Feedback will be sent to Microsoft: By pressing the submit button, your feedback will be used to improve Microsoft products and services. Privacy policy. This topic describes how to use My. Ports to send strings to the computer's serial ports in Visual Basic. This example sends a string to the COM1 serial port. You may need to use a different serial port on your computer.
I have interfaced a microcontroller board MSPG on Launch Pad to the serial port using a null modem cable like this. The Controller waits for a character to be received and lights up the corresponding LED. The code for MSP is included in the zip file. Privacy Policy. All rights reserved. All trademarks and service marks are the properties of their respective owners. Skip to main content. Search form Search.
Request new password. Net for Beginners. Serial Port Programming using Visual Basic. Submitted by Rahul. Sreedharan on 25 March - am. Hello folks, Visual Basic. Press OK Now your file name under your Solution Explorer as well as the Module name on the Source file will be changed to the one specified by you here SerialCommWrite Now you can add your code for controlling the serialport under Sub Main and start Programming.
Adding Files to Your Project and Compiling Instead of typing your own code under the Sub Main you want to add the downloaded source files from github directly under your Project Tree. Imports System 'To Access Console. WriteLine Imports System. GetPortNames Console. Note the use of enumeration to specify the parity and the number of stop bits. The keyword "WithEvents" makes it possible for subroutines to subscribe to events from this object COMPort by means of the keyword "Handles". This is described later in the chapter "Event Keywords".
For simple VB data types you don't need the keyword "New". The object is build when you assign a value to it, but not before! Net, all strings are immutable, meaning you cannot change its length or contents once it is declared.
Therefore, all string manipulations has to be done by making a new instance of the String class, copying the string s and then abandon the previous string.
This is a very time consuming procedure, which also leads to a serious fragmentation of the memory. The immutable strings is actually one of the less clever things of. A string declaration could easily have a maximum length like arrays, but unfortunately this is not the way it is done. NET, all applications and services are build by means of one or more Forms.
A form may be visible in case of applications or invisible in case of services. Controls are used in the designer to create the look of the UI. A control is an object that has a predefined appearance and behavior. For example, a Button control looks and behaves like a push button. Each control in VB has its own purpose. For example, a TextBox control is used for entering and showing text, while a PictureBox control is used for displaying pictures.
When designing your UI, you drag controls from the Toolbox, drop them onto a form, and then position them and resize them to create the desired look. You can further change the name, appearance and behavior of forms and controls by setting properties in the Properties window. For example, the ShowInTaskbar property of a form determines if the form will appear in the Windows taskbar when the program is running.
You can also make your own controls. The object Inheritance hierarchy of forms and controls is shown below: System. Object System. MarshalByRefObject System. Component System. Control System. ScrollableControl System. ContainerControl System. Form Derived Classes You can forget all classes except for controls and forms, but it is important to notice that forms are actually just an advanced control, so basically everything is just windows on top of windows or even more correct - controls on top of controls.
In the following "window" means form or control and a "form" is a top-level window. Because all controls are derived from System. Control, they all have at least the same basic properties, methods and events. Multitasking and Multithreading. Windows is capable of keeping many programs in memory at once and give each of them an opportunity to run.
This is called multitasking. Windows NT, and XP runs each program in a separate process. This is an artificial division that gives the programs mutual isolation so that no problem can indirectly affect the operation of other programs. The process is started when the program starts and exists for as long as the program is running.
Each process manifest itselves as an application or a service. The difference between applications and services is that services don't usually have a user interface in the following just UI , whereas applications do.
Windows is able to run more parts threads of the various processes virtually simultaneously. This is called multithreading. A thread is in effect an execution pointer, which allows Windows to keep track of which line of your program is running at any one time.
This pointer starts at the top of the program and moves through each line, branching and looping when it comes across decisions and loops and, at a time when the program is no longer needed, the pointer steps outside of the program code and the program is effectively stopped.
Application software uses more threads primarily to deliver a better user experience, because it allows some programs like spell checkers, print spoolers etc. Service software uses more threads to deliver scalability and improve the service offered, for example to be able to handle more channels in parallel.
