To check whether or not an RS-232 serial port is working, perform an RS-232 loopback test by doing the following:
- May 13, 2010 Software Product Description This application runs a loopback test of the selected serial port. The user must simply connect the TX and RX signal lines and then execute the test.
- Dec 06, 2016 Serial Port Debugging 101: Loopback Test. Necessary Software. Get unlimited access to the best stories on Medium — and support writers while you’re at it. Just $5/month.
- For example the Serial Ports panel will then show you the status of each serial port. Alternately you can get this serail port status information from the command line by typing pmshell –signals –l port1 Hardware loopback testing the ports Each console server has Loopback Test facilities built in.
- The optional set of parallel port and serial port signal loopback TEST Plugs (one 25-pin parallel, one 9-pin serial & one 25-pin serial) are used with the 'external' parallel port and serial port tests. The specific tests are noted in Tests &.
- Oct 09, 2013 Soft loopback testing is accomplished with a set of Cisco IOS ® software configuration commands on the Cisco gateway. The commands cause the WAN interface card (WIC) driver to automatically send traffic back towards the sending T1/E1 port. Soft loopback does not require any hardware changes or re-configuration, as shown in this figure.
To check whether or not an RS-232 serial port is working, perform an RS-232 loopback test by doing the following: If your serial port is not female, convert it by taking a female/female cable or gender changer and plugging it into the serial port. Take a metal paperclip or wire and cross pins 2 and 3.
- If your serial port is not female, convert it by taking a female/female cable or gender changer and plugging it into the serial port.
- Take a metal paperclip or wire and cross pins 2 and 3. If you look closely at the female end, the pins should be numbered.
- Open a telnet session on the COM port number of the device that you are testing. To find out the COM port number, refer to the following FAQ: https://www.startech.com/faq/com-port-listing-windows.
Note: To open a telnet session on the COM port, you need a telnet client like PuTTY or Hyper Terminal. Windows XP comes with Hyper Terminal.
- When the session is open, anything you type into it you should see. The loopback test fails when you cannot see what you are typing.
If the loopback test fails, make sure that the serial cable or gender changer that you are using works and that the adapter is in the correct port.
You can check multiple ports at the same time by opening multiple sessions, putting the loopback adapter on one port, and trying to type into each session. When you can see what you are typing, you know that the COM port is working and you can see which port number the physical serial port is. Close the window for the port that you just tested to speed up the testing of the remaining serial ports.
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(Redirected from Loopback interface)
Loopback, or loop-back, refers to the routing of electronic signals, digital data streams, or flows of items back to their source without intentional processing or modification. This is primarily a means of testing the communications infrastructure.
Many example applications exist. It may be a communication channel with only one communication endpoint. Any message transmitted by such a channel is immediately and only received by that same channel. In telecommunications, loopback devices perform transmission tests of access lines from the serving switching center, which usually does not require the assistance of personnel at the served terminal. Loop around is a method of testing between stations that are not necessarily adjacent, wherein two lines are used, with the test being done at one station and the two lines are interconnected at the distant station. A patch cable may also function as loopback, when applied manually or automatically, remotely or locally, facilitating a loop-back test.
Where a system (such as a modem) involves round-trip analog-to-digital processing, a distinction is made between analog loopback, where the analog signal is looped back directly, and digital loopback, where the signal is processed in the digital domain before being re-converted to an analog signal and returned to the source.
- 3Virtual loopback interface
Telecommunications[edit]
In telecommunications, loopback, or a loop, is a hardware or software method which feeds a received signal or data back to the sender. It is used as an aid in debugging physical connection problems. As a test, many data communication devices can be configured to send specific patterns (such as all ones) on an interface and can detect the reception of this signal on the same port. This is called a loopback test and can be performed within a modem or transceiver by connecting its output to its own input. A circuit between two points in different locations may be tested by applying a test signal on the circuit in one location, and having the network device at the other location send a signal back through the circuit. If this device receives its own signal back, this proves that the circuit is functioning.
