Looking at the responses collected by Nuala Brady <[log in to unmask]>, it
would appear as if the G4-ASL interface were more complicated than it
really is. Had my response been less sketchy, there would have been three
responses rather than five, since Wolfgang Lehmann mentions a
different system and Jeff Pelz only uses a serial interface for
controlling the record/pause/stop XDAT functions. Allow me to clarify
things:
- Brian Griffith (Applied Science Laboratories - [log in to unmask]) is
actually describing our system at the Toronto Western Hospital, based on
our conversation this past June. As he mentions, we use VPixx for stimulus
generation and to run our psychophysical testing. Peter April, its
creator, modified VPixx so that it can write to the USB port connected to
the ActiveWire card, which in turn is connected to the XDAT port. VPixx
requires that the ActiveWire driver, which comes with the card, be saved
in the Extensions folder.
- As Kevin Purdy (University of Derby, [log in to unmask]) points out,
the ASLŐs XDAT port is a parallel port. It has a 25-pin connector of which
only 16 are used, the others grounded. The ActiveWire card is a USB to 16-bit
parallel I/O board. To connect to the XDAT port, you just need a 25-pin
connector in which some pins are not used.
- The 16 bit XDAT port allows VPixx to write any number between 0 and
65535 (Binary 0 to 1111 1111 1111 1111) to it. In our version, VPixx uses
this capability to send the x,y coordinates of the center of stimulus
regions to the XDAT port, as well as other numbers that identify what is
being shown on the MacŐs screen, or other events in an experiment. In
this way, the experimenter can control each one of the I/O channels
independently. Kevin Purdy rightly identifies this as a necessary requirement
of a useful interface.
- The XDAT port is designed to accept TTL rather than USB inputs, but the
ASL system handles the difference in voltage very well.
- The connection from the Mac to the ActiveWire board is a simple USB
cable, but a special one has to be made in order to connect the board to
the XDAT port. The pin assignment is complicated and the ActiveWire
documentation includes irrelevant information pertaining only to PCs.
- Matlab is a powerful alternative to VPixx, but we have not explored its
use in interfacing the Mac to the ASL system.
I hope this information is helpful.
Esther G. Gonzalez, Ph. D. [log in to unmask]
Vision Science Research Program
Toronto Western Research Institute Tel: 416 603 5800, ext 2515
Toronto Western Hospital Fax: 416 603 5126
399 Bathurst Street
Toronto, Ontario M5T 2S8
Canada.
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