what's the difference between LFLine Fee and NL New line 4448

Paul Gilmartin wrote, in part:

Yes. (Sort of.) Well, that's UNIX's problem.

The characters CR and LF made their existence felt back when page printers were made which used a character code based on the five-unit code of Donald Murray, with some modifications. This code is often called "Baudot", but while Emile Baudot's code was a five-unit code, it was still very different from ITA-2, the code used for Telex, for RTTY, and for TDD.

And CR returned the printing element to the beginning of a line, while LF advanced the paper by one line.

That's just what those characters did. When ASCII came into being, CR and LF did exactly the same thing on the Teletype Model 33 Keyboard Send-Recieve and Automatic Send-Receive and Receive-Only teletypewriters.

People started connecting ASCII terminals from the Teletype Corporation (now no longer gracing Skokie, Illinois with its presence) and other manufacturers to computers. Usually, when they did so, they followed the convention that the user would press the "Carriage Return" key to signal that he (or she) was finished typing a line of data. The computer would then echo first a carriage return code, and then, while the carriage was busy moving along the width of the platen, it would use the wasted time to transmit a line feed.

It did not need to transmit nulls as well, but if a less intelligent device than a computer was at the other end - say a paper tape reader - then a few of those would tend to be included at the end of each line also.

For some bizarre reason, the default UNIX convention - which, fortunately, can be changed depending on one's terminal type - was to have the user type a "Line Feed" rather than a "Carriage Return" for this purpose. This complicated software somewhat, since now it wasn't possible to first echo every character typed, and then figure out what to do extra later. Also, not all ASCII terminals have a "Line Feed" key, or, at least, they don't have one in a convenient place. (Note that, even "in UNIX environments", terminals did what they always did with a line feed character, and thus the computer running the UNIX operating system still had to echo a carriage return as well as a line feed to the terminal - after receiving, as was its wont, a line feed from the user.)

In the mean time, at the other end of the world... IBM took its world-famous Selectric typewriter, and from it, it crafted the 2741 terminal. (Actually, before the 2741, there was the 1050, and the IO Selectric, and so on.)

With a Teletype Model 33, you might have a switch to change it from "full duplex" to "half duplex" operation. In "full duplex" mode, what you typed on the keyboard just went to the computer; if you were to see what you typed, the computer would have to send the same characters back to you. In "half duplex" mode, what you typed was also sent directly to the printing part of the terminal, so the computer at the other end didn't need to - and shouldn't, of course - bother.

The 2741, being made from a typewriter, only ran in half duplex mode. The carriage return key on a 2741 did exactly what the carriage return key on a typewriter - as opposed to a Teletype Model 19 - does, it both caused the printing element to return to the left margin, and it indexed the paper. So the character transmitted to the computer when it was pressed was called NL, or "New Line". As well, with the 2741, when the computer wished to type, it locked the keyboard on the terminal, and then, when it was finished, it unlocked it again. Since the computer wasn't doing the typing when the keyboard was not locked, the code to lock the keyboard was the same as that for one of the *printable* characters.

John Savard