Machining Monday

Punched Tape: Why Your G-Code Still Starts With a Percent Sign

You have typed a percent sign at the top of a program a hundred times and never thought about it. It is a leftover. Before machines had memory, programs came in on a roll of paper tape, and the percent sign marked where the tape began and ended.

The percent sign is not the only piece left over. The block, the end of block and the habit of thinking about a program as a stream fed in one line at a time all came from tape. Here is where it came from and why it stuck.

A Mill That Read Paper

In the U.S., numerical control grew out of an Air Force contract. According to ASM International, the U.S. Air Force contracted the Parsons Corporation of Traverse City, Michigan, to develop a better way to machine complex parts, and Parsons subcontracted the control to the MIT Servomechanisms Laboratory.

In 1952 MIT showed the result: a three-axis Cincinnati Milling Machine Company Hydro-Tel mill under numerical control. As Make summarized a Scientific American account, its program came in on perforated paper tape similar to the tape used in teletype systems. A unit called the director read the tape and sent commands to three servomechanisms on the machine. About 10 feet of tape ran the mill for roughly an hour.

That set the pattern. The program was not in the machine. It was on the tape, and the machine did what the holes said.

One Inch, Eight Channels

The tape that took over NC was simple. East Tennessee State University's NC programming notes describe it: one inch wide, eight channels of holes across, with a row of small sprocket holes that pull the tape through the reader. Rows are spaced a tenth of an inch apart, and each row is one character.

A photoelectric reader shone light through the tape onto eight photocells, one per channel. A hole let light through, and the pattern of light told the control which character it was reading. Thin paper could let light through where there was no hole, which is why shops moved to opaque black paper or Mylar tape.

Odd, Even and the Parity Error

There were two codes. The EIA's RS-244 code used odd parity, meaning every character had an odd number of holes, and it marked the end of each block with a hole in channel 8. The ASCII-based RS-358 code, published by the Electronic Industries Association, used even parity and added lowercase letters and symbols such as parentheses.

Parity was the error check. If a hole was missing or a speck punched an extra one, the count came out wrong, and the control stopped with a parity error instead of running a bad command. The ETSU notes give the old trick for telling the codes apart: count the holes in a few rows. All odd is EIA. All even is ASCII.

The Percent Sign

Here's the part most machinists never think about: the program you send to a modern control is still formatted like a tape.

The percent sign at the top and bottom marks the beginning and end of the program. On tape, it told the control where the program started and stopped. Many modern controls do not need it, as one G-code library's documentation notes, but you will still find it at the top and bottom of most programs.

The block is the same story. Each line of a program is a block because a tape program was read as blocks, each ending in an end-of-block character. When you see a program as a column of lines read top to bottom, you are seeing a tape laid out on a screen.

Writing the Tape

Somebody had to punch the holes. In MIT's 1959 Science Reporter program on APT, short for Automatically Programmed Tools, the workflow ran like this: a draftsman made the drawing, a part programmer wrote statements in APT, a keypunch operator put them on cards, and an IBM 704 computer at MIT turned them into punched tape for the machine.

Douglas T. Ross, who led the lab's computer applications group, explained the language on the broadcast. MIT said one aircraft plant had cut the programming time for one part from 200 man-hours to five. The output of all that work was still a roll of tape.

How Tape Faded Out

Tape lasted a long time. Many shops were still punching it into the 1980s, according to the Wikipedia history of direct numerical control, long after the computer industry had dropped it. Controls with more memory and serial ports gradually let programs come straight from a computer.

For older controls built around a tape reader, the fix was a behind-the-tape reader. It plugs in between the reader and the control and imitates the tape reader's signals, so the control thinks it is reading tape while the program actually comes from a computer. When a control did not have the memory for a long program, the computer could send it a block at a time, which shops call drip feeding.

The Tape Is Still There

You do not see paper on the shop floor anymore, but you still write for it. The percent sign, the block and the end of block all came from a strip of paper an inch wide.

Every program you post is still a tape. It just never gets punched.

Frequently Asked Questions