Integrating Virtual Serial Port ActiveX Control with .NET and Delphi

Top Features of Virtual Serial Port ActiveX Control ExplainedVirtual Serial Port ActiveX Control is a developer component that simulates standard COM (serial) ports in software, allowing applications to communicate over virtualized serial links without physical hardware. This article explains its top features, typical use cases, integration patterns, and implementation considerations so you can decide whether and how to use it in your projects.

What is a Virtual Serial Port ActiveX Control?

A Virtual Serial Port ActiveX Control exposes an API compatible with serial COM port behavior but creates virtual device pairs in the operating system. Each pair behaves like a hardware RS-232 connection: data written to one end is immediately available to read on the other end. Because it acts like a real COM port, legacy applications and drivers that expect serial ports can interact with virtual ports without modification.

Key Features

1. Creation and Management of Virtual COM Port Pairs

One of the core features is the ability to create, enumerate, and remove pairs of virtual COM ports programmatically. The control typically provides methods to:

Create a new pair with specified port names (e.g., COM5 <-> COM6).
Delete existing pairs.
List all virtual ports currently installed.

This makes it easy to set up point-to-point communication channels on demand.

2. Compatibility with Standard Serial APIs

Virtual Serial Port ActiveX Controls present themselves to Windows and applications as standard serial ports, compatible with:

Win32 serial APIs (CreateFile, ReadFile, WriteFile, SetCommState, etc.).
Common language bindings (COM/ActiveX interfaces), so they’re usable from environments like VB6, VB.NET, C#, Delphi, and C++.
Terminal and serial communication utilities (PuTTY, RealTerm).

This compatibility ensures minimal changes when integrating into existing systems.

3. Data Loopback and Pairing Behavior

The virtual port pairs implement reliable loopback behavior: whatever data is sent to one port appears at the other. This enables a variety of scenarios:

Inter-process communication on the same machine.
Testing and debugging serial applications without hardware.
Simulating devices for development and CI systems.

Some controls offer configurable buffering and packetization to better mimic real device timing.

4. Event and Callback Support

Advanced controls provide event-driven mechanisms or callbacks for:

Data received.
Port opened/closed.
Errors and flow control events.

Event support simplifies asynchronous programming and reduces busy-waiting, which is especially helpful in GUI apps and services.

5. Flow Control and Line Signal Emulation

To emulate realistic serial behavior, virtual port controls often support management of serial line signals:

Hardware flow control: RTS/CTS.
Software flow control: XON/XOFF.
Status lines: DTR, DSR, DCD, RI.

This lets developers test behavior dependent on line states, such as modem emulation or handshake-driven protocols.

6. Configurable Baud Rate and Serial Parameters

Although virtual ports are not limited by physical signaling, controls generally let you set and report common serial parameters:

Baud rate, data bits, parity, stop bits.
Timeouts and buffer sizes.

Setting these parameters lets applications behave identically to when using real serial ports and allows simulation of timing-sensitive communication.

7. Multi-Instance and Cross-Process Support

A robust ActiveX control supports multiple simultaneous virtual port pairs and allows different processes (or threads) to open opposite ends. This is crucial for:

Multi-client testing environments.
Complex test harnesses integrating multiple simulated devices.

Access controls and concurrency handling ensure ports can be shared or exclusively opened as needed.

8. Persistent Port Creation and Driver-Level Installation

Some implementations offer the option to create virtual ports that persist across system restarts by installing kernel-mode drivers or using system-level services. Persistent ports are helpful when:

You need stable COM port names for third-party software.
Virtual devices must appear permanently to the OS.

Developer-mode (temporary, in-process) vs. driver-mode (persistent) operation choices let you balance ease of use and system integration.

9. Security and Access Control

Quality controls include features to manage which processes can open or configure virtual ports, preventing unauthorized access or accidental conflicts. Administrative privileges may be required for creating persistent ports or installing drivers.

10. Diagnostic Tools and Logging

Built-in diagnostics, logging features, and utilities (port monitors, packet loggers) assist in debugging communication issues, verifying data flow, and auditing usage. This is valuable during development and in production troubleshooting.

Typical Use Cases

Legacy application modernization: Let old software interact with modern systems by bridging virtual COM ports to network services or USB devices.
Automated testing: Simulate serial devices for unit, integration, and system tests without physical hardware.
Development and debugging: Rapidly prototype device firmware or application behavior using loopback and simulated signals.
Industrial automation: Integrate equipment that uses serial protocols into newer infrastructures.
Education and training: Teach serial communication concepts without multiple hardware units.

Integration Patterns

In-process ActiveX: Embed control within application code (VB6, VB.NET via COM interop) for direct programmatic control.
Out-of-process server: Run the control as a background service that manages persistent ports and provides a networked API.
Bridge/gateway: Use the control to connect a COM-based application to a TCP/IP service or USB-serial converter.

Example (conceptual) flow for using from C# via COM interop:

Instantiate the ActiveX object.
Create a virtual COM pair with specified names.
Open one end in the device emulator and the other in the application under test.
Use events to handle incoming data and send responses.

Performance and Limitations

Latency is generally low (sub-millisecond to millisecond range) but can vary with buffering and system load.
Throughput is usually limited by OS scheduling and buffer sizes rather than physical constraints.
True hardware behavior (like electrical noise) cannot be fully reproduced; specialized simulators are needed for those cases.
Administrative rights often required for creating persistent ports or installing drivers.

Implementation Considerations

Choose between temporary (process-bound) and persistent (driver-installed) modes based on deployment.
Verify compatibility with target OS versions and 64-bit vs 32-bit applications.
Ensure proper cleanup: delete temporary ports and unregister drivers during uninstall.
Test flow control and signal handling if your application relies on hardware-style handshakes.
Use event-driven APIs to avoid polling and reduce CPU usage.

Example Development Checklist

Confirm supported languages and binding examples (VB, C#, Delphi, C++).
Test basic create/open/read/write/close cycle.
Validate parity, stop bits, and baud rate handling.
Verify RTS/CTS and XON/XOFF behavior if used.
Check persistence behavior across reboots if required.
Run concurrency tests with multiple processes accessing different port pairs.

Conclusion

Virtual Serial Port ActiveX Controls provide a flexible, low-effort way to simulate COM ports for development, testing, and integration. Their key strengths are compatibility with standard serial APIs, configurable behavior (flow control, baud, signals), event-driven design, and options for persistence. When chosen and configured appropriately, they significantly reduce the need for physical hardware and speed up development and testing cycles.