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Archipp Zakharov
Archipp Zakharov

Data Reading From Serial Port Forms 6i _VERIFIED_

I've been assigned to display weight from weighing scale (CAS CI-201A) into a textbox using C#. The weight will be sent via serial port RS-232 or USB converter. The scale is with me but I don't know where to start. How can I achieve my goal?

Data Reading From Serial Port Forms 6i

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Firstly, before you start to code anything, I would check whether you're using the right cable. Try open a serial terminal of your choice (HyperTerm, putty) and check whether there is any data at all.

I am usingyaohua xk3190-a9Weighing Scale indicator connected to my serial port. And after trying lots of codes, follwing code finally worked for me. I am pasting the code here so that if anybody is using the same device can get help.

First I'll briefly discuss why serial communications is hard. After reading that chapter you'll probably be convinced as well that you need a class, which deals with serial communication. The classes provided in the library are not the only classes, which handle the serial communication. Many other programmers wrote their own classes, but I found many of them too inefficient or they weren't robust, scalable or suitable for non-MFC programs. I tried to make these classes as efficient, reliable and robust as possible, without sacrificing ease of use too much.

Serial communication in Win32 uses the standard ReadFile/WriteFile functions to receive and transmit data, so why should serial communication be any harder then just plain file I/O? There are several reasons, which I'll try to explain. Some problems are solved in this library, but some others cannot be solved by a library.

Handshaking is much more difficult, because it's more difficult to detect problems in this area. Handshaking is being used to control the amount of data that can be transmitted. If the sending machine can send data more quickly then the receiving machine can process we get more and more data in the receiver's buffer, which will overflow at a certain time. It would be nice when the receiving machine could tell the sending machine to stop sending data for a while, so it won't overflow the receiver's buffers. This process of controlling the transmission of data is called handshaking and there are basically three forms of handshaking:

Because the GetMessage function blocks until there is a message in the message queue, there's no way to wake up when a serial event occurs. Of course you can set a timer and check the ports there, but this kind of polling is bad design and certainly doesn't scale well. Unfortunately the Win32 serial communication API doesn't fit in this event driven model. It would be easier for GUI applications that the Win32 API posted a message to a window when a communication event occurred (this is exactly what the 16-bit implementation looked like).

This code is much more complex then the simple message pump displayed above. This isn't that bad, but there is another problem with this code, which is much more serious. The message pump is normally in one of the main modules of your program. You don't want to pollute that piece of code with serial communication from a completely different module. The handle is probably not even valid at all times, which can cause problems of its own. This solution is therefore not recommended. MFC and OWL programmers cannot implement this at all, because these frameworks already their own message pumps. You might be able to override that message pump, but it probably requires a lot of tricky code and undocumented tricks.

Using serial communications in a single-threaded event-driven program is difficult as I've just explained, but you probably found that out yourself. How can we solve this problem for these types of applications? The answer is in the CSerialWnd class, which posts a message to a window (both the message and window can be specified by the programmer) whenever a serial event occurs. This makes using a serial port in GUI based applications much easier. There is also a very thin MFC wrapper class, which is called CSerialMFC but it's that thin, that it's hardly worth mentioning.

If you're not using a message pump in the thread that performs the serial communication, then you should use the CSerial or CSerialEx classes. You can use blocking calls (the easiest solution) or one of the synchronization functions (i.e. WaitForMultipleObjects) to wait for communication events. This approach is also used in most Unix programs, which has a similar function as WaitForMultipleObjects called 'select'. This approach is often the best solution in non-GUI applications, such as NT services.

The CSerialEx adds another thread to the serial object. This frees the main thread from blocking, when waiting for serial events. These events are received in the context of this worker thread, so the programmer needs to know the impact of multi-threading. If all processing can be done in this thread, then this is a pretty efficient solution. You need some kind of thread synchronization, when you need to communicate with the main GUI thread (i.e. for progress indication). If you need to communicate a lot with the main GUI thread, then it is probably better to use the CSerialWnd class. However, if you don't communicate a lot with the main thread, then this class can be a good alternative.

GUI applications, which want to use the event-driven programming model for serial communications should use CSerialWnd. It is a little less efficient, but the performance degradation is minimal if you read the port efficiently. Because it fits perfectly in the event-driven paradigm the slight performance degradation is a minimal sacrifice. Note that you can use CSerial in GUI based applications (even MFC/WTL based), but then you might block the message pump. This is, of course, bad practice in in a commercial application (blocking the message pump hangs the application from the user's point of view for a certain time). As long as you know what the impact is of blocking the message pump, you can decide for yourself if it is acceptable in your case (could be fine for testing).

Using the serial classes can be divided into several parts. First you need to open the serial port, then you set the appropriate baudrate, databits, handshaking, etc... This is pretty straightforward. The tricky part is actually transmitting and receiving the data, which will probably cause the most time to implement. At last you need to close the serial port and as a bonus if you don't then the library will do it for you.

The interesting part is inside the main routine. At the top we declare the serial variable, which represents exactly one COM port. Before you can use it, you need to open the port. Of course there should be some error handling in the code, but that's left as an exercise for the reader. Besides specifying the COM port, you can also specify the input and output buffer sizes. If you don't specify anything, then the default OS buffer sizes are being used (older versions of the library used 2KB as the default buffer size, but this has been changed). If you need larger buffers, then specify them yourself.

Setting up the serial port is also pretty straightforward. The settings from the control panel (or Device Manager) are being used as the port's default settings. Call Setup if these settings do not apply for your application. If you prefer to use integers instead of the enumerated types then just cast the integer to the required type. So the following two initializations are equivalent:

Finally, the port is closed and the program exits. This program is nice to display how easy it is to open and setup the serial communication, but it's not really useful. The more interesting programs will be discussed later.

Like in real life it's easier to tell something what to do then listening to another and take appropriate actions. The same holds for serial communication. As we saw in the Hello world example writing to the port is just as straightforward as writing to a file. Receiving data is a little more difficult. Reading the data is not that hard, but knowing that there is data and how much makes it more difficult. You'll have to wait until data arrives and when you're waiting you cannot do something else. That is exactly what causes problems in single-threaded applications. There are three common approaches to solve this.

The first solution is easy. Just block until some data arrives on the serial port. Just call WaitEvent without specifying the overlapped structure. This function blocks until a communication event occurs (or an optional time-out expires). Easy, but the thread is blocked and only wakes up for communication events or a time-out.

The last solution is one which will be appreciated by most Windows GUI programmers. Whenever something happens a message is posted to the application's message queue indicating what happened. Using the standard message dispatching this message will be processed eventually. This solution fits perfect in the event-driven programming environment and is therefore useful for most GUI (both non-MFC and MFC) applications. Unfortunately, the Win32 API offers no support to accomplish this, which is the primary reasons why the serial classes were created. The old Win16 API uses the SetCommEventMask and EnableCommNotification to do exactly this, but these were dropped from the Win32 API.

Blocking is the easiest way to wait for data and will therefore be discussed first. The CSerial class exposes a method called WaitEvent, which will block until an event has been received. You can (optionally) specify a time-out for this call (if overlapped I/O is enabled), so it won't block forever if no data arrives anymore. The WaitEvent method can wait for several events, which must be registered during setup. The following events can occur on a COM port:

When a serial port is opened, then the EEventBreak, EEventError and EEventRecv are being registered. If you would like to receive the other events then you have to register them using the SetMask method.

Reading can be done using the Read method, but reading is trickier then you might think at first. You get only an event that there is some data, but not how much. It could be a single byte, but it can also be several kilobytes. There is only one way to deal with this. Just read as much as you can handle (efficiently) and process it.


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