Telnet COM connection theory and practice - serial to IP software

Telnet is a TCP/IP protocol used to create connections to remote computers. It is also a user command, and by using Telnet, users or administrators have the ability to access remotely located machines. There are other protocols used on the Web, such as HTTP and FTP that allow you to make file requests from remote computers without logging in as a user of that machine. When using Telnet, you log in as a user of the remote machine and are granted those user’s privileges to applications and data that reside on it.


  1. Telnet usage
  2. The problem when connecting serial to telnet
  3. Telnet serial port solution
  4. Software solution for telnet COM forwarding

Telnet usage fields

The format of a Telnet command is as follows (use your actual computer name when connecting):


After entering this command you would be presented with a login screen and need to provide a userid and password. If your credentials are correct, you are then logged in as if you were a local user.

Telnet is primarily used to satisfy the need to access designated applications or data that are located on a particular computer that is located remotely. Serial devices that are attached to a remote server can be accessed by using the Telnet RFC 2217 protocol. Examples of these devices are:

  • industrial monitoring devices such as water flow control monitors
  • medical monitoring equipment
  • serial printers and scanners
  • copiers, cash registers, and other serially attached equipment

The problem

Let’s take a look at an application that utilizes a serially connected device, and employs a COM port, like a GPS reader.

We would like to be able to use a device connected to a computer (server) to be accessed by an application that is running on a network-connected remote computer (client). An example would be having a device connected to the small, single-board PC ODROID. After connecting the ODROID to the network, we want to read the device’s data on our local desktop.

If, as we suspect, the application must use a serial port to communicate with the device, then the client computer needs a virtual serial port for the application to use.

Then the application interacts with the client machine’s serial port and is not concerned by the fact that the data is being sent over the network.

The solution in theory

One of the solutions to this problem is to use Telnet with RFC2217 - Telnet Com Port Control Option. There are many software tools that supports Telnet+RFC2217 serial port forwarding, enabling you to run server and client machines on the Linux or Windows platforms. The interoperability of RFC2217 and Telnet means that a Linux server and Windows client can communicate using completely different software packages, as the underlying protocol is the same. You also have the ability to configure parameters, for example to multiplex the serial ports or enact data encryption when using RS232 to telnet.

A great Windows alternative to passthrough Serial to Telnet is the software solution from Eltima - Serial to Ethernet Connector. The software lets you create an unlimited number of virtual COM ports in a system. No additional hardware, cables, or wires are required to connect to serial device over Telnet with this tool.

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Serial to Ethernet Connector

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Serial to Ethernet Connector

The solution in practice for Windows

Here is a great solution for Telnet serial port forwarding on both client and server Windows machines. Serial to Ethernet Connector is a software application that offers a Windows 10 telnet server alternative. It provides an easy path to connecting to a remote computer using the TCP/IP protocol, and supports the RFC 2217 Telnet extensions allowing COM port control. Follow these steps to create a Telnet IP client connection:

  1. Select New connection → Client connection on the toolbar
  2. Specify the name for the new connection.
  3. Select the local serial port that you will use for the connection.
  4. When creating a virtual serial port simply Check the Create as virtual port option. Virtual ports can be created in any number and be named however you like. They act as if they were actual physical ports and fully emulate their setting while offering an advantage in transmission speed.

    Note: a virtual serial COM port can be named with the same name as an existing physical port.

    The virtual port will “overlap” the real one in this situation, and the physical port will be inaccessible to other applications.

  5. Enable Strict baudrate emulation if you are creating a virtual port and are planning to send large blocks of data of a size of 2 KBs or greater. This forces the baud rate of the virtual port to replicate that of the physical port and help minimize possible data loss.
  6. Under Remote host name and Port specify the name or IP address and the listening port of the server(s) you are connecting to.
  7. Configure any advanced settings that you need (more details in the next section).
  8. Press Create connection to complete the process.

  9. General connection settings:

    1. Open local real port only when connection is established – this option is available when you create a connection on a real telnet RS232 port. You can maintain this port’s availability for use by local applications when it is not connected to a remote machine. Serial to Ethernet Connector
    2. Connect to remote end only when local virtual port is open – this setting is used when the connection uses a virtual telnet serial port. When enabled, the local COM port must be opened by an application before any connection attempts are made.
    3. On error retry to establish connection every – when this option is chosen, retries of failed Telnet to COM connection attempts will be made based on a specified time interval.

    Network protocol settings:

    • ★ Raw data transmission protocol: Use of this transmission mode enable the establishment of a raw TCP session. All data is subsequently sent with no protocol-specific or application layer formatting.

    • ★ Use Telnet protocol (RFC 2217):Telnet with RFC 2217 extensions allows server and client to exchange the telnet ip port configuration information. The remote end can be notified of changes made to the settings or the state of the local serial port.

    • ★ RFC 2217 mode was originally designed to be used with modems, but can also be used for serial printers, scanners, laboratory monitoring devices, and office equipment.

    Serial to Ethernet Connector allows you to forward any serial port device to Telnet simply and easily when using RFC2217. Use the instructions above and you can soon be working with serial devices over the network.