Timing issues - Serial Response Box

[David McFarlane]

Valerio,

In this context, "baud" refers to bits-per-second 
while transmitting data (see 
http://en.wikipedia.org/wiki/Baud ).  "CPS" here 
stands for characters-per-second, which in this 
case means bytes-per-second.  So every 1000/800 = 
1.25 ms the SRBox sends one byte of data (each of 
8 bits indicating the state of one switch), with 
1000/19200 = 52 µs for transmitting each bit within that byte.

For our purpose, the baud rate is irrelevant as 
long as it is fast enough to handle the CPS, so 
the CPS is all we care about.  Note that, once 
started, the SRBox transmits data continuously at 
800 CPS, which means that the latency from 
response to transmission will be 0-1.25 ms 
(depending on when the response happens within 
that 1.25 ms window).  While that much 
uncertainty results from the design alone, other latencies may also occur.

-----
David McFarlane
E-Prime training 
online:  http://psychology.msu.edu/Workshops_Courses/eprime.aspx
Twitter:  @EPrimeMaster (https://twitter.com/EPrimeMaster)

/----
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At 7/30/2013 09:35 AM Tuesday, Vaaal wrote:
>I was reading your post and I realized that this part is not clear to me:
>"CPS-setting, which defines the number of bytes 
>are send to the computer per second. This 
>defines the resolution data flow (i.e. refreshes 
>per second) and is 800 by default?"
>What is the difference between this value and 
>the baud rate? When can I change this last value?
>
>On Tuesday, 30 July 2013 13:12:23 UTC+1, Paul Groot wrote:
>Hi Valerio,
>
>You best option with eprime is to use the serial 
>button box on a *NATIVE* RS232 port. I.e., keep 
>away from USB to RS232 converters if timing 
>accuracy is critical. RS232 ports use a baud 
>rate to specify throughput (in this case 
>configurable to 9600 or 19k2, if I remember 
>correctly). Specific to the serial response box 
>is the so called CPS-setting, which defines the 
>number of bytes are send to the computer per 
>second. This defines the resolution data flow 
>(i.e. refreshes per second) and is 800 by 
>default. Native RS232 ports are interrupt 
>driven, so latencies should be minimal. Although 
>PST doesn't give any details about this, I think 
>that EPrime has some built-in intelligence to 
>increase the accuracy of the reported RT's. (The 
>data stream has a fixed CPS-rate, so it is 
>possible to fix for small 'hiccups' in the 
>interrupt handling.) Such a mechanism could also 
>minimize RT-variance when using a USB converter.
>
>If USB is your only option, you should have a look at the following page:
><http://www.pstnet.com/support/kb.asp?TopicID=1835>http://www.pstnet.com/support/kb.asp?TopicID=1835
>
>Also, I personally think that the 1ms 
>requirements are overrated for almost all experiments...
>
>cheers
>Paul
>
>
>On 30 July 2013 13:42, Vaaal <valerio.... at gmail.com> wrote:
>Hello to everyone,
>
>I am struggling to get accurate reaction time 
>without starting put my hand in engineering 
>stuff, since I don't have the required knowledge to do like that.
>I would like to buy a Serial Response Box, but 
>the technical details in the page are really not 
>enough. I am particularly worried about the serial port connection.
>I know that the pooling rate of the usb port is 
>normally 125 hz, that is 8 milliseconds. I am 
>able to overclock that value in order to have a 
>pooling rate of 1000Hz, which is 1 milliseconds, 
>which is quite fine. But, I am still not sure of 
>the accuracy of my measurement, just because 
>using a normal keyboard can add some latency in 
>the process of transmitting information.  This 
>is why I am willing to buy a professional device.
>
>The problem is that I don't know the technical 
>specification of a serial port. There is 
>anything similar to a pooling rate of a serial 
>port? If it is so, how much it is, and there is 
>any way to overclock it? Basically how much accurate is the response box?
>
>The other option is to use DirectIN High Speed 
>Button-Box here 
><http://www.empirisoft.com/directin.aspx>http://www.empirisoft.com/directin.aspx, 
>which actually uses a USB port that I can 
>overclock. BUT since I use E-Prime I would like 
>to buy a piece of hardware that is designed to 
>be used with E-Prime, just to avoid unnecessary headache in the future.
>
>So generally my question is: what are the limit 
>and the advantages of having a serial port? Do 
>you think that the Serial Response Box is really 
>1 ms accurate? Do you use any other equipment?
>
>Thank you for your reply,
>Valerio

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