[Coco] usb to serial adaptors and drivewire

Aaron Wolfe aawolfe at gmail.com
Sat Aug 28 23:37:16 EDT 2010

On Sat, Aug 28, 2010 at 6:03 PM, Steven Hirsch <snhirsch at gmail.com> wrote:
> On Sat, 28 Aug 2010, Aaron Wolfe wrote:
>> In my experience, they work fine most of the time.  I've used both DW3
>> and DW4 with a variety of adapters on a variety of computers and OSes.
>> Never found a combination that doesn't work at all, but Windows 7 + a
>> cheap Prolific adapter would cause a blue screen of death every few
>> days of continuous operation.   It seems almost all adapters have
>> either Prolific or FTDI chipsets.  The cheaper ones use Prolific.
>> There are some notes in the DW4 wiki that can help you get better
>> speeds (+%30) with FTDI adapters, they seem to be set up for very high
>> speed by default, and need to be adjusted for 115.2k.  Prolific seem
>> to perform at the same speed as a real serial port without any
>> adjustment.
> I'm able to get reliable 460k Baud operation using a Prolific adapter on my
> x86_64 Ubuntu box.  Obviously the client is FPGA CoCo on an Altera DE-1
> board :-).

Some random thoughts/comments on serial stuff with DW...

When working on the code that became DriveWire 4, my primary dev
platform included a Prolific adapter, so I had adjusted the parameters
used in the serial I/O to work with it.  At some point I obtained an
FTDI adapter and started testing with that, and also started testing
on another computer that had a built in 16550 based serial port.  I
found that a couple changes allowed all three types to work pretty
much fine, but that performance was significantly worse on the FTDI,
despite it being widely accepted that the FTDI adapters are "better".
 No amount of tweaking in the RXTX calls made the FTDI come up to par
with the other two serial options I had.  Some research on the web
revealed that this is a common problem at lower speeds and FTDI
chipsets.  The fix is simple, you just change the "receive buffer
latency timer" to 4ms (from the default 16ms). In Windows, this is
done in the properties of the adapter, accessible from device manager.

Although I haven't verified it, I strongly suspect that DriveWire 3
will also have about 20-30% slower overall performance with FTDI
adapters, unless this change is made.  I only have the one FTDI
adapter to test with, but from what I read, this change would improve
performance on most if not all FTDI based adapters.  If you have FTDI
and either version of DW, consider changing that setting.

When Gary added the support for 230 and 460k operation to CoCo3FGPA, I
was concerned that DW4 might not be quick enough, but so far it seems
to work perfectly with both FTDI and Prolific adapters.  Now we need a
460k+ solution for a real CoCo :)


> Steve
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