Update, Saturday July 4th, 2009: All preorders are closed.

A probe cable makes it easy to connect the Bus Pirate to a circuit and get hacking. Good test clips make quick connections on cramped PCBs without causing short circuits. We made two cables for the Bus Pirate v2, keep reading for an overview of our designs and list of part suppliers.

Friday, July 3, 2009 is the last day to pre-order a Bus Pirate. There’s only two days left to get your own Bus Pirate, fully assembled and shipped worldwide, for only $30.

Overview

We use these cables to connect the Bus Pirate’s I/O pins to a microchip or test circuit. A cable consists of a 2×5 connector, a cable, and some kind of attachable probe like an alligator clip or test hook.

The gray cable (top) is a ‘junk box’ cable, we recycled it from scrap parts and old computer hardware. The ‘expensive’ cable (bottom) uses high quality and special-order parts.

2×5pin female connector

The Bus Pirate’s I/O header is two rows of five 0.1″ spaced pins. We used a 2×5 arrangement because 2×5pin female ribbon cable connectors are common and cheap. We decided against a single row of 10 pins because the connector is an expensive specialty item.

The pin names are shown above, and are silk-screened on the bottom of the PCB. See the Bus Pirate page for detailed descriptions of each pin function.

The junk box cable uses a 2×5pin female connector from an old PC ISA card.

The expensive cable uses a black connector with a reinforced cable holder. Mouser has gray connectors ($0.69) and black connectors ($1.15).

Ribbon cable connectors have internal pins that pierce the cable when the top part is pressed onto the bottom part.

Ribbon cable

Standard 2×5pin female connectors attach to 0.05″ 10-strand ribbon cable. The wire thickness is usually 22, 24, or 26 AWG. We think 12inches (30cm) is a useful length that doesn’t get in the way.

Grey ribbon cable is pretty common. We salvaged a piece from an old computer connector, you might get lucky and find one with a 2×5 connector already attached.

A color coded cable makes it easy to identify each connection. DigiKey has 5 foot sections ($3.03), Mouser has it by the foot ($1.16, $1.19).

Ribbon cable is cheap and readily available, but it tends to tangle and kink. A really nice probe could use a ribbon cable stub attached to thicker test leads.

Test clips

Test clips are the most important part of the cable. They have to be easy to position, and maintain contact with the circuit. Alligator clips work, but there’s a lot of exposed metal that can create short circuits. Professional test clips have a grabber that retracts into the probe leaving less metal exposed.

Alligator clips

The junk box cable has alligator clip probes, we pulled them off test leads like these (40 leads for $12). You could also use loose red and black clips (20 for $2.30).

Remember to put the rubber housing on the cable before soldering the wire to the alligator clip, it won’t go on later. In the photos you can see that some of our covers are cut to fit over the front of the clip because we forgot.

Round test hooks

This is the classic, round-bodied test hook. These are great for grabbing onto 0.1″ pin headers, wires, and the leads of through-hole components. The hooks are usually too big to use with surface mount components, and the round body makes it hard to fit more than a few in a small space.

Test hooks are easy to position. Squeeze the probe to extend a single metal hook, grab something, then release. The hook retracts into the body of the probe, securing it in place and preventing short circuits.

Most hooks come apart by pulling the top away from the body. Put the test lead through the hole in the cap and solder it to the metal tab. Push the halves together when the joint is cool.

DigiKey ($17.26) and Fry’s ($14.95) have multi-colored hooks in sets of 10. Deal Extreme has dirt-cheap 10 packs of yellow ($2.30)  and black ($2.33) hooks, but the reviews say the quality matches the price so buy extra (via [haku]).

Flat test tweezers

Tweezer-probes are great for clipping onto the legs of through-hole, surface mount, and many smaller chips. They usually have a flat body so they fit better in tight spaces than round hook probes.

This type of probe has tiny tweezers instead of a hook. Accidental short circuits are rare because there’s so little exposed metal when the tweezers retract.

Most tweezer-probes pull apart and have a metal solder tab inside. Run a cable strand through the hole in the cap, solder it to the metal tab, and then press the halves back together.

Tweezer quality varies dramatically among brands, we’ve used no-name probes that bend easily or don’t grip well. The X- series micro-hooks from E-Z-Hook are the Cadillac of tweezer-probes, we first used the XKM version that comes with the Saleae Logic. They’re intended to fit specialty test leads, but it’s easy to solder a wire to them instead. About $2 each, available directly from the E-Z-Hook website.

Conclusion

We highly recommend a cable with hook or tweezer-probes for secure connections without causing shorts. The right probe depends on the parts you use. Round test hooks work best with through-hole parts and wires. Flat test tweezers attach well to small, surface mount chips.

Please share any additional part sources in the comments. We did our best to provide a variety of sources, but there’s going to be some great places we’ve missed.

Friday, July 3, 2009 is the last day to pre-order a Bus Pirate. There’s only two days left to get your own Bus Pirate, fully assembled and shipped worldwide, for only $30.