Windows uses timeslicing to switch between the various threads. Because the execution of a thread is interrupted due to an external factor such as time-slicing, this is called preemptive multithreading. The run-time each thread gets before a new thread is selected is not fixed, but depends on the priority and varies dynamically to obtain the best possible performance. The run-time for each thread does not depend on the process, so a process with many threads will get more execution time than a process with only a few threads.
To keep the UI responsive, all UI threads described below have higher priority than for example thread pool threads and most background threads. A high priority thread always preempts a lower priority thread, but to prevent low priority threads from getting starved and solve nasty priority-inversion problems, the scheduler continuously adjusts the actual priority from the programmed priority.
The advantage of timeslicing is that the processes don't have to actively participate in this process and that it is not possible for one process to block the operation of other processes.
For example, a timeslice system may waist a lot of time on threads, which cannot utilize the full timeslot, but even worse - there is no guarantee that a subroutine or function has finished the use of some data before another part of the program can change these data.
Therefore, a subroutine of function cannot just be called with a reference to the data ByRef , but must be called with its own copy of the data ByVal. All this data copying takes a lot of time and increases the necessary stack size. It may even lead to a serious fragmentation of the memory if the data cannot be located on the stack.
Therefore, preemptive multitasking is usually used in the PC world where there is a lot of memory and a very fast CPU and where programs from many different vendors must work together, and cooperative multitasking is used in the embedded world where speed and efficiency is a primary concern. No process can write directly to the data area of another process, but threads within the same process may share the same global variables.
However, more threads should of course not write to the same data area at the same time as this may create unpredictable results. In a time-sliced system there is also a risk that a method called from one thread does not finish its job before it is called again from another thread.
To prevent this,. NET unlike previous versions of VB simply do not allow such a cross thread call except for four so-called thread safe methods - Invoke, BeginInvoke, EndInvoke and CreateGraphics - plus some hiden methods to set some properties.
A piece of code is thread-safe if it functions correctly during simultaneous execution by multiple threads. In particular, it must satisfy the need for multiple threads to access the same shared data, and the need for a shared piece of data to be accessed by only one thread at any given time. Because shared data must be located somewhere and everything is running on a thread, you may also say that a method is thread safe if it is able to safely use data area shared data of another thread.
It is not defined anywhere which properties you are allowed to set from another thread, but it is probably the ones, which may be contained in a single bit value like BackColor , since there is now way such a property can be half updated on a bit system. One of the cardinal rules of Windows GUI programming is that only the thread that created a control like for example a TextBox can access its methods or should modify its contents except for simple properties like for example the background color etc.
Try doing it from any other thread and you'll get unpredictable behavior ranging from deadlock, to exceptions to a half updated UI. It is very important to notice that all controls and forms have the four thread safe methods see last chapter , so if you for example want to call BeginInvoke to invoke a method on the UI thread, you can use any of your controls on the UI thread like for example TextBox1. Every application and service has one primary thread - Main.
This thread serves as the main process thread through the application or service. It's this thread that calls the Main method, which is the first code to run when your application or service has been loaded.
Main serves as the starting point and overall control for your application or service. A file that runs on its own usually with extension.
For a console application, you can write Main to accept user input and process them. All windows forms or controls belong to the thread on which they are created. This thread is called the UI thread and it is usually the main thread. Threads are a limited resource. They take approximately , cycles to create and about , cycles to destroy.
By default they reserve 1 megabyte of virtual memory for its stack and use 2,, cycles for each context switch. It is possible to make and control threads yourself, but usually it is neither necessary nor appropriate due to the huge use of resources. Windows has a pool of 25 general useable threads called thread pool threads, which may be used instead for background jobs. The pool is generated the first time it is used. You may regard this pool as a company with 25 workers, which you can hire to do a job for you.
If you request a thread from this pool and no thread is available, Windows generates a new pool thread up to a total of threads. Requests beyond that limit is queued until a thread becomes available.
The extra threads are destroyed as soon as they are no longer needed until the number of threads in the pool reaches the minimum level of Because it takes approximately , cycles to create and destroy a thread, it is obvious that you should try to avoid that more thread pool threads than 25 are needed.
