A COM port redirector (tty port redirector under Unix/Linux) is specialized software (often including device driver and user application) that includes the underlying network software necessary to access networked device servers that provide remote serial devices or modems.
- 2Variants
- 2.1Virtual serial port
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Overview[edit]
The purpose of the redirector is to make the virtual COM port exhibit behavior that closely resembles that of a 'real' COM port, i.e., a COM port driver for local serial port hardware. A virtual COM port itself is a relatively simple software mechanism that can be implemented by driver software similar to that of a conventional COM port driver. The main challenges arise in two other areas: the network connection to the device server and the behavior of the device server. These issues are described in the Technology section below.
Applications use a COM port redirector through one or more virtual COM ports that the redirector creates, as configured by the user. When the application opens the virtual COM port, the redirector makes an IP network connection to the device server at the specified IP address and TCP/UDP port number that corresponds to the remote device on the server. The COM port redirector then begins relaying the application data stream between the virtual COM port and the device server.
A redirector will typically permit creation of many (at least 256) virtual COM ports, but simultaneous use of hundreds of ports is often practically limited by a number of factors, including the memory and processor requirements of the redirector, limits on operating system resources, and the performance of the network stack.
A redirector for the Windows operating system is typically configured using a control-panel style graphical user interface for creating virtual COM ports, configuring settings for individual COM ports, and configuring global settings affecting all COM ports. The redirector GUI typically also includes displays of virtual COM port activity and various diagnostic aids.
The performance of a COM port redirector is determined by both its implementation and the network it uses to reach device servers. The performance drawbacks of simple redirector implementations can be largely addressed by kernel-level drivers that avoid context switches. Network packet loss or excessive packet times have dramatic effects on redirector operation and must be avoided.
COM port redirector software products have been offered by at least 30 vendors dating back to the early 1990s. Compatible networked device servers are currently available from a large number of manufacturers, with a heavy concentration of revenue in the top players, who are based in the North America and Asia/Pacific regions.
The equivalent software for a Unix/Linux operating system is commonly called a tty port redirector and most of the information on this page also applies to it.
Redirectors address a number of issues related to the network connection, including:
- Network protocol support that is compatible with the device server.
- Most device servers are accessed with TCP connections (raw or using the Telnet protocol) to gain reliable delivery of the application's data stream in order of transmission. The majority of server manufacturers use public TCP protocols (raw, Telnet, or Telnet with RFC 2217 extensions). Several of the larger server manufacturers use proprietary protocols in addition to, or instead of, public protocols. Device servers for certain applications, such as those that use wireless networks, use UDP instead of TCP to gain performance at the risk of network reliability.
- Initiating the network connection to a device server and determining that server is ready to relay application data.
- Accepting inbound connections initiated by device servers running in client mode and routing the data stream to a waiting application.
- Flow control of the network data stream to prevent overrun of the server. (This is not the same as hardware/software flow control of the serial device itself.)
- Data rate limiting of the application data stream to provide the performance expected for the baud rate currently in effect on the virtual COM port, which is slower than the maximum speed of the network connection to the server.
- The timing effects of the TCP protocol stack, e.g. network packetization and Nagle's algorithm.
- Network connections through proxy servers.
- Management of the IP routing table to avoid loss of an IP route to the device server.
- Detection and handling of network interruptions, possibly with an automatic attempt to reconnect to the device server to resume application data flow.
Redirectors must also deal with the feature differences of networked device servers related to:
- Visibility and control of serial line signals such as DSR, DCD, CTS, DTR. The redirector may be able to sufficiently emulate these signals.
- Relay of BREAK signals.
- Settings for hardware or software flow control.
- Handling of the network connection when serial devices or modems disconnect.
Because COM port redirectors emulate COM ports on the operating system abstraction level, rather than hardware serial ports, old software which is not written to utilize the operating system, but instead interface directly with a serial microcontroller (hardware ports 0x3F8 to 0x3FF for COM1, for example) will not function with COM port redirectors.
Variants[edit]
Specialized types of redirectors have been offered to meet the needs of certain applications.
A redirector may support back-to-back operation, in which two computers run copies of the redirector and an outbound connection from one results in an inbound connection to the other. In effect, this technique creates a serial communications tunnel through a network connection. In practice, this configuration works only for certain applications but offers potentially lower costs and higher performance using the Internet to carry serial communications instead of modems between two computers.[citation needed]
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A redirector may include a modem emulator that allows the application to use 'AT' modem commands even though no physical modem is present. The 'number' dialed is an IP address, and the connection is a TCP network connection instead of a modem telephone call. This type of redirector is generally used by applications in originating client software needs to use a modem but the destination for the connection is a network endpoint. Back-to-back operation of this type of redirector can, in some cases, function as a replacement for modems on two computers for some applications. Network effects on timing of the data stream generally preclude the use of this method for transmitting faxes. Additionally, this method is also not reliable if used for PPP connections (such as dial-up networking) due to architectural limitations of TCP, a topic discussed in technical literature related to TCP-over-TCP.
Virtual serial port[edit]
One variant of a COM port redirector is a virtual serial port. A virtual serial port is a redirector without network software support which is usually used to create a pair of back-to-back virtual COM ports on the same computer. Two legacy applications can then communicate using virtual serial ports instead of conventional inter-process communication mechanisms such as named pipes.
