Null modem testen

Start the program and open the " Pair " window. There you'll see the ' Add pair ' option. Now, you can either select the virtual serial ports from the dropdown list or specify them manually.

What is especially convenient is that you can assign any custom names to the ports you create with Virtual Serial Port Driver except for those port names already used in your system.

Just make sure that the apps that will work with these COM ports support the port names you've chosen. Click "Create" and that's it. The number of virtual COM port pairs you can create with the software is not limited. COM port testing: troubleshooting guide. How to test a serial port with HyperTerminal One of the possible workarounds is using a null-modem cable, which allows connecting one program, e.

After that, open the File Menu and select 'New Connection'. Enter a name, e. Make sure the properties are kept default and press OK. Next step is creating a connection to the other port - COM4. Keep the properties default and press the 'OK' button. After the program is started, you will want to establish a new connection to a serial interface.

They need to be identical to the connection settings of the serial port that will be accessed remotely. After communication is established, the serial port needs to be opened using one of these three options. You need at least one unoccupied serial port in order to open it for use with the app.

This means no other program can be using the interface when the connection is established. Once the port is opened, the connection can be used to send data in text, binary, octal, decimal or hexadecimal format. 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 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. Virtual null modem emulator. What is a null modem? 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. If a reference points to null , then no value is associated with it. On the other hand, a value is, by definition, the value itself. There is no pointer involved. A value type is stored as the value itself.

Therefore the concept of null doesn't exist for value types. The following picture demonstrates the difference. On the left side you can see again the memory in case of variable name being a reference pointing to "Bob".

The right side shows the memory in case of variable name being a value type. As we can see, in case of a value type, the value itself is directly stored at the address A0A1 which is associated with variable name. There would be much more to say about reference versus value types, but this is out of the scope of this article.

Please note also that some programming languages support only reference types, others support only value types, and some e. C and Java support both of them. The concept of null exists only for reference types. It doesn't exist for value types. Suppose we have a type person with a field emailAddress. Suppose also that, for a given person which we will call Alice, emailAddress points to null.

As we have seen already, what we can assert is that no value is associated with emailAddress. But why is there no value? What is the reason of emailAddress pointing to null?

If we don't know the context and history, then we can only speculate. The reason for null could be:. In practice we often know the application and context.

We intuitively associate a precise meaning to null. In a simple and flawless world, null would simply mean that Alice actually doesn't have an email address. When we write code, the reason why a reference points to null is often irrelevant. We just check for null and take appropriate actions. For example, suppose that we have to write a loop that sends emails for a list of persons. The code in Java could look like this:. We just acknowledge the fact that there is no email address, log a warning, and continue.

If a reference points to null then it always means that there is no value associated with it. In most cases, null has a more specific meaning that depends on the context. In this case, returning null or an empty list is ambiguous. Does it mean that the patient doesn't have allergies, or does it mean that an allergy test has not yet been performed?

These are two semantically very different cases that must be handled differently. Or else the outcome might be life-threatening. Just suppose that the patient has allergies, but an allergy test has not yet been done and the software tells the doctor that 'there are no allergies'.

Hence we need additional information. We need to know why the function returns null. It would be tempting to say: Well, to differentiate, we return null if an allergy test has not yet been performed, and we return an empty list if there are no allergies. The different semantics for returning null versus returning an empty list would need to be well documented. And as we all know, comments can be wrong i.

There is no protection for misuses in client code that calls the function. For example, the following code is wrong, but it compiles without errors. Moreover, the error is difficult to spot for a human reader.

And there is no protection against forgetting any one of these changes.