Slow data rate FT245BM to PC

I am working on a PIC project that uses an FT245BM to transfer data to the PC over USB. I am transferring about 3MB of data and want to do it as fast as possible. However, my current solution is way too slow. Maybe someone can point out what I am doing wrong.

Let me start off by saying that I know VB isn't a speed demon, but I don't think it is the bottle neck here. I am running this on a P4

Here are the symptoms of my problem. The 245's TXE seems to go high (signaling that the internal buffer is full) for "long" periods of time even though I am doing FT_Reads as fast as possible on the PC. I've run into issues like this before using the FTDI chips, but was able to solve them by tweaking the timeouts, latency timer, and buffer sizes. However, this time, tweaking the settings just seem to slow things down even more.

Here are the basic init commands I am using (note this is not runnable code)

FT_OpenEx("Device Name", FT_OPEN_BY_DESCRIPTION, lngHandle) FT_SetTimeouts(lngHandle, 500, 0) FT_SetLatencyTimer(lngHandle, 1) FT_SetChars(lngHandle, 126, 1, 0, 0) FT_SetUSBParameters(lngHandle, 16384, 0) FT_Purge(lngHandle, 3)

Here is the code I use to poll the device:

--------------------------------------------------------------------- Do lngBytesRead = 0 queue_length = 0 ftStatus = FT_GetQueueStatus(lngHandle, queue_length) If queue_length > 32000 Then queue_length = 32000 End If ftStatus = FT_Read(lngHandle, strReadBuffer, queue_length, lngBytesRead)

If (ftStatus = FT_OK) Or (ftStatus = FT_IO_ERROR) Then If lngBytesRead > 0 Then bigarray() = StrConv(strReadBuffer, vbFromUnicode) ReDim Preserve bigarray(lngBytesRead) CopyMem imgarray(data_buffer_pointer), bigarray(0),UBound(bigarray) data_buffer_pointer = UBound(bigarray) + data_buffer_pointer lngTotalBytesRead = lngTotalBytesRead + lngBytesRead Else

End If Else flFatalError = True End If DoEvents Loop Until (lngTotalBytesRead >= 3145728) Or (cancel_but = True) Or (flFatalError = True)

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The code functions well, it is just a bit slow. It can take up to 30 seconds to transfer 3 megs. I tried not using the FT_GetQueueStatus and just used a fixed number for the FT_Read. This seemed to be less efficient.

Any suggestions would be helpful. Thanks!

Zach