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A hardware loop is a simple device that physically connects the receiver channel to the transmitter channel. In the case of a network termination connector such as X.21, this is typically done by simply connecting the pins together in the connector. Media such as optical fiber or coaxial cable, which have separate transmit and receive connectors, can simply be looped together with a single strand of the appropriate medium.
A modem can be configured to loop incoming signals from either the remote modem or the local terminal. This is referred to as loopback or software loop. Intel motherboard manual download.
Serial interfaces[edit]
A serial communications transceiver can use loopback for testing its functionality. For example, a device's transmit pin connected to its receive pin will result in the device receiving exactly what it transmits. Moving this looping connection to the remote end of a cable adds the cable to this test. Moving it to the far end of a modem link extends the test further. This is a common troubleshooting technique and is often combined with a specialized test device that sends specific patterns and counts any errors that come back (see Bit Error Rate Test). Some devices include built-in loopback capability.
A simple serial interface loopback test, called paperclip test, is sometimes used to identify serial ports of a computer and verify operation. It utilizes a terminal emulator application to send characters, with flow control set to off, to the serial port and receive the same back. For this purpose, a paperclip is used to short pin 2 to pin 3 (the receive and transmit pins) on a standard RS-232 interface using D-subminiature DE-9 or DB-25 connectors.
Virtual loopback interface[edit]
Implementations of the Internet Protocol Suite include a virtual network interface through which network applications can communicate when executing on the same machine. It is implemented entirely within the operating system's networking software and passes no packets to any network interface controller. Any traffic that a computer program sends to a loopback IP address is simply and immediately passed back up the network software stack as if it had been received from another device.
Unix-like systems usually name this loopback interface
lo
or lo0
.Various Internet Engineering Task Force (IETF) standards reserve the IPv4 address block 127.0.0.0/8, in CIDR notation and the IPv6 address ::1/128 for this purpose. The most common IPv4 address used is 127.0.0.1. Commonly these loopback addresses are mapped to the hostnames, localhost or loopback.
MPLS[edit]
One notable exception to the use of the 127.0.0.0/8 network addresses is their use in Multiprotocol Label Switching (MPLS) traceroute error detection, in which their property of not being routable provides a convenient means to avoid delivery of faulty packets to end users.
Martian packets[edit]
Any IP datagram with a source or destination address set to a loopback address must not appear outside of a computing system, or be routed by any routing device. Packets received on an interface with a loopback destination address must be dropped. Such packets are sometimes referred to as Martian packets.[1] As with other bogus packets, they may be malicious and any problems they might cause can be avoided by applying bogon filtering.
Management interface[edit]
Some computer network equipment use the term 'loopback' for a virtual interface used for management purposes. Unlike a proper loopback interface, this type of loopback device is not used to talk with itself.
Such an interface is assigned an address that can be accessed from management equipment over a network but is not assigned to any of the physical interfaces on the device. Such a loopback device is also used for management datagrams, such as alarms, originating from the equipment. The property that makes this virtual interface special is that applications that use it will send or receive traffic using the address assigned to the virtual interface as opposed to the address on the physical interface through which the traffic passes.
Loopback interfaces of this sort are often used in the operation of routing protocols, because they have the useful property that, unlike real physical interfaces, they will not go down when a physical port fails.
Other applications[edit]
Serial Port Loopback Test Software Download
![Loopback Loopback](/uploads/1/2/4/8/124856644/264876500.jpg)
The audio systems Open Sound System (OSS), Advanced Linux Sound Architecture (ALSA) and PulseAudio have loopback modules for recording the audio output of applications for testing purposes. Unlike physical loopbacks, this does not involve double analog/digital conversion and no disruption is caused by hardware malfunctions.
See also[edit]
References[edit]
This article incorporates public domain material from the General Services Administration document 'Federal Standard 1037C' (in support of MIL-STD-188).
Loopback Tester
External links[edit]
Serial Loopback Test
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