I'm embarrassed to have to ask such a question, but I'm having a rough time figuring out how to reliably read data over a serial port with the .NET SerialPort class.
My first approach:
static void Main(string[] args)
{
_port = new SerialPort
{
PortName = portName,
BaudRate = 57600,
DataBits = 8,
Parity = Parity.None,
StopBits = StopBits.One,
RtsEnable = true,
DtrEnable = false,
WriteBufferSize = 2048,
ReadBufferSize = 2048,
ReceivedBytesThreshold = 1,
ReadTimeout = 5000,
};
_port.DataReceived += _port_DataReceived;
_port.Open();
// whatever
}
private void _port_DataReceived(object sender, SerialDataReceivedEventArgs e)
{
var buf = new byte[_port.BytesToRead];
var bytesRead = _port.Read(buf, 0, buf.Length);
_port.DiscardInBuffer();
for (int i = 0; i < bytesRead; ++i)
{
// read each byte, look for start/end values,
// signal complete packet event if/when end is found
}
}
So this has an obvious problem; I am calling DiscardInBuffer, so any data which came in after the event was fired is discarded, i.e., I'm dropping data.
Now, the documentation for SerialPort.Read() does not even state if it advances the current position of the stream (really?), but I have found other sources which claim that it does (which makes sense). However, if I do not call DiscardInBuffer I eventually get an RXOver error, i.e., I'm taking too long to process each message and the buffer is overflowing.
So... I'm really not a fan of this interface. If I have to process each buffer on a separate thread I'll do that, but that comes with its own set of problems, and I'm hoping that I am missing something as I don't have much experience with this interface.
Jason makes some good points about reducing UI access from the worker thread, but an even better option is to not receive the data on a worker thread in the first place.
Use port.BaseStream.ReadAsync to get your data, event-driven, on the thread where you want it. I've written more about this approach at http://www.sparxeng.com/blog/software/must-use-net-system-io-ports-serialport
To correctly handle data from a serial port you need to do a couple of things.
First, don't handle the data in your receive event. Copy the data somewhere else and do any processing on another thread. (This is true of most events - it is a bad idea to do any time-consuming processing in an event handler as it delays the caller and can introduce problems. You also need to be careful as your event is raised on a different thread to your main application)
Secondly, you can't guarantee that you will receive exactly one packet, or a complete packet when you receive data - it may come to you in small fragments.
So the upshot of this is that you should create your own buffer (big enough to hold several packets), and when you receive data, append it to your buffer. Then in another thread you can process the buffer, looking to see if you can decode a packet from it and then consume that data. You may have to skip the end of a partial packet before you find the start of a valid one. If you don't have enough data to build a full packet, then you may need to wait for a bit until more data arrives.
You shouldn't call Discard on the port - just read the data and consume it. Each time you are called, there will be another fragment of data to process. It does not remember the data from previous calls - each time your event is called, it is given a small burst of data that has arrived since you were last called. Just use the data you've been given and return.
As a last suggestion: Don't change any settings for the port unless you specifically need to for it to operate properly. So you must set the baud rate, data/stop bits and parity, but avoid trying to change properties like the Rts/Dtr, buffer sizes and read thresholds unless you have a good reason to think you know better than the author of the serial port. Most serial devices work in an industry standard manner these days, and changing these low-level options is very likely to cause trouble unless you're talking to some unusual equipment and you intimately know the hardware.
In particular setting the ReceivedBytesThreshold to 1 is probably what is causing the failure you've mentioned, because you are asking the serial port to call your event handler with only one byte at a time, 57,600 times per second - giving your event handler only 0.017 milliseconds to process each byte before you'll start to get re-entrant calls.
DiscardInBuffer is typically only used immediately after opening a serial port. It is not required for standard serial port communication so you should not have it in your dataReceived handler.
