[fpc-pascal] USB serial interface

Klaus Hartnegg klaus at fischer.brain.uni-freiburg.de
Wed Dec 27 10:28:55 CET 2006

> Am Dienstag, 26. Dezember 2006 18:57 schrieb Marco van de Voort:
> > > The best USB-serial Interfaces for Linux are the ones with FTDI Chip
> > > inside. http://www.ftdichip.com/
> > > http://www.ftdichip.com/Products/EvaluationKits/USB-Serial.htm
> >
> > Do they have the same latency as a normal serial port? I've tried several
> > usb-serial ports, and they all have horrible latencies compared to an
> > onboard (or PCI card).
> >
> May be it is
> also a question of the Kerneldriver and the driver will improve more in the
> future.

It's more likely a general problem of USB.
A real serial port is always ready to receive and it triggers an 

interrupt as soon as it has received a character, so the PC 
immediately fetches the received data.

On USB the converter must buffer all incoming data until
the PC specifically asks it to do its next transfer.
On USB1 there is no guarantee that the PC can do
1000 transfers per second. 
USB1 can transfer a lot more than 1000 bytes per second, 
but not in 1000 transfers of only 1 byte each.
I don't know the exact number for USB2. 
Also it depends on which other USB devides are connected 
because they all share the bus.

Klaus Hartnegg, Brain Research Group, University Freiburg, 
Hansa-strasse 9, D-79104 Freiburg, Germany, hartnegg at uni-freiburg.de
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