Null modem necessary direct data link

Null modem cable: RS null modem pinout and wiring As we have seen, null-modem connections are made possible through the use of a null mode cable. Below are the most common schemes of null modem cables. Virtual null modem emulator As we stated previously, a null-modem cable connection will not provide high-speed data transfer.

RS null modem pinout and wiring. Get a download link for your desktop Submit your email address to get a link for quick download on your desktop and get started! The download link was sent to you, check your email. Latest version Standard. Latest version Standard: v Category: Serial Port Software. Demo limitations. Demo limitations: day fully-functional trial. Languages: English.

Upgrade Discount. With serial cables now considered obsolete, the odds of running into one of the other types is practically non-existent unless you happen to be using very old, very specific equipment. A standard pinout featuring the two most common null modem cables, DB9 and DB Null modem cables are still somewhat common today, having been used on many machines for a few decades. Along with full-fledged cables, there are also null modem adapters that can be used to change a straight-thru cable into null modem.

When using an adapter, make sure to only put a null modem adapter on one end. The adapters change the pin configuration from straight-thru to null modem. So if null modem adapters are installed on both sides, the pins will line up like a straight-thru cable again. One downside of null modem cables is their limited distance. AT cables can easily cover distances of up to feet while null modem cables have a maximum distance of feet.

This foot limit also applies when using a null modem adapter with a serial cable. Internet Explorer will soon be unsupported on this site. Please upgrade to a newer browser to continue using our site. Posted: April 04, Categories: Serial , null modem. Depending on the type of the software used, some kind of handshaking may also be required.

This type of cable includes seven wires, and this RS wiring scheme has essentially become standard. As we already mentioned, a null-modem cable is not designed to provide high data transfer speed. In practice, however, this speed is even lower, since it depends heavily on the length of the cable.

To make your null modem data exchange faster and more efficient, you may use null-modem emulator - the software that is able to connect COM ports with a virtual null modem cable. Pairs of virtual serial ports created with Virtual COM Port Driver can communicate data in such a way that sent to one COM port, serial data is immediately available in the other port and vice versa. A virtual cable used by the virtual null modem software is a good alternative to a physical one.

More than that, the app makes it easy to emulate the break of a serial line. Electronic Team, Inc. By continuing to use this site, you agree to our cookie policy. Click here to learn more. The simple null modem cable without handshaking shows incompatibilities with common software. The main problem with this cable is that there is a possibility for the software to hang if it checks the modem signal lines in a proper way. To overcome this problem and still be able to use a cheap null modem communication cable with only three lines in it, a fake null modem cable layout has been defined.

The null modem cable with loop back handshaking resulted from this. The main purpose of this null modem cable is to let well defined software think there is handshaking available, with a null modem cable which has no provisions for it. Consider first the DSR signal pin 6. This input indicates that the other side is ready to start communicating. In the layout, the line is linked back to the DTR output pin 4. The same holds for the CD input pin 1. The assumption is, that if software has been written to check the DSR line to test communication availability, it will probably also set the DTR output to indicate its own state.

The same trick is used with the CTS input. There is no possibility of a software hangup because of dangling RTS requests. The null modem cable with loop back handshaking is often advised as the best low cost available null modem cable.

But, is it really so good? The simple null modem cable without handshaking has the disadvantage that it does not permit proper written software to communicate with it.

Software which is aware of the lack of handshaking signals can however use it without problems. The null modem cable with loop back handshaking can be used with more software, but it has no functional enhancements over the simple cable! If the software is designed for using hardware flow control it seems to work with this null modem cable, but on unpredictable moments, data loss may occur. This means that the null modem cable allows communication as long as no flow control is needed, but when data speeds reach the limit the receivers can handle, communication may stop immediately without an assignable reason.

Therefore, although this null modem cable is cheap and easy to make, use it with care! The simple null modem cable and the null modem cable with loop back handshaking are useful, but have no provisions for hardware flow control. If it is absolutely necessary that hardware flow control is used, the null modem with partial handshaking can be an alternative. This null modem cable is the best of two worlds.