Related
Excuse me, quick question:
I have this hardware setup:
Same machine: "Com3" -> USB -> To Serial -> To USB -> "Com4"
And I followed MSDN SerialPort Class and MSDN SerialPort.ReadLine() to build this routine:
SerialPort SendSerialPort = new SerialPort("Com3", 9600);
SerialPort ReceiveSerialPort = new SerialPort("Com4", 9600);
SendSerialPort.Open();
ReceiveSerialPort.Open();
SendSerialPort.WriteLine("Test");
var message = ReceiveSerialPort.ReadLine(); // control stops here
SendSerialPort.Close();
ReceiveSerialPort.Close();
Console.WriteLine(message);
However, when I tend to ReadLine(), my control stops and just waits. I did not expect that.
I am expecting to receive the string Test and assign it to my var message. Could you please tell me what am I doing wrong here?
EDIT:
I tested my hardware using the Serial Port Utility Application and it worked just fine.
I've altered from the example you linked:
To actually have both ports running to read and write back and forth you will actually need to implement threading for reading and writing for both.
It can be a good idea to use a timer.
public static void Main()
{
SerialPort SendSerialPort = new SerialPort("Com3", 9600);
SerialPort ReceiveSerialPort = new SerialPort("Com4", 9600);
StringComparer stringComparer = StringComparer.OrdinalIgnoreCase;
Thread readThread = new Thread(Read);
// Set the read/write timeouts
_serialPort.ReadTimeout = 500;
_serialPort.WriteTimeout = 500;
SendSerialPort.Open();
ReceiveSerialPort.Open();
bool _continue = true;
readThread.Start();
Console.Write("Name: ");
name = Console.ReadLine();
Console.WriteLine("Type QUIT to exit");
while (_continue)
{
message = Console.ReadLine();
if (stringComparer.Equals("quit", message))
_continue = false;
else
SendSerialPort.WriteLine(String.Format("<{0}>: {1}", name, message));
}
readThread.Join();
SendSerialPort.Close();
}
public static void Read()
{
while (_continue)
{
try
{
string message = ReceiveSerialPort.ReadLine();
Console.WriteLine(message);
}
catch (TimeoutException) { }
}
}
Usually there will be a beginning and end value within the written data to tell the other port that the message is finished and also for the ports to validate that they are reading data they should be, usually with commands of what to do with that data. (out of scope for this question).
Also lacking and important is the intialisation of your ports.
I prefer to use the default constructor (preference only)
SerialPort Constructor ()
And then set any values like so:
_serialPort.BaudRate = SetPortBaudRate(_serialPort.BaudRate);
_serialPort.Parity = SetPortParity(_serialPort.Parity);
_serialPort.DataBits = SetPortDataBits(_serialPort.DataBits);
_serialPort.StopBits = SetPortStopBits(_serialPort.StopBits);
_serialPort.Handshake = SetPortHandshake(_serialPort.Handshake);
All the constructors will give these values:
This constructor uses default property values when none are specified. For example, the DataBits property defaults to 8, the Parity property defaults to the None enumeration value, the StopBits property defaults to 1, and a default port name of COM1.
Even the handshake has a default value. If you look at the source code.
private const Handshake defaultHandshake = Handshake.None;
The problem with your code is in this line
var message = ReceiveSerialPort.ReadLine();
You block your code to wait for a line, if the line never arrives it will remain here forever or the value set to ReadTimeout
So why does the line never arrive?
The problem can be an error in WriteLine("Test");, you should handle errors, or it can be that your in are blocking your code ReadLine() before the WriteLine("Test") manage to come through, you could insert a Thread.Sleep(100) between, but this is not really improving the code.
Note: Your code will also work as is sometimes, depending on these race conditions.
This synchronized / blocking reading from serial ports seems simple in code just one line; but it creates a lot of negative side effects in your communication protocol's.
A much better solution (considering that you like to Read / Write data from a microcontroller) is to either use a thread as Yvette suggested or use asynchronously reading Stream.BeginRead (Byte[], Int32, Int32, AsyncCallback, Object) which I would prefer.
The asynchronously reading will throw an event when something is incoming on the serial port. The basic idea of this programming strategy is to not do step programming but expecting what ever result and then handle it correctly.
In communications protocol with asynchronously reading the AutoResetEvent is very useful, hence you send something, then you start the AutoResetEvent, if asynchronously the expected result is arriving you will set this event and your code can continue, if it does not arrive the AutoResetEvent will timeout and you can handle this.
