Sunday, 26 May 2013

Octals and Octuples

Younger readers will be amused to hear that bits once were so precious that we did't group them profligately in fours. Rather, we arranged them sparingly in threes.

The four-bit groups conveniently were abbreviated into hexadecimal numbers, now widely used in computing. The three-bit groups didn't require counting to use the rather unnatural "numbers" A : F evoked in hexadecimal, as three binary digits could simply be written in base 8, using the familiar symbols 0 : 7. We called that numerical system "octal".

Way back then, computers were often using word lengths and internal architectures which were multiples of three - so the base 8 octal system was great. Today, our digital machines usually are built around parallel groups of bits of length divisible by four and NOT by three - so octal has fallen into a dusty corner of the dictionary. Pity.

All this talk of 8 arises because I have just made what the world calls an "octal" bi-directional level converter, extending my earlier "quad" level converter (as was mentioned here). The world shouldn't call it this - it should call it an "octuple level converter", but we all know how dumb the world is!

For those wishing to brush up on the correct names of things, take a look here. 

Enough pedantry - let's look at the new gadget.

It is built as an n-tuple extension (pedant !) of the circuit originally used in the development of my Si570 DDS module . I previously had made up a plug-in module (regular readers know I have a passion for these) hosting quadruple converters but there are times when four simply isn't enough. [There is nothing to forgive when the world calls this a "quad converter", as "quad" is an abbreviation of "quadruple" - not a misunderstanding of "quartal". Pedant !]

 Here's the copper layout, for a single-sided, homebrewed, octuple converter...


The two lines of 0.1 inch pitch pins are 0.6 inches apart (making the plug-in module like a "wide" DIP) but the overall board width isn't nearly as attractive as the commercial equivalents of this device, due to the constraints imposed by my clumsy, single-sided PCB. That's the price you have to pay for being a cheapskate! (Note, if you followed that "commercial equivalents" link, that Lady Ada doesn't fall into the "octal" misnomer trap, staying safe with "8-channel".) 

Here's the finished module, along with its quadruple smaller sibling...


You might be able to see from the photo above that my new octuple module is less than twice the "length" of the quad, which is why I made the 8-channel version. I've already made three quads (hard for me to say as the father of twin daughters), but they're wasteful of breadboard area (the modules - not the twins!). The new octuple squeezes more into less.

Note also there are a couple of unpopulated boards lurking in the background, waiting for the next time I feel strong enough to engage in some microscopic work. That work has been greatly eased by the purchase of some Xcelite 412 "anti-tweezers" (as Tom and I call them), which give me some chance of holding the tiny devices for long enough to solder them in place. Get yourself a pair - they work well.

The imminent arrival of new spectacles (currently being made up at the opticians) might even allow me to see the objects I'm trying to solder too!

 ...-.- de m0xpd


  1. I guess, there are jumper's on this Board to. Is it possible to share the overlay to show these jumpers.

    But surely good work done on these board.

    1. Thanks for your interest. I put the Eagle files on this page:

      There are links on the (bottom) side of the board - they are indicated in the Eagle files by a design for the bottom layer of copper - I make the board single sided and add these conductors as "jumpers".