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Messages - Me123

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Sorry, I was caught up with other things for the last week.  I've sent the email now.

Yes I have played with adjusting the transfer size, but I see that as a separate question, at the moment I just wanted to understand what the behaviour of event characters should be as it is not clear to me from reading the programmers guide or AN232B-04.  However, whilst on the subject of the transfer size, one thing I was curious about is what happens if it is set to e.g. 64 bytes and I send, say 70 bytes of data?  Is there a danger of losing the last few bytes or will all the data simply be made available in two packets?

Many thanks.

OK thank you.  I will send an email to support.  Just to be clear though, if I were to send 62bytes of data without an event character then this absolutely is the expected behaviour.  This is because I am sending data faster than the set latency timeout and so data is not available until the USB buffer is full, so only when it has received 4096 bytes.  This is described in section 3.3 of application note AN232B-04.  I was expecting event characters to flush the USB buffer but they appear not to do that.

Hi.  I am sending 62 bytes including the event character.  It is not sent immediately, at least it is not available to be read out by my application and as I stated in my last post - because the latency timer never times out no data is available to be read until the USB buffer is full.  For me this happens 6.4 seconds after I start sending data, which seems a bit extreme.

Hi, thanks for the reply. 

Yes I have seen that application note and read it several times, however it is still not clear to me what the behaviour should be when a packet of data is sent that ends in an event character and is 62 bytes long.

You state that if the event character is enabled and it is detected in the data stream, then the contents of the devices buffer is sent immediately. However, this is not true if the packet of data you are sending is 62 bytes long, at least not in my testing. This, combined with the fact that if you are sending packets of data with a smaller interval than the latency timer setting then the buffer will not be transmitted until it is full (default 4096bytes), means even if you are using event characters there can be very long delays between receiving data.  I would like to know if this is the expected behaviour, because it does not seem like it should be to me.


Hello.  I'm using the FTD2XX.dll to interface to a FT232RL chip and was noticing some odd behaviour.  I've turned on event characters and when I send a series of packets that are exactly 62 bytes (including the event character) then there is a very long delay before the data is available to my application.  I've read application note AN232B-04 a few times and this behaviour makes sense if I don't have event characters turned on, as the rate I am sending data back and the value of the latency timer means data wouldn't be sent until the USB buffer is full.  But as I have event characters turned on shouldn't the USB buffer be flushed every time an event character is seen in the data stream, or have I misunderstood something?

Many thanks

Just a quick update. After plugging in a few more devices they suddenly started to be given different COM port numbers.  Not sure why this started working but thank you very much for the help.

Thanks very much for that information.  The devices do not have the same serial number but all the USB ports on my laptop are 3.0 so that probably explains the source of the weird behaviour.

I've just got a new laptop (Windows 7 64bit) and I'm having an issue whenever I plug in a new FTDI USB to serial device it installs with the same COM port number.  I can have two devices plugged in at the same time with the same port number, which is obviously a problem.  This didn't use to happen on my old 32 bit Win 7 laptop, and it doesn't happen on some other laptops with 64 bit Win 7 I've tested it out on so I presume it is some issue with my laptop, I just can't think what it is.  I've tried uninstalling/reinstalling the drivers but that didn't help, I'm not sure what else to try. Does anyone have any idea?  Thanks.

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