a minimum total loop resistance of 250 ohms (between 250 and 1100 ohms); without adequate loop resistance, the FSK voltage signals are attenuated and the modem will fail to establish communication.

Engineer FAQ

Where do I configure the COM port for a USB-HART modem in PCS 7?

In the SIMATIC PDM Process Device Network View, under the Object Properties of the 'HART Modem' network object.

Why can't I find the COM port setting in HW Config?

HW Config is strictly for PLC station hardware. Point-to-point field modems are managed exclusively in PDM Process Device Network View.

What loop resistance is required for HART communication?

A minimum of 250 ohms loop resistance is required across the 4-20 mA signal wires for HART FSK communication.

What HART polling address is standard for point-to-point modems?

Address 0 is standard for 4-20 mA analog instruments. Addresses 1 to 63 are used in multi-drop mode.

What should I do if Windows assigns a high COM port number like COM18?

In Windows Device Manager, open the port properties, navigate to Port Settings > Advanced, and reassign the COM port to an unused number between COM1 and COM8.