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. Null modem cable DB9-DB9. Pinout and signals for building a serial RS nullmodem cable.
Crossover or "Null Modem" vs. Set your store and be able to check inventory and pick up at your local store. A simple but useful mechanism allowing flow control in one direction. In the past, this was true. Modem speeds of baud were common and baud was seen as a high speed connection. For further control of the information flow, both devices have the ability to signal their status to the other side. These signals are in one direction only. It is not used directly for flow control, but mainly an indication of the ability of the modem device to communicate with its counter part.
This signal indicates the existence of a communication link between two modem devices. How to use the handshaking lines in a null modem configuration? In that situation, only the data lines and signal ground are cross connected in the null modem communication cable. All other pins have no connection. An example of such a null modem cable without handshaking can be seen in the figure below.
If you read about null modems, this three wire null modem cable is often talked about. Yes, it is simple but can we use it in all circumstances? These signals normally define the ability of the other side to communicate.
As they are not connected, their signal level will never go high. This might cause a problem. If the software on both sides is well structured, the RTS output is set high and then a waiting cycle is started until a ready signal is received on the CTS line. This causes the software to hang because no physical connection is present to either CTS line to make this possible. This does however not mean, that this null modem cable is useless.
Communication links like present in the Norton Commander program can use this null modem cable. This null modem cable can also be used when communicating with devices which do not have modem control signals like electronic measuring equipment etc. As you can imagine, with this simple null modem cable no hardware flow control can be implemented.
The simple null modem cable without handshaking shows incompatibilities with common software. The simplest type of serial cable has no hardware handshaking. This cable has only the data and signal ground wires connected.
All of the other pins have no connection. With this type of cable flow control has to be implemented in the software. Because of the compatibility issues and potential problems with a simple null modem cable, a solution was developed to trick the software into thinking there was handshaking available.
However, the cable pin out merely loops back, and does not physically support the hardware flow control. This cable could be used with more software but it had no actual enhancements over its predecessor.
The software would work thinking it had hardware flow control but could suddenly stop when higher speeds were reached and with no identifiable reason. In this cable the flow control lines are still looped back to the device. The only way the flow control signal would reach the other device is if the opposite device checked for a Carrier Detect CD signal at pin 1 on a DE-9 cable and pin 8 on a DB cable. As a result, only specially designed software could make use of this partial handshaking.
Software flow control still worked with this cable. With suitable software, the cable is capable of much higher speeds than its predecessors. It also supports software flow control. A virtual null modem is a communication method to connect two computer applications directly using a virtual serial port.
Unlike a null modem cable, a virtual null modem is a software solution which emulates a hardware null modem within the computer. There are some advantages to this:.
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