BUS PIRATE: pin order and cable colors

The Bus Pirate pinout was supposed to be intuitive, except for one Arduino-like mistake.

Each protocol uses the same pin for similar functions, and the pins used are supposed to “walk” up the row. 1-Wire uses Master Out Slave In (MOSI, pin number 1). I2C uses MOSI (1) and CLOCK (2). UART uses MOSI (1) and Master In Slave Out (MISO, 3). SPI uses MOSI (1), CLOCK (2), MISO(3), and Chip Select (CS,4).

It should have been a nice intuitive progression, except for the unfortunate use of a hideous 2x5pin IDC connector. It’s hard to recall why we used this connector. Probably to keep the board small, provide a keyed connector, and likely because it was in our parts box.

The IDC connector was a poor choice. Not only is it ugly, probes end up using crappy ribbon cable that makes it look even worse. The connector was added without regard for the proper pin order, and used something hard to remember – MISO, CS, MOSI, CLK. Once it was loose in the wild we were stuck with that convention, and that’s how Bus Pirates have been produced for over 10 years!

Bus Pirate Ultra uses a 1x10pin connector called TJC8 2.54mm or 2543 by Chinese suppliers. It’s keyed, but also fits common 2.54mm “DuPont” connectors laying around most workshops. The pinout is DIO1 to DIO8, Vout, and Ground. Each protocol mode is in charge of naming the DIO pins, and the labels are displayed on the LCD above the connector.

We also want to give some thought to the color codes used on the pinout display and probe cables. Typically cable manufacturers stock wire in ten colors: red, orange, yellow, green, blue, purple, gray, white, black, and brown.

Goal one is to make the power and ground pins an intuitive color pair. Black and red, white and black, maybe even red and green. A color pair that a beginner in electronics has probably seen somewhere before.

Goal two is to follow the rainbow. Most people are probably familiar with ROY G BIV, the acronym for the order of colors in a rainbow. We want to start with red and progress downwards in a logical order so that pin one is instantly obvious, and the pins can be identified in a tangled messy probe cable without tracing them back to the source. Indigo and violet colored cable isn’t standard and the colors are hard to tell apart, so they’re usually substituted with purple and brown.

Pin 1 (DIO1) is assigned red. DIO2 to DIO7 are assigned orange, yellow, green, blue, purple and brown. With three colors remaining (gray, black, white), white and black are the obvious choice for the power/ground pin pair. Grey is assigned to DIO8.

Eventually we’ll need to choose good quality wire and some decent probe hooks for the cable. Sigrok, the open source logic analyzer software project, has a good overview of probe hook options. The rest of this week we’ll work on getting the firmware cleaned up and Ultra v1c board routed. As always, you can follow our latest progress in the forum.

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