Help the electronics newb

January 13, 2007 1:01 PM

Help an electronics newb with his quest to make a cable and learn a little bit about electronics on the way.

So long story short, I bricked my Linksys WRT54G and I'd like to revive it. Doing so requires hooking up a JTAG cable to a 12 pin header on the circuit board. There are plans here on how to make a cheap and short unbuffered JTAG cable. I'd like to make one but have no electronics experience. So my question is, what parts do I need? Where do I get them?

For instance, the cable will need 4 100ohm resistors. Through some googling I found some people seemed to like digi-key for electronic stuff so I went over there and started looking at resistors. Do I want a carbon resistor...ceramic...etc? I figured carbon since it seemed to be the most common, and filtered to 100ohms. Now I'm faced with the question of what all the other categories mean. The different packages, series, and tolerances I have no idea what to make of. For instance, is 20%..25% tolerance "good enough" for this project? How do I tell? Side adjustment / top adjustment? Someone please make some sense of this stuff for me.

Preferably, someone explain to me what I need and the easiest/cheapest place to get them online way of getting them. I already have access to a soldering iron and basic tools (screw drivers, wire cutters, etc).

posted by crypticgeek to technology (13 comments total)

Digi-Key is normally good, but they charge a $5 fee if you order less than $25 worth of parts.
posted by drezdn at 1:18 PM on January 13, 2007

Actually I just realized what I was looking at earlier were "adjustable resistors"...oops. Yeah, I found standard 1/4 Watt 100ohm resistors on there just fine. How do I find the other stuff?
posted by crypticgeek at 1:19 PM on January 13, 2007

I like jameco for smaller parts orders, and because they have pictures of most of their items.
posted by drezdn at 1:19 PM on January 13, 2007

I would try these, resistors, the plans don't say anything about tolerance, so I wouldn't be too concerned. If you're just looking to build that single thing, you should be able to get by with just a soldering iron, solder and a razor blade (for stripping the wire).
posted by drezdn at 1:27 PM on January 13, 2007

Yes, Jameco is great when you want to explore. Digikey and Mouser are good when you know exactly what you want. (Though I do recommend getting Digikey to send you a physical catalog for bathroom reading. :)

As for as which (fixed) resistors you need, usually you want just the cheapest resistors you can find. The main choices then are physical type (with wire leads or surface mount) and the amount of power they can handle (1/8 W, 1/4 W, 1/2 W etc.). For most hobby things, 1/4 Watt resistors are the way to go.

Note that for small cheap things like resistors, you'll be buying them in large-ish quantities. For instance, at Jameco, you get 100 count bag of 100 ohm resistors for a dollar.
posted by todbot at 1:33 PM on January 13, 2007

100 ohms is a very small resistance. In this application the resistors help to reduce the noise. Tolerance isn't really a big deal - anything under 1000 ohm would probably do you just fine.
posted by three blind mice at 2:06 PM on January 13, 2007

Resistor tolerance probably will not matter one bit here, you could probably get away with anything in the hundreds of ohms range.

This PDF has pictures.

For the parallel port you probably want a 25 pin d-sub male connector. For the jtag connector, you'll need a standard male header (don't worry about the size, you can just clip it to the desired length if it's too long) and then use any old ribbon cable with the standard two-row black connector that you find in a junk bin as long as it's equal to or greater the number of pins needed. Or you can order a 12 pin flat ribbon connector and make your own.

That jtag cable you're building is a real hack, it's certainly at the edge of what would work so be sure to heed the advice to keep it at 6" or less.
posted by Rhomboid at 2:21 PM on January 13, 2007

Err, and I was assuming there that you'd have to solider on the header to the PCB. If it's already there then you can skip that, just find a matching ribbon cable header.
posted by Rhomboid at 2:23 PM on January 13, 2007

Thanks rhomboid. As I don't really have any junk to pick through laying around here, could you tell me if the below would work for ribbon cable / connector?
posted by crypticgeek at 2:40 PM on January 13, 2007

That looks fine, except that if you were buying more than a couple I'd say get the jameco house brand (p/n 153947) but that's probably neither here nor there. For crimping this to the ribbon cable you're supposed to use a specialized crimper tool, which I would suggest you buy if you were working with the 40 or 50 pin size, but for 14 pins you can probably get by with pliars and some practice (or better, a small vice.) Definitely order a couple spares if you've never crimped an IDC type connector -- it's very simple but it never hurts to have an extra if you foul up.
posted by Rhomboid at 2:54 PM on January 13, 2007

Resistors

Wikipedia has a pretty good resistor article. Particularly, resistors have non-ideal characteristics. The various types, like carbon and metal-film, exist because each is closer to ideal in some area (or combination of areas). This resistor tutorial goes into much more detail on the specifics of the different types available. Also, here is a great great capacitor tutorial.

If anything, for your application, select for low inductance first. However, the standard hobby choices are 5% carbon film and 1% metal film, and they will both suit your application just fine.

What you really need is a local electronic component store, because shipping and handling will be far more expensive than the actual parts.

Connectors

You can get solderless D-sub and IDC connectors for use with ribbon cable. They are far more expensive than the choices Rhomboid made, but they are easier.
posted by Chuckles at 5:47 PM on January 13, 2007

Rhomboid already figured out that all you need is the 12-pin female IDC connector (I've only selected that site because the pictures are good, and it was easy to google). Note that the female crimp on solderless IDCs for ribbon cable are much cheaper than the male counterpart.
posted by Chuckles at 5:52 PM on January 13, 2007

Well the reason for not just using an IDC d-sub connector instead of soldier cup was that he was going to soldier the resistors inline, which would be a lot more difficult that way, as he'd have to cut and splice into the middle of the ribbon cable.
posted by Rhomboid at 4:42 AM on January 14, 2007