Why not forget the VCP drivers ?
What I remember from them, is that a full duplex at 115 kBaud is not 
realiable
(and also depends strongly on the chozen commportdriver in windows,
and what other tasks windows is doing).
For over a year we're now using the Dx2xx drivers,
standard  full duplex at 1.25 MBaud (form FTDI to PC at  full bandwidth 
the otherway around at a few % bandwidth), without any trouble,
with a latency of about 10 msec.

Stef Mientki

Jim Robertson wrote:

>
> Has anyone here got the FTDI 232BM chip and the VCP
> driver to transfer data downstream (PC to FTDI chip ) with a
> high efficieny  rate (95%+)  when using higher UART baud rate
> settings like 115.2k?
>
> The expected time to transfer 131072 bytes @ 115.2k baud is
> ~ 11 seconds.
>
> Both the native comport and my prolific chip set based USB-RS232
> convertor transfer the full 131072 bytes in ~ 11 seconds.
>
> However the FTDI chip took ~16 seconds!  That is a ~80k bit/sec
> transfer rate or only 69% efficiency  This was using the same hub as
> the other USB-RS232 bridge.
>
> This is not an latency timer issue as the latency timer is for upstream
> transfers only.
>
> Upstream transfers are not much of an issue. I can the FTDI chip to
> send 131072 bytes upstream at a rate close to 15.2k bits/sec.
>
> However, there does seem to be an issue with the downstream transfer
> rates of the FTDI chip and the VCP driver once you go over 57600 baud.
> (57600 baud transfer was near 100% efficient.)
>
> So, has anyone found a way to get high speed downstream transfers
> using the FTDI -VCP driver combo and 115.2k baud or greater?
>
> I have two olimex MCP programmers here. One is the USB model
> the other is the older comport model. I really, really bugs me that I can
> program a 18F8722 in 16 seconds with the comport model while the USB
> model takes 21 seconds with the same firmware and baud rate...
>
> -Jim
>
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