This type of software is capable of emulating all serial port functionality, including Baud rate, data bits, parity bits, stop bits, etc. Additionally, it allows the data flow to be controlled, emulating all signal lines (DTR / DSR / CTS / RTS / DCD / RI) and customizing pinout.
Serial port emulation[edit]
- Serial port emulation is useful especially when there is a lack of available physical serial ports. Communication between software and/or devices which would otherwise require extra physical connections, can be benefited by using a virtual COM Port emulator.
- Virtual serial ports let you send or receive data over a TCP/IP port using any serial communication program. This facility allows creating a full-fledged client-server architecture which provides multiple connections and data sharing possibilities between different applications. Such a connection is the best way to allow use of rare and expensive serial devices by different users simultaneously.
- Using serial port emulation one can split a real COM port between a number of virtual serial ports. This makes it possible to supply data from a single serial device to a number of different applications. Such necessity arises when several applications compete for a single GPS connection and the user must close one program to allow another to access a single GPS device.
- A virtual serial port may have the same name as physical one. This facility allows real serial port overlapping (mapping) and receiving data from a physical port through virtual port. In other words, you can map any serial port or virtual port to any other existing port in your system. In this case applications will work with virtual ports, but, in fact, they will receive data from overlapped real ports.
Specification[edit]
- RFC 854 - Telnet protocol specification
- RFC 2217 - Telnet Com Port Control Option
See also[edit]
References[edit]
SerialToIP - open source Terminal Server software
![Serial Port Emulator Serial Port Emulator](https://images.sftcdn.net/images/t_app-cover-l,f_auto/p/c728bacc-96d7-11e6-8067-00163ed833e7/1195708222/free-virtual-serial-ports-emulator-screenshot.jpg)
Further reading[edit]
- Serial Port Complete: COM Ports, USB Virtual COM Ports, and Ports for Embedded Systems; 2nd Edition; Jan Axelson; Lakeview Research; 380 pages; 2007; ISBN978-1-931-44806-2.
Employing virtual devices to simulate the behavior of their physical counterparts has long been a widely accepted method of developing, testing and debugging software.
Serial port emulation is used by developers of complex serial communication systems. Programmers can use a variety of software tools that provide operating systems (primarily Windows OS) with virtual serial ports. These software solutions allow testing and monitoring of serial port communications between RS232 and RS484 interfaces.
Even applications designed to work with hardware COM ports that are resident on client computers can be tested without connecting to physical serial ports, Dedicated software tools enable the creation of pairs of virtual COM ports that communicate by way of a virtual null-modem cable. You can also develop a serial port emulator yourself to simplify the testing and monitoring of data traffic between your program and a physical COM port.
Serial port emulation: main principles
- Programmers can use multiple virtual serial ports, allowing them to diagnose and debug software on systems containing no physical serial ports.
As an example, consider that a programmer developing and debugging an application for serial communication will usually need at least two physical serial ports. Currently, most computers provide at most one serial port, forcing a developer to either use two machines or obtain a PCI card containing a COM port. By using the serial port emulator the programmer can now create multiple virtual COM ports on one computer, allowing any serial communication program to be developed on any computer that can run the emulator.
By emulating COM ports, developers no longer need to use complex software to control interprocess communication and the sharing of physical resources. The speed of data transmission is not dependent on the physical COM port but relies on your system specifications as traffic moves between the virtual ports.
Virtual serial ports can replicate all the parameters of physical serial ports and provides strict baudrate emulation.
Virtual COM Port Driver by Eltima Software is a powerful serial port emulator that offers advanced functionality built on a simple, user-friendly interface. All recent versions of the Windows operating system are supported by VSPD. The tool is even more stable when run on Windows 10 as it is digitally signed with WHQL. These are just a few of the many reasons that make Virtual Serial Port Driver one of the best and effective serial port emulate tools available.
Serial port simulation with different programming languages
To simplify working with files and devices, standard options such as CreatePair, DeletPair, SetStrictBaudrate and others are fully supported by VSPD. Below are some examples of how you can take advantage of Virtual Serial Port Driver with some popular programming languages.
C/C++
Load the DLL file dynamically and then choose and call your desired functions to effectively use virtual port emulator. Here’s how it looks in Visual C++ :
Visual Basic
With Visual Basic, you need to paste these lines into your source file to access the functionality of VSPD:
With the VSPDCTL.DLL in your application’s directory, the functions can be called directly.
Serial Port Emulator Freeware
Delphi
Usb Comm Port Emulator
Here are the function declarations for Delphi:
Com0com
ArmAccess can be loaded statically or dynamically. For dynamic loading, use this sample code:
This module has a full integrated Linux OS, so it's happy to respond on its mini-USB port, to PuTTY/Telnet type connection requests and offers a full account security capability etc.
I'd like to connect my laptop (running Win XP SP3 32-bit) to this module, using a USB port in my laptop with a cable that has a mini-USB port on one end, and a normal USB connector on the other end.
What I want to ask about is what do I need, in terms of software or device drivers, to make this happen?
Here's a crude diagram of what I'm trying to connect together:
[MODULE with RS-232 connection using mini-USB port] <-------- Mini-USB connector -- CABLE -- Normal USB connector --------> [Laptop with USB port]
Will it work straight away or do I need some kind of Serial-over-USB device driver/emulator software or other software that will allow my USB port (on my laptop) to 'act' as a serial port and connect to the module's mini-USB?