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VB Forum / Win API / June 2008RS232 flowcontrol
Dear all I have written a communication app. in VB6 that uses the Winapi functions to access the serial Interface(createfile,etc). In general my app works OK but occassionally I get bufferoverruns and therefore I loose some data ALTHOUGH I use RTS/CTS flowcontrol. All the wiring is OK and I can also see that my app deactivates the RTS signal periodically in case the buffer gets full. I made some investigations and found out that my communication device still can send upto 256databytes once it detects a deactivated RTS signal. On the device there is no way to prevent this. These additional 256bytes seem to be the cause for the bufferoverrun. I tried to adjust the parameters Xonlim and Xofflim of the structure DCB to have enough room for these additional databytes in the RXbuffer this didn't help and I still get CE_overrun error. Increasing the buffersize(I use 2048bytes) to 4096 also didn't help. Has anyone ever run into that problem and knows how I can prevent the overrun error? Regards Peter I think I have already found the reason for this problem: The usb-V.24 converter I use seems to add a hugh delay to the RTS line which will fill up my rxbuffer. The flowcontrol mechanism works fine on a native RS232 comport in my PC. Regards Peter > Dear all > I have written a communication app. in VB6 that uses the Winapi functions [quoted text clipped - 15 lines] > Regards > Peter Hi, What you are seeing is two-fold. First, the USB driiver software (virtual serial port) UART probably implements a 96-byte (or possible deeper) transmit FIFO. This FIFO is emptied, even if CTS is lowered. There isn't anything that you can do in your application code if this is, in fact, the case -- except to work around it yourself (see below). Second, the FIFO in your PC UART is only 16-bytes in depth, so it sounds like the receiving hardware can handle 16-bytes, but not the larger number. So, what's the work-around? The obvious. Leave CTS/RTS flow control in place, then... Write a routine that breaks transmit data into small chunks, of less than 16-bytes. Check the status of CTS in your transmit routine. If CTSHolding = True, then send 16-bytes. Add a small delay between each 16-byte "chunk" to allow CTS to propogate from hardware to your software. If CTSHolding = False, then stay in a delay loop until it goes back to True. Continue until all data have been sent. I'd start with about a 1 mS or so delay between each "chunk," but a much larger delay inside the loop (perhaps 50 mS). Keep decreasing the large delay until you encounter a problem, then bump it back up some, until it is reliable.  SignatureRichard Grier, MVP Hard & Software Author of Visual Basic Programmer's Guide to Serial Communications, Fourth Edition, ISBN 1-890422-28-2 (391 pages, includes CD-ROM). July 2004, Revised March 2006. See www.hardandsoftware.net for details and contact information. Thanks for your answer. But I have problems to understand it. I don't have any possibilities to modify the sending behaviour of my serial device(a GPRS modem). It is specified that after my VB6 app has lowered signal RTS it my still send additional 256bytes until it stops sending data. Inbetween I think that these additional 256bytes are not the main cause of my problem because if I use the native COM-Port of my PC(instead of a USB-Comm converter) everything works fine. Instead, I think that either the USB driver or the USB hardware adds a certain delay to signal RTS. In case the PC's receive buffer gets full the "serial driver" instructs RTS to go low but due to the additional delay(coming from the USB converter) much more data can still access the receive buffer which gets overflown. Now I try to play with the two parameters Xonlim and Xofflim of the DCB structure because it seems that they also define the levels when RTS should be lowered. I hope I can adjust the flowcontrol mechanism(also by increasing the size of the receive buffer) to accept the additional delay of signal RTS on the USB converter. Regards Peter > Hi, > [quoted text clipped - 21 lines] > until you encounter a problem, then bump it back up some, until it is > reliable. Hi Should XoffLim and XonLim of the DCB structure work in case I use RTS/CTS flowcontrol(fRtsControl=RTS_CONTROL_HANDSHAKE and fOutxCtsFlow=true)? Or does XoffLim and XonLim only work with Xon/Xoff Flowcontrol? What ever value I set for XoffLim and XonLim, the app does not set/reset RTS earlier or later in case the receive buffer gets full. Could it be that these values are ignored for RTS/CTS flowcontrol? Any other reason why it is not working? In MSDN the following discriptuion con be found: ***************************************************** XoffLim Maximum number of bytes allowed in the input buffer before flow control is activated to allow transmission by the sender. This assumes that either XON/XOFF, RTS, or DTR input flow control is specified in fInX, fRtsControl, or fDtrControl. The maximum number of bytes allowed is calculated by subtracting this value from the size, in bytes, of the input buffer. ***************************************************** Regards Peter > Hi, > [quoted text clipped - 21 lines] > until you encounter a problem, then bump it back up some, until it is > reliable. Hi, These settings do not affect the operation of the FIFO. They only control the standard Windows serial driver -- some drivers for USB devices may use them, others may not. Unfortunately, MSDN says only what the article author thinks "should be true," and has no control over the reality of what really is true. Experience (and experimentation) are the only real world guides. Without "hands-on" the actuall hardware and drivers, I can only offer advice. Dick SignatureRichard Grier, MVP Hard & Software Author of Visual Basic Programmer's Guide to Serial Communications, Fourth Edition, ISBN 1-890422-28-2 (391 pages, includes CD-ROM). July 2004, Revised March 2006. See www.hardandsoftware.net for details and contact information. Thanks, Dick But XoffLim and XonLim also do not work on my native serial Commports(no USB)? Do you have any experience whether these parameters should work with RTS/CTS flowcontrol at all? Regards Peter > Hi, > [quoted text clipped - 10 lines] > > Dick |
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