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(→Sending a an external trigger into the box)
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[http://www.cedrus.com/xid/a4_set_io_direction.htm Input/output lines have to be set] so that touching the wires together will register an event in the box
== Timing Precision ==
== Timing Precision ==
Revision as of 01:15, 6 February 2009
The button-box we have used for our "sensorimotor synchronisation" experiments is a Cedrus RB-730 response pad. It is on loan from Nenad and the IT office. It connects to the computer via a USB port, and is controlled by the computer as a serial port (or something like that). . To work, you must install a driver from the Cedrus website here:, and surprisingly, I also had to install a USB to serial port driver from a company called Keyspan: . I used the driver for their USA-28x product.
I had all of the little dip switches in back in the down position.
Basic programming with the box
To use the button-box with a python program, we put dipswitches 1 and 2 down so the box used XID (eXperimental Interface Device) communication protocol, which provides timestamps of button press times. Then you need these lines at the start of your code:
from psychopy import serial from struct import unpack
And to initialize the box:
s = serial.Serial('/dev/tty.usbserial-FT3ERKOD', 115200)
To erase all info in the box about previous button-presses:
To reset the button-box internal clock to 0:
Critically, the box records both the press and release of the button and sends a signal in exactly the same way (at least how it's set up now). To avoid this, you need to record both separately. In the attached file that shows how we tested the precision of the box relative to the computer clock, you can see how I did this. But basically, to check if a key has been pressed or released, you check this:
if s.inWaiting() > 0: ......
To get out the data about the button press or release:
response=s.read(6) formatted_response=unpack('<cBI', response) #'<' means little-endian order, 'c' means char, 'B' means unsigned char, 'I' means unsigned int time_in_msec = formatted_response #unsigned int, long?
Example code to increment a counter of presses and button-releases:
pressEventCodes = (48,80,112,144,176,208,240) #7 buttons from left to right releaseEventCodes=(32,64,96,128,160,192,224) #7 buttons from left to right eventType = formatted_response if (eventType in pressEventCodes): pressEvents += 1 elif (eventType in releaseEventCodes): releaseEvents += 1
Sam T tried to do an experiment collecting multiple responses per trial, but seemed that the Cedrus RB-730 measured multiple responses inaccurately, accumulating a timing error of roughly 500ms over a 20-tap-piloting trial.
Sending a an external trigger into the box
s.write('a10') #switch button box to general purpose mode i hope s.write('a11') #or switch button box to reflective mode i hope s.write('_a1'); #ask what mode you're in msg= s.read(6) # get answer of what mode it's in numWaiting= s.inWaiting(); print 'numWaiting=',numWaiting msg=s.read(numWaiting) print msg
Input/output lines have to be set so that touching the wires together will register an event in the box
Info about messaging format e.g. ASCII vs. binary
A file summarizing our tests of the precision of the timing is here media:buttonBoxPrecisionSummary.doc
For on-line, during-experiment timing verification we would like to trigger a photodetector by drawing on the screen at a known time, and have the photodetector press a button on the button box using what? an "Active Switch Closure" lead?Another way to verify timing and precision would be with this expensive device.