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4004 – Signal Boxes by Ploy – Setter’s Blog

Posted by Listen With Others on 14 Nov 2008


Listener No 4004    Signal Boxes  by  Ploy

I’d been aware for some time that Chambers 2003 contained entries for five logic circuit elements or “gates” (AND, NAND, NOR, NOT, and OR), with clear explanations of their functions.  Having spent my working life in electronics research, this appealed to me as a promising theme for a Listener crossword.  If it were not for simple electronic items such as these, many of the everyday devices we now take for granted would not exist.


My idea was that the grid would contain a logic circuit built up from a number of gates, with the solver having to select a self-consistent set of binary signal values (0 or 1) to complete the grid.  I also wanted to include references to George Boole (logic) and Claude Shannon (circuits), both key figures in this subject.  I decided to use one of each of the five gates in Chambers, and exploit some of the O’s and I’s in Boole, logic, Shannon, and circuit as signal values zero and one.  Surprisingly quickly I arrived at a suitable basic layout containing the key components, both electronic and lexical.  This had ten O/I clashes, five of them resolved by thematic names/words, the remaining five being uniquely determined by the nature of the gates.  Any prospect of having a symmetrical bar pattern was ruled out by the amount of thematic material I was trying to include.  The grid fill was then completed using Ross Beresford’s excellent Tea & Sympathy software, and took eight attempts.

The choice of title and preamble wording was intended to (misleadingly!) suggest a railway connection, my previous Listener puzzle, “Travelling Light”, having had such a theme.  In the event, however, a fair amount of time passed before “Signal Boxes” was published, though a number of solvers did comment on an initial, apparent connection.

Clue writing was straightforward, there being no gimmicks to take care of, and I was able to introduce a scattering of railway terminology in them, just for fun!

While I was fairly certain that most solvers would make progress by initially noticing “logic” or “Boole” in the grid, I appreciated that there would be much less familiarity with the name “Shannon”.  I was relieved to learn later that there is an entry for Claude Shannon in Chambers Biographical Dictionary, so any complaints about obscurity would not be on very strong grounds.

I was delighted to find that overall the puzzle received a very good reception, and that nobody seemed to mind that a crossword required the solver to perform some mathematical processing.

The most common error was, perhaps predictably, incorrect assignment of one or more of the five “free” signal values.  Opinion was divided as to whether the final stage was straightforward or not, as is often the case with puzzles where there is some “business” to attend to once the grid has been filled.  Comments on the clues showed that they were regarded as being on the easy side by Listener standards.

Digital circuits exploit other logic gates apart from the five I used, but in fairness I stuck to those with entries in Chambers 2003.  A solver’s remark on the absence of XOR in the puzzle prompted me to check the new edition of Chambers (2008), when I found that the two “missing” gates, XOR and XNOR, have now been added.  The puzzle might have looked different had I been setting it in 2009!

Phil Lloyd  (Ploy)

10th November 2008


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