It is possible to stop a thread temporary by means of System. Sleep time. The time is specified in milliseconds, but the accuracy is much lower. If you for example try to make a 1 mS delay by means of Sleep 1 , the thread is temporary suspended. Even if there are no waiting threads, it may take up to 15 mS - the next time slice tick - before the thread runs again.
Therefore, you won't be sure when you get control back, so Sleep cannot be used for accurate timing. Sleep 0 can be useful to tell the system that you want to forfeit the rest of the thread's timeslice and let another, waiting, thread run. If there are no other threads waiting to run you still relinquish your timeslice.
This is similar to the typical Release call in a cooperative multitasking system. Sleep 0 may be very useful in a game loop. Sleep 1 is similar, but keeps the CPU cool.
Sleep 1 is very useful in soft real-time systems. You'll need this when you have to rely on polling to discover anything happening in the system. This is very common with imperfect drivers, which can't generate an event. If Sleep is called from the UI thread, the user interface will be completely dead for the specified amount of time.
Therefore, calling Sleep from the UI thread is not recommended except to solve "don't know enough about the state of the object" problems. For example, the SerialPort class has a nasty problem where you can't open a port too quickly after closing it.
There is a background thread, which needs to exit before you can successfully open the port again. This is a flaw in the class design. Sleep can solve this, but the solution is not perfect. The exact amount of time you need to be sure the thread exits the amount of time needed before the background thread gets scheduled again depends on system load, but because all time consuming, high priority UI jobs are temporary suspended, Sleep is usually enough although Sleep is recommended for high reliability applications.
If you don't sleep the UI thread, but for example use a timer instead, you must use a considerably longer time to ensure that the background thread gets enough time to close down - for example 2 seconds! It is possible to interrupt a thread by means of a so-called interrupt. When you make any action such as to click a mouse button, move the mouse, type on the keyboard, resize or move a window etc.
This start a long, rather complicated chain of actions. If you are a beginner, just skip this chapter. It will just confuse you. The interrupt signal is received in an integrated circuit called the interrupt controller. The controller gives each interrupt request IRQ its own priority so that the interrupt with the highest priority can be executed first in case there are more interrupts.
Each interrupt has an interrupt vector, which points to the interrupt routine, which shall be executed when the interrupt becomes the one with the highest priority. On most PC's there are only 15 hardware interrupts, which is by far not enough for all devices, so it may be necessary for many devices to share the same interrupt. Therefore, the interrupt vector does not point directly to the interrupt service routine ISR.
Instead, each device has an interrupt object with a dispatch code. The interrupt vector points to this code. The dispatch code is generated when a device driver is loaded. If more device drivers want to use the same hardware interrupt, that is, same interrupt vector, the interrupt objects are linked together in a chain. Windows does not depend entirely on the hardware interrupt priority. It has its own priority system with 31 interrupt request levels IRQL. Windows uses IRQL's for implementing a priority mechanism similar to the one in the interrupt controller.
If IRQL is 0, all interrupts are enabled. The dispatch code in the interrupt object calls KiInterruptDispatch if there is only one interrupt object for this vector. If there are more objects it calls KiChainedDispatch. KiInterruptDispatch or KiChainedDispatch raises the IRQL to the level specified in the interrupt object to block for all other interrupts with the same or lower priority. The ISR reads short information like status etc.
The ISR acknowledge the interrupt. The ISR is not allowed to do any time consuming actions. APC routines are used to execute some code asynchronous in the context address space of a particular thread. Therefore each thread has its own APC queues - one for kernel mode and one for user mode.
This will course execution of the APC routines in the right address space. By means of APC routines, one thread can execute code in the address space of another thread - even a thread from another process.
A callback method is not executed immediately on the thread from which it is called, but delayed and executed on the thread, which shall receive the result. Thread switching is also done by means of a DPC routine. All thread synchronization is therefore done at dispatch level IRQL 2 , so no other thread related jobs must take place at this or higher levels.
Instead it must post an APC routine, which can do the job. In practice, interrupt handling is even more complicated than described here - especially on a multi processor system, but these are the basic principles on a bit system with all minor details omitted. On a bit system, the interrupt system is similar, but with some modifications. This is important when working with serial port because you must have a hardware FIFO buffer First In, First Out , which can hold all characters you receive until the DPC routine in the driver has transfered these data to a software buffer in the driver.
