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How to make your own parallel port response boxes

I’ve previously written about the importance of response hardware for doing timing-accurate experiments – in a nutshell, anything you connect via USB is likely to be sub-optimal,* because the operating system only polls (or looks for an input) on the USB port some of the time – in Windows the USB polling rate is 125Hz, or every 8ms.

So, for accurate timing of responses, we want to use something other than the USB port. There are various options – my personal favourite is to use the 25-pin parallel (or ‘printer’) port. Some newer desktop models unfortunately don’t have parallel ports anymore, as they’re largely obsolete for connecting peripherals, however any model older than two or three years should have one – and you generally don’t need a super-fast, up-to-date computer for running stimulus programs and collecting data.

I needed a couple of response boxes for a project recently, and decided to just make them up myself. I came across this fantastic little paper  (PDF) which describes a simple method of taking apart a couple of standard computer mice, and rewiring the switches into a parallel port plug – this gives you up to six buttons. The circuit diagram is really, really simple:

Circuit Diagram for a six-button, parallel port response box. Reproduced from Voss, Leonhart and Stahl (2007).

It’s just the switches, and a 100-ohm resistor for each one, wired up to different pins on the data register of the port, with a common ground (pin 18). Honestly, if you were being lazy, you could even just forget about the resistors and it would still work fine. I decided not to take apart any mice, but just to use some buttons I bought off-the-shelf, as I only needed two for each box. Getting the right buttons is really important for this kind of thing – you want them to be a decent size, and have a good clicky-action, without being too difficult to depress. I also got some small plastic boxes, some multi-core cable (I used standard network cable as it’s quite stiff and robust, but almost anything will do), and some parallel port plugs. You can buy everything you need from Maplin or Radio Spares (Radio Shack, if you’re in the US) for about £10-15. I just drilled some holes in the boxes fairly roughly and secured the buttons there with a dab of epoxy resin, but you can get as fancy as you want in that respect.

The only really tricky bit is deciding which pins on the parallel port you want to wire your switches up to. This will largely be determined by which pins the software you’re going to use can read from. Psychology software like Inquisit or E-prime is able to read inputs from pins 2-9 on the data register (see below diagram) but it’s worth doing a bit of reading about the different pins on the parallel port and what they’re used for. A good place to start is here. Probably what you want to do is use one of the data pins for one pole of each switch, and wire the other pole to a common ground pin, as in the above diagram.

Parallel port pin diagram showing the three registers (data, status and control) and the ground pins (in green)

So there you have it – the most simple, inexpensive and accurate solution for recording response times in cognitive experiments. If you’re at all handy with a soldering iron you can probably knock up a couple of these in half an hour or so. If you’ve never done any electronics or soldering before, then this would be an ideal first project to get started with! This was my finished article:

Nice, huh? Happy soldering! TTFN.

*Not quite true – some of the expensive button-boxes you can buy from psychology software companies are USB, but have their own electronics inside them to get around this and time things accurately.

The HP DM1-3200SA – the perfect student computer?

A while ago I did a post on how to choose a computer for studying but shied away from making any specific recommendations. This is partly because reviews of specific machines would date pretty quickly, and I didn’t really want to make this site into a load of hardware reviews.* However I recently got my hands on one of these little beauties:

The Hewlett Packard DM1-3200SA. Oooh... sexy.

It’s a Hewlett Packard Pavilion DM1-3200SA laptop. It’s basically like a netbook-on-steroids – small enough to carry around really easily, but with enough power under the hood to get the job done. The screen is 11.6 inches – this might be a little small for a lot of people, but personally I find the 12-13″ range to be the sweet-spot in terms of the compromise between portability and usefulness; it’s also only 1.6kg in weight. It’s got an AMD 1.6Ghz dual-core processor and 3GB of RAM (up to 1.4Gb of which can be allocated to video RAM) so it can handle pretty much anything you want with grace and ease. The 320Gb hard drive and 1366×768 screen are pretty standard features, but not bad at all. HP reckons you can get up to 9.5 hours from the battery, but 7-8 would probably be more realistic – still excellent though. The keyboard is near-as-dammit full-size, and pretty comfortable for typing. My only complaint about using it was the trackpad was a little small (understandably, it’s a small computer after all) and the multi-touch implementation (two-finger scrolling, etc.) was a bit unresponsive. It’s possible this will be fixed in a future driver update though, and to be fair I’m comparing it to the multi-touch trackpad on my MacBook Pro which is best-in-class. Actually, having any kind of multi-touch trackpad on a laptop at this price point is pretty impressive.

Which brings me to arguably its best feature – the price. It’s available now from Play.com and a few other places for £319 – an absolute honest-to-goodness, stone-cold bloody bargain! Particularly when you consider there’s a Sony model also available at the moment which has an almost identical specification, but is more than twice the price. So, for my money, I reckon this laptop might be the perfect student computer – light and with enough battery life to make lugging it to a whole day of lectures possible, but powerful enough to handle chewing through editing videos of drunken nights out if necessary.

Here’s the full spec sheet on the HP site, and here’s a more in-depth review from the gadget gurus at Engadget (they gave it 8 out of 10). One last thing – confusingly, there’s an older HP model also called the DM1 with a silver keyboard – don’t get that one, the spec isn’t quite as good – make sure you get one with the black keyboard! Also, if you do get one, the first thing you should do is spend a couple of hours uninstalling all the crapware that HP puts on it as standard – Media players/editors, and Norton trials etc. Ugh – wish manufacturers would stop doing that.

Happy laptopping! TTFN

*Although if any manufacturer wants to send me any of their new sexy gear to review, that would make me very happy. No? Oh well… worth a try.

TTL-Technology – Getting Things Talking to Each Other

A common issue in psychology research is getting various bits of hardware linked up in the right way. To take a simple example, I mentioned in this post that you can build a simple response box for your experiments, but (deliberately) neglected to mention what happens with the signals from the response box at the computer’s end. How does the computer ‘know’ that your participant has pressed one of the keys? The answer is TTL (Transistor-Transistor-Logic) signals. TTL signalling was invented back in 1961, and intended as a standard way for a piece of electronic equipment to send bipolar logical signals (i.e. 1=on and 0=off) to another piece of equipment. Long before ethernet, the internet or TCP/IP, this was how computers communicated with each other. TTL-type signals can be presented through most computer’s parallel and serial ports. Read the rest of this entry