Binary-coded information

Roman signaling

The ancient Romans developed a telegraph system using racks of flags on poles. Each rack had five poles and the racks were used in pairs. One letter of the alphabet was sent at a time from one signaling post to another. This was done by raising some flags on each rack (from one to five). To encode and decode the flags, the alphabet was written in a five-by-five grid with each letter in one of the squares. One rack represented a column of a grid and the other rack a row of a grid. Using the modern English alphabet as an example, the numbers 1 and 1 would represent the letter A. The numbers 1 and 2 would represent the letter B. The numbers 2 and 1 would represent the letter F, and so on. This telegraph could handle an alphabet with 25 letters and it would take several seconds send each letter.1

This is one of the earliest known examples of binary-coded information. It was binary—meaning to have two states—because each flag could be raised or not raised (two possibilities). This type of binary coding is very similar to how text information is encoded in computers today.

The American Standard Code for Information Interchange (ASCII) is used to encode text information in countries that use alphabets similar to the English alphabet. To see how ASCII is similar to Roman signaling, imagine the racks of flags having four flags each. Instead of raising from 1, 2, 3 or 4 flags, any flag can be up or down regardless of the other flags. With four flags, there are 16 possibilities for which flags are up and which are down. With two racks of four flags each, you could represent 16 rows and 16 columns of a table. This makes a table with 256 squares. This is enough to represent every letter of the alphabet (both upper and lower-case), the numerals 0 through 9, punctuation and common symbols. All of that uses less than half of the available squares in the table.

This is part of the ASCII code table. Flag-down is represented with a hyphen (-) and flag-up is represented with an upper-case I. Using
the ASCII code the flags are representing the Upper-case letter A.

No encoded information has any meaning until two or more people agree on what it means. In the case of the Roman signaling, the Roman signal corps had to know what the flags meant. In the case of ASCII, companies that operated Teleprinter services agreed upon the meaning of the code. When Teleprinters were attached to computers, ASCII became the de-facto code for computers to represent text.

Paul Revere

A famous use of binary-coded information involved Paul Revere. Preceding the battles of Lexington and Concord in the American Revolutionary War, Paul Revere was charged with warning John Hancock and Samuel Adams when the British army moved upon Lexington. On the night of April 18, 1775, when the armies were expected to move, Revere was stationed in Charleston where he could watch for a signal from the Old North Church in Boston. By previous agreement with Robert Newman, the sexton of the church, Newman would place a single lantern in the church tower if the army took an overland route. If the army crossed the Charles River instead, he would put two lanterns in the tower (one if by land, two if by sea).

Once again we have a binary code used to convey information. In this case, to make the code meaningful, Revere, Newman, William Dawes (Revere's backup rider, who took a different route in case Revere was captured) and some people in Charleston knew what the code meant. In this case, the two states consisted of lanterns, each of which could be shown or not shown.

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