It cannot block when there is data available. What you sent either got stuck in the transmit buffer, got lost due to a wiring mistake, triggered an error or was ignored. If it works with another program then a wiring mistake can't be the problem.
Do keep in mind that just setting the Baudrate is not enough, you must also use set the DataBits, Parity and Stopbits properties to match the device settings. A mismatch can trigger an error, the kind you can only see when you write an event handler for the ErrorReceived event. Never skip that event, confounding problems can occur if you never check.
And above all the Handshake property must be set correctly. The proper value depends on how the ports are wired together, it is too common to not connect them. Start by setting it to Handshake.None so a wrong state for the DSR and CTS signals can't block reception and a wrong state for the DTR and RTS signals can't block transmission. Beware that it is common for another program to enable hardware handshaking, a mismatch is guaranteed to cause communications to stall.
If you use synchronous reads instead of the DataReceived event then you should in general deal with the possibility that a device is not responding. Either because it is powered off, not connected at all or malfunctioning. Use the ReadTimeout property for that so your program cannot hang. Aim high, 10000 milliseconds is a reasonable choice.
Beware the randomness of this problem, putzing around with another program can easily get the port configured correctly and now it will suddenly work. And beware that starting a thread accomplishes nothing, it will now be that thread that gets stuck and the Join() call will deadlock.
I'm taking data from a serial instrument for plotting on a chart. The data stream is 230 kbps, and the serial pipeline is less than 50% full, data arrives about 100 kbps and actually doesn't vary really and rate or quantity.
Having used just a serial terminal program, like Teraterm, on the same computer; I can capture data and prove that both the source of the data as well as the test reception method are fine and I see no errors to the captured data.
The Windows Forms application I'm developing loses data. I've reduced it from receiving, capturing (in parallel), parsing, and plotting, to just receiving and capturing. And have found that I still see lost data in the capture.
I'm not a long experienced Windows person, so therefore may not know of better ways to accomplish the same functions. Here are the actions I'm taking to perform receive actions:
I'm using a System.IO.Ports.SerialPort class.
I modify the .DataReceived event via:
+= new SerialDataReceivedEventHandler(comPort_DataReceive);
I then call the open() method.
Note: I may be doing something incorrect here, I never clear the .DataReceived event with a -= at any point, instead each time I open, the event is added yet again. Nevertheless, these problems occur even when I've only talked to the port once.
Here's my code for the data receive function. RxString is a string.
private void comPort_DataReceive(object sender, SerialDataReceivedEventArgs e)
{
RxString = comPort.ReadExisting();
this.Invoke(new EventHandler(ParseData));
}
private void ParseData(object sender, EventArgs e)
{
// Save to capture file, if capture is enabled
if ((WriteToFileEnabled == true) && (WriteToFileName != null))
{
writeFileHandle.Write(RxString);
}
return;
// Previously would parse and plot data
}
So, how would persons execute a receive in this situation to get this data without losing it?
Follow on questions are things like: How big is the buffer for serial receive, or do I need to worry about that if I have a reasonably responsive application? Flow control is irrelevant, the remote device is going to send data no matter what, so it would be up to my computer to take that data and process it or ignore it. But how would I know if I've lost data or experienced framing errors and stuff? (I ask that last one without having searched much on the SerialPort class structure, sorry.)
Lets assume that your device is sending messages that are 85 bytes in length. The DataReceive event handler may or may not fire once to receive those 85 bytes. Since it might fire more than once your code must account for that. The DataReceive event handler should read the bytes available and append them to a buffer that is processed later.
Also, only one of the events raised by the SerialPort class can execute at a time. In the example assume the handler has to fire three times to receive the 85 bytes. While processing the first part the other two can't execute. If while processing the first part one of the other events, PinChanged or ErrorReceived, are needed they can't be executed either.
My first two experiences with the SerialPort class were a 9600 bps terminal and a 1 Mbps bluetooth device. What worked for the slower did not work for the faster, but when I figured out how to get the faster to work the slower could use the same methodology.
