The adapter contains the necessary crosslinks between the signals. Below is a very common wiring diagram for a null modem cable to interconnect two DTEs e. The original application of a null modem was to connect two teleprinter terminals directly without using modems.
As the RS standard was adopted by other types of equipment, designers needed to decide whether their devices would have DTE-like or DCE-like interfaces. When an application required that two DTEs or two DCEs needed to communicate with each other, then a null modem was necessary. Null modems were commonly used for file transfer between computers, or remote operation. Under the Microsoft Windowsoperating system, the direct cable connection can be used over a null modem connection. Both pieces of software allow the mapping of a hard disk on one computer as a network drive on the other computer.
No Ethernet hardware such as a network interface card or a modem is required for this. The popularity and availability of faster information exchange systems such as Ethernet made the use of null modem cables less common. In modern systems, such a cable can still be useful for kernel mode development, since it allows the user to remotely debug a kernel with a minimum of device drivers and code a serial driver mainly consists of two FIFO buffers and an interrupt service routine. This can also provide a serial console through which the in-kernel debugger can be dropped to in case of kernel panics, in which case the local monitor and keyboard may not be usable anymore the GUI reserves those resources and dropping to the debugger in the case of a panic won't free them.
Another context where these cables can be useful is when administering 'headless' devices providing a serial administration console i. An example of embedded systems that widely use null modems for remote monitoring include RTUs, device controllers, and smart sensing devices.
These devices tend to reside in close proximity and lend themselves to short run serial communication through protocols such as DNP3, Modbus, and other IEC variants. The Electric, Oil, Gas, and Water Utilities are slow to respond to newer networking technologies which may be due to large investments in capital equipment that has useful service life measured in decades. Serial ports and null modem cables are still widely used in these industries with Ethernet just slowly becoming a widely available option.
This can be done with a separate device and two cables, or using a cable wired to do this. One feature of the Yost standard is that a null modem cable is a 'rollover cable' that just reverses pins 1 through 8 on one end to 8 through 1 on the other end. The simplest type of serial cable has no hardware handshaking.
The teletypes communicated to each other through the modems. This asymmetric connection using the RS required a modem on one side and a data source or consumer at the other end.
Null-modem connection are not standardized, allowing for multiple possible wiring configurations. In this type of connection, the transmission and reception lines are connected crosswise, enabling two-way data transmission. Today, null modems are used primarily to exchange data between older computers or laptops.
These machines may not have a USB port or network card. For these types of machines, a null modem is the only option when data transmission is required. As we have seen, null-modem connections are made possible through the use of a null mode cable. This is the simplest solution to connecting the two computers. But what is a null modem cable? The cable is comprised of three lines.
One wire serves as the signal ground with the other two lines fostering the transmitting and receiving of your data. Based on the software used in this connection, some kind of authenticating handshake may be necessary. RS cable wiring scheme by Microsoft has become the industry standard, using a cable made up of seven wires.
As we stated previously, a null-modem cable connection will not provide high-speed data transfer. Using a null modem emulator allows effective and higher-speed data transfer using your null modem. This is a software application that connects COM ports by way of a virtual null modem cable.
Virtual Null Modem software allows you to create pairs of virtual serial ports. 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. As with the loop back null modem cable, these signals are not connected to the other device, but directly looped back on the same connector. Only when the software at the other side checks the CD signal at pin 1, the RTS information will reach the other device. This would however be only the case in specifically developed software which uses the CD input for this purpose. By cross connecting these lines, their original function is simulated pretty well.
The DTR output is used to signal the other device that communication is possible. This information is read on the DSR input, the same input used for this purpose with modem communication. Because of this cross connection, the DTR output line can be used for simple flow control. Incomming data is allowed when the output is set, and blocked if the output is not set. Most software however will also check the DSR line and in that case—when using the null modem cable with partial handshaking—the best possible hardware flow control can be achieved which is still compatible with the original use with modems.
The most expensive null modem cable is the null modem cable suitable for full handshaking. In this null modem cable, seven wires are present. Only the ring indicator RI and carrier detect CD signal are not linked. The cable is shown in the following figure. The null modem cable with full handshaking does not permit the older way of flow control to take place.
When the full handshaking null modem cable is used, there is no request anymore. The lines are purely used for telling the other side if communication is possible.
The main advantage of this cable is, that there are two signalling lines in each direction. This makes it possible to achieve very high communication speeds with this type of null modem cable, provided that the software has been designed for it. This is the type of cable Microsoft recommends for the direct cable connection in their knowledge base article. The right null modem cable to choose mainly depends on the application and the software that will be used.
As a general guide line, I would advise the following. The null modem cable with partial handshaking works in most cases. If you are developing software which must work with all kinds of cables, it is best to use software flow control only and ignore all modem control inputs. Home Interfacing Software About Sitemap. EN NL. About the Author: Lammert Bies is a dad, husband and polyglot.
Simple null modem without handshaking.
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