Ladder Diagram?

     It's incredibly geeky, and (at least in part) out of date: what's a "ladder diagram" or a "control ladder," anyway?

     To make matters worse, in the ones I know best, they laid the "ladder" on its side.

     But it's not intentionally arcane: in the days when all the control and logic in electronic devices was done with relays, a convention arose of drawing the schematic diagram of it with the power supply (AC or DC, one side grounded or not) as a pair of lines at the right and left, with all the "stuff" in between: switches, relay coils, relay contacts, lights and buzzers, electrical motors or controlled equipment, looking something like strange rungs of a ladder, with the power supply lines as the rails.*  The order of operations generally ran from top to bottom, and left to right for each "rung."

     It makes for simple, clear diagrams, or as simple and clear as they get for that sort of thing, and over time, it evolved its own specialized set of symbols, which came to be used interchangeably with the more usual set in some applications.†  RCA tended to rotate the diagrams, so the rails were at the top and bottom of a long blueprint and the order of the "rungs" ran from left to right -- this is a better fit to a long drawing table.  By the 1970s, it was starting to run up against advances in technology: the fifteen foot drawing I mentioned yesterday had a gap in it, with dotted lines leading to a box marked "Solid State Logic."  A card cage tucked behind a meter panel held eight or ten plug-in cards, loaded with early optical isolators, solid-state relays, and logic gates built with discrete resistors, diodes and transistors‡ (2N4401 and 2N4402, if memory serves, an NPN/PNP pair) -- and those circuits were not at all friendly to ladder-type diagrams.  They had their own set of drawings.  It was the beginning of the end -- and the start of twisty, hard-to-follow multi-page schematics that were drawn as a guide to manufacturing and passed along to the techs who had to service the equipment with little thought given to clarity. 

     You'll still find ladder logic in industrial controls, and ladder-type diagrams are used as a kind of programming language to set up the workflow for programmable logic controllers; but in my line of work, it's a lost art, as dusty as Ancient Egyptian hieroglyphs.    
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* One of several convergent uses of the term "rail" in connection with power supply.  The primary source of this is probably the live third rail of an electric train -- but any power bus in electrical or electronic applications can be casually referred to as a "rail," and the people who work with it will know the meaning.
 
† This, in turn, eventually resulted in a change to the general set of electronic symbols: prior to WW II, capacitors were represented by a pair of parallel lines with connections coming off the sides, -||-.  Ladder logic used a similar symbol for normally-open relay contacts: -| |-.   Context was usually sufficient to tell them apart -- until wartime needs meant a huge number of workers and techs had to learn in a hurry!  After the war, capacitors were given a curved line for one side: -|(-.  Since many of them are round in one dimension or another, it made sense.  And just to make things interesting, there's an entirely different set of symbols for relay contacts in general electronics, which have been used all along.
 
‡ Troubleshooting in the solid state logic was difficult -- a clamp-on card extender would let you move one card at a time to an easier-to-reach position, but it was awkward and didn't always make good electrical contact.  I started adding "state monitor" LEDs to the cards as I worked on them and eventually, opening up the cover and looking at the little red lights would usually localize problems to one card -- or eliminate them as the cause and point to any of the several dozen relays in the control ladder.