My methodology:
Before opening the serial port I start two other background threads that run in a do loop. The first one(Receive) reads all available bytes from the serial port, adds them to a buffer, and signals the second thread on every read. The second one(Protocol) determines if a full message has arrived, does any byte to string conversion, updates the UI, etc. Depending on the application I may start a third thread that handles errors and pin changes. All of these threads are throttled by a Threading AutoResetEvent.
My DataReceive event handler has one line in it, a Set on the AutoResetEvent that is throttling Receive.
A VB example of this can be found here SerialPort Methodology. Since adopting this methodology I have not had any of the problems that seem to plague other SerialPort users and have used it successfully with speeds up to 2Mbps.
I am currently stuck at a problem which I cant find any answer for. I am controlling a device that is attached to the computer via a Virtual-ComPort (USB). For this communication I use the SerialPort class in System.IO.Ports.
This is how I initialize the port:
SerialPort SerialPort = new SerialPort();
SerialPort.BaudRate = BaudRate;
SerialPort.DataBits = 8;
SerialPort.Parity = Parity.None;
SerialPort.PortName = SerialPortName;
SerialPort.StopBits = StopBits.One;
SerialPort.ReceivedBytesThreshold = 9;
SerialPort.WriteTimeout = 1500;
SerialPort.ReadTimeout = 1500;
SerialPort.DataReceived += dataReceivedHandler;
SerialPort.Open();
I set the received Threshold to 9 because I know that all the data I can receive is 9 bytes long and I want to minimize the number of callbacks. Can this cause any harm?
This does work really well in 99,99% of all cases. But after a random amount of time/commands (typically after several thousand operations) I don't get a callback when data is send to the PC. The DataReceived handler is just not called in these cases. I checked this by adding a Debug.WriteLine("Callback received!") at the very beginning of the dataReceivedHandler.
I am sure the data is send by the device I like to control. What is the cause of this seemingly "random" behavior and how can I fix it?
I am grateful for any suggestions.
Best
Nano
Using ReceivedBytesThreshold is a convenience, but it only works when the Sun is shining and you've got a stiff breeze in your back. You are skipping the things you need to do to ensure it can work:
You need to implement flow control so there is no scenario where the device can overflow the input buffer. Even though modern machines are plenty fast enough to support a high baudrate, they cannot guarantee that they are always fast enough. Losing the processor for many hundreds of milliseconds is very possible. You should always set the Handshake property so this can never happen. The proper value depends on what the device supports, always start with Handshake.RequestToSend if you don't know.
Never checking for errors is pretty much a guarantee that you cannot diagnose a mishap like this. Implementing the ErrorReceived event handler is not optional. It is the only way you can diagnose mishaps, like a UART or device driver input buffer overflow, the kind that cause data loss and cause the kind of trouble you are trying to troubleshoot.
ReceivedBytesThreshold always gives you a synchronization problem. You have to make sure that you start listening at just the right time, in between two chunks of 9 bytes. That is of course pretty hard to do if you have no control over the startup order of your program vs the device, you rarely do. A decent protocol always gives you a way to synchronize, typically with a dedicated byte value that indicates the start of a message. Once you have that, the value of the ReceivedBytesThreshold completely disappears.
Start with the ErrorReceived event so you know when things go wrong. Shopping for another USB emulator also ought to be high on your list of possible workarounds, drivers for them are generally pretty cruddy.
I'm writing a message layer for my distributed system. I'm using IOCP, ie the Socket.XXXAsync methods.
Here's something pretty close to what I'm doing (in fact, my receive function is based on his):
http://vadmyst.blogspot.com/2008/05/sample-code-for-tcp-server-using.html
What I've found now is that at the start of the program (two test servers talking to each other) I each time get a number of SAEA objects where the .Buffer is entirely filled with zeroes, yet the .BytesTransferred is the size of the buffer (1024 in my case).
What does this mean? Is there a special condition I need to check for? My system interprets this as an incomplete message and moves on, but I'm wondering if I'm actually missing some data. I was under the impression that if nothing was being received, you'd not get a callback. In any case, I can see in WireShark that there aren't any zero-length packets coming in.