As this may easily take mS, the software buffer in the driver should be at least times as big as the hardware buffer. The interrupt system DPC routine is not allowed to perform very time consuming or thread related operations, so all further processing must be done by non-interrupt routines.
There are two ways to do this. Each event can be executed on its own thread. This is usually done by means of a background thread, which waits for one or more events. It can then grap a thread from the thread pool and use this for further processing of the various events so that the background thread immediately gets ready for the next event. This is the method used in SerialPort. More events can be executed on the same thread. This is the method used by the UI. All UI related actions gets translated to a message.
It is possible to post up to 10, messages to one queue. This ensures that a window receives its input messages in the proper first in, first out FIFO sequence. The routine that reads from the message queue is called the message pump.
The loop then performs any message translations by means of TranslateMessage. These virtual-key messages contain a code that identifies which key was pressed or released, but not its character value, which might depend on the state of the Shift or Alt key or the character Language. At last, the message pump calls DispatchMessage, which calls the message handling procedure - WndProc - of the control window specified in the message.
Note that in case of one of the four thread safe methods - Invoke, BeginInvoke, EndInvoke and CreateGraphics, it is WhdProc of the control you have used in the call, which will receive the message - for example Button2.
WndProc in case of Button2. BeginInvoke - even though Button2 may have absolutely nothing to do with the message! Message Select Case m. NET 1. This means that if a program keeps sending such messages to the message queue, the window will not react on e. Therefore it is very important not to overload the message queue with these kind of messages!
Note that both events are raised before the form is closed so that it is possible to cancel the operation instance. When the associated method like OnClick or OnPaint has done whatever it has to do, it must call any other methods, which shall also be executed when this event occurs.
These methods are called event handlers, and the method, which executes the event handlers, is RaiseEvent. It is a very confusing name because it makes you believe that it raises a new event like raising a flag or an interrupt. It doesn't. It just execute some event handlers from a list, but it is a convenient way of describing things. You write some event handlers and when you want to execute these, you just "raise the event". The event has a name similar to the message and the associated method except that no prefix is used like for example just Click.
When the method and any event handler s have finished their jobs, DispatchMessage returns and the loop calls GetMessage again to wait for the next message. Let us take an example. If the user moves or resizes a window, some part of the window - a rectangle, a region or maybe the whole window - needs to be redrawn.
The method, which has moved or resized the windows, tells which part it is by calling Invalidate. However, there may be other UI events, which has occured before the window has been changed and also needs to be executed, so the redraw is not done immediately.
If you for example has created graphics with CreateGraphics, this graphics may need to be redrawn so you must make a handler for the Paint event, which can do this. The OnPaint method and any other methods associated with a message are overridable so that they may be changed if you derive a class from another.
All applications or services, which use ChildClass, can then subscribe to the Paint event of this class if they have any graphics to redraw in case the window is invalidated. A typical GUI application, after doing some initialization in the Main method, will then start running the message pump. Because the message pump is an infinite loop it will run until the application closes, so the Main thread can't do anything else once the message pump starts.
The System. Run method takes care of message pumping. If no form is specified, it begins running a standard application message loop on the current thread usually the UI-thread for the application or service.
If a form is specified, it also makes the specified form visible like: Shared Sub Main ' Called from the primary thread Application. EnableVisualStyles Application. Run New Form1 ' Start the application and make Form1 visible End Sub In case of an application, a form is usually specified or else there is no UI the user can click, but it is also possible to show the form later by means of NameOfForm.
Show if an event to do this is available. If you want to hide a form, you can call NameOfForm. This will set the Visible property to false. If you want to close a form, you can call NameOfForm. Each application can have many UI-threads, which again can have many windows, but the thread that creates a window must also be the thread that handles all messages send to that window. All the controls in a single window must be created on that windows UI thread, but if you have multiple top-level windows, you can have multiple UI threads - as many as you have windows if you really want.
It has the advantage that if one window is blocked the others may still be functional. Stack Overflow for Teams — Collaborate and share knowledge with a private group.
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