'real time' issues

[Paul Gr]

Hi Leisha,

The serial port is fine. Although in most cases the parallel printer port 
and the game port are also usable. This is the case when the buttons are 
connected 'directly' to the input lines of the port (not the serial Rx/Tx 
transmission lines) and a 'Port' device is used in EPrime to scan the 
corresponding IO register. (See also 
http://www.psy.vu.nl/download/menu/index.html)

Note that the PST Serial Response Box uses a different technique to send the 
button states to the serial port. In this case a sequence of bytes is 
transferred to the Rx line of the serial port. It is not very likely that 
your button box uses this serial protocol because it requires additional 
electronics and software.

best,
paul



>From: Leisha Wharfield <leisha at decisionresearch.org>
>To: Paul Gr <pauls_postbus at hotmail.com>
>CC: eprime at mail.talkbank.org
>Subject: Re: 'real time' issues
>Date: Fri, 06 Jan 2006 17:32:23 -0800
>
>So your advice is to stick with the serial port? Thanks for the wikipedia 
>link.
>
>Leisha
>
>Paul Gr wrote:
>
>>
>>hello Leisha,
>>
>>A small note on ‘real time’ issues:
>>
>>Since marketing people like to use ‘sexy’ words to promote additional 
>>functionality or suggest superior specifications, we have to be a bit 
>>careful how to interpret the words ‘real time’ correctly. The actual 
>>meaning depends on the context: Most hardware and software engineers use 
>>the words ‘real time’ to indicate that the time allowed to execute some 
>>kind of operation is limited to a known maximum. Some engineers also 
>>differentiate between so called hard and soft real time. In hard real time 
>>systems it is an absolute system failure if the real time criteria are not 
>>met. (Ie. the air bag in a car is definitely a hard real time system.) In 
>>soft real time systems the real time criteria are less strict. This is the 
>>case with EPrime where it is sufficient to keep track of such timing 
>>failures. (See also http://en.wikipedia.org/wiki/Real_time) Anyway, this 
>>kind of real time is about the response time limits of a system, not its 
>>execution speed or bandwidth.
>>
>>Real time streaming media protocols on the other hand, are said to play 
>>media in ‘real time’ when it is possible to play the media stream at the 
>>correct speed (ie. not in slow motion.) So, in this case marketing people 
>>and engineers refer to the large bandwidth or execution speed of the 
>>system, not maximum response latency. In other words: even though Firewire 
>>and USB standards support large bandwidths, this does not mean that they 
>>support short (<1ms) transmission delays. Furthermore, the complex 
>>protocol stacks that are used to implement those communication systems 
>>make it almost impossible to realize short latencies. But even if you are 
>>willing to accept timing errors, it is not very trivial to develop 
>>hardware that connects through USB or Firewire. RS232 and parallel ports 
>>are much easier to use in both hard- and software.
>>
>>There are some real time systems that use another common connection 
>>between de external hardware and the computer: Ethernet. A (dedicated) 
>>Ethernet connection supports high bandwidth and short transmission 
>>latencies. However, as with USB and Firewire, it is not very easy to 
>>develop hard- and software for such a connection. (Note that PST writes it 
>>will support some kind of Network Socket Device in version 2.)
>>
>>Paul Groot
>>
>>
>>
>>>From: Leisha Wharfield <leisha at decisionresearch.org>
>>>To: Alison Wright <alison.wright at kcl.ac.uk>, 'E-Prime' 
>>><eprime at mail.talkbank.org>
>>>Subject: Laptop replies + new issue #5
>>>Date: Thu, 05 Jan 2006 10:33:00 -0800
>>>
>>...
>>
>>>
>>>5. A final issue to add: We use input devices that were made for our RT 
>>>experiments. They are large switches or buttons, one for each hand, that 
>>>can be pushed easily and give a solid clicking sound. They are wired into 
>>>a serial device because when we began this series of RT computer 
>>>experiments, we were told that the serial port was the cleanest, quickest 
>>>way to get to the processor and we are measuring very fine time 
>>>differences. Now serial ports are hard to come by on laptops, and when I 
>>>read about new laptop connectors I find that the now-standard FireWire 
>>>IEEE 1394 is a port that was designed for using a computer in "real 
>>>time," that is, to play 3D games or to interact with streaming video. 
>>>Wouldn't this now be the best port for measuring very fine time 
>>>differences? Or is serial still the best way to go?
>>>
>>>I'll post another synopsis of replies received.
>>>
>>>Thanks for all your help.
>>>
>>>Leisha
>>>
>>
>>
>>
>
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>
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