I've found the following when I Googled it, but I'm not sure my problem is the same:
http://social.msdn.microsoft.com/Forums/en-US/ncl/thread/40fe397c-b1da-428e-a355-ee5a6b0b4d2c
http://go4answers.webhost4life.com/Example/socketasynceventargs-buffer-not-ready-121918.aspx
I am sure not what is going on in the linked example. It appears to be using asynchronous sockets in a synchronous way. I cannot see any callbacks or similar in the code. You may need to rethink whether you need synchronous or asynchronous sockets :).
To the problem at hand stems from the possibility that your functions are trying to read/write to the buffer before the network transmit/receive has been completed. Try using the callback functionality included in the async Socket. E.g.
// This goes into your accept function, to begin receiving the data
socketName.BeginReceive(yourbuffer, 0, yourbuffer.Length,
SocketFlags.None, new AsyncCallback(OnRecieveData), socketName);
// In your callback function you know that the socket has finished receiving data
// This callback will fire when the receive is complete.
private void OnRecieveData(IAsyncResult input) {
Socket inSocket = (Socket)input.AsyncState; // This is just a typecast
inSocket.EndReceive(input);
// Pull the data out of the socket as you already have before.
// state.Data.Write ......
}
I'm working on a SerialPort app and one very simple part of it is giving me issues. I simply want to read a constant stream of data from the port and write it out to a binary file as it comes in. The problem seems to be speed: my code has worked fine on my 9600 baud test device, but when carried over to the 115200bps live device, I seem to be losing data. What happens is after a variable period of time, I miss 1 byte which throws off the rest of the data. I've tried a few things:
private void serialPort1_DataReceived(object sender, SerialDataReceivedEventArgs e)
{
bwLogger.Write((byte)serialPort1.ReadByte());
}
or
private void serialPort1_DataReceived(object sender, SerialDataReceivedEventArgs e)
{
byte[] inc = new byte[serialPort1.BytesToRead];
serialPort1.Read(inc, 0, inc.Length);
bwLogger.Write(inc);
}
and a few variations. I can't use ReadLine() as I am working with a constant stream of data (right?). I've tried fiddling with the buffer size (both serialPort1.ReadBufferSize and the hardware FIFO buffer). Ideally, for usability purposes, I'd handle this on the software side and not make the user have to change Windows driver settings.
Any ideas?
If the problem seems to be that you can't process the data fast enough, what you could try would be to double-buffer your data.
1) Allow one thread to read the serial port into one buffer. This may involve copying data off the port into the buffer (i'm not intimately familiar with .NET).
2) When you are ready to handle the incoming data, (on a different thread) make your program read into the 2nd buffer, and while this is happening you should write the first buffer to disk.
3) When the first buffer is written to disk, swap it back to the serial port buffer, and write the 2nd buffer to disk. Repeat process, continually swapping the buffers.
You might try enabling handshaking, using the Handshake property of the SerialPort object.
You'll have to set it on both the sender or receiver. however: if you're overflowing the receiver's UART's buffer (very small, 16 bytes IIRC), there's probably no other way. If you can't enable handshaking on the sender, you'll probably have to stay at 9600 or below.
I'd try the following:
Set the Buffer-Size to at least 230K Bytes
Set the Incoming Threshold to 16K, 32K or 65K
Write this fixed blocks of data to the file
I'm not sure if this might help, but it should at least take the pressure of the framework to fire the event that often.
I would check the number of bytes read which is returned by the Read(Byte>[], Int32, Int32) method and make sure it matches what you expect.
Make sure you are listening for SerialErrorReceivedEventHandler ErrorReceived events on the port object. An RXOver error would indicate your buffer is full.
Check the thread safety on your output buffer. If the write to the output buffer is not thread safe, a second write may corrupt the first write.
Is your bwLogger a BinaryWriter class? You might try using it with a BufferedStream to make the disk I/O nonblocking.
Also, if your packets have a known ending character, you can set the SerialPort.NewLine property to enable you to use ReadLine/WriteLine, although I don't think that would make much of a performance difference.
The machines I've been working with recently all send a stop code (in my case ASCII code 3 or 4). If you also have this feature, you can make use of ReadTo(string) off your SerialPort object.