Eight Ways to Read Punched Tape

PAUL RUTTKAY, Burroughs Corp.

Here are eight basic techniques now in use to read the digital information from punched paper or plastic tapes. Three different mediums are used to sense a hole in the tape: air, mechanical fingers, and light. Only the method of producing an output signal that represents the presence of a hole is shown - design details of complete reading stations or additional electronics that may be necessary have been omitted.

AIR SENSING

	1. An old method of reading punched tape pneumatically, used widely in the player piano. Tapes are wide, of coarse pitch. The technique is simple, rugged, not very fast. When a hole in the tape opens the port in the metal bar, atmospheric pressure allows the spring to expand the bellows to give a direct mechanical push.
	2. The hot wire anemometer technique is the newest way being used to read punched tape. A small wire, about 0.0005 inch diameter by about 1/16th inch long, is supported in a duct and heated by current. When hole in tape opens, air flow changes wire temperature, hence resistance, providing a low-level electrical signal. Capable of very high reading rates.

MECHANICAL FINGERS

3. A conventional method of reading stepped tape. The tape is supported by a block with a hole for each channel. While the tape is in motion, bail holds contact levels up. When tape stops, fingers rest on tape. Where there is a hole, finger enters it, making electrical contact. This design has many variations, such as offset fingers for more contact room, double-contact blades for isolated channels, etc. Contact blade spring period can be critical. Cost moderate. Speed about 5 pitches per second. Tape about 1 inch wide, 8 channels plus spocket on 1/10th inch centers; other sizes can be used.

4. A conventional method of reading program tape. Tape can travel one direction only - reversal will damage contacts. A wide variety of contacts readily available from stock. Maximum speeds with stepped tape motion about 40 steps per second; with continuous tape motion, as high as 60 inches per second. Contact life several million cycles. As many as 50 channels can be placed on a 3-inch wide tape, each channel controlling one circuit. Sprocket pitch about 1/10 inch. Although contacts can operate small current relays, electronic equipment may be better. Mechanical cost moderate, electronic cost can vary.

LIGHT SENSING

	5. The basic technique for reading punched tape photoelectrically. Fast, reliable, quiet, with no moving parts. May be used with stepped or continuously moving tape. Uses one photoelectric cell or photomultiplier tube for each channel. Prisms, mirrors, light pipes, lenses, etc., used to spread light beams from channels to obtain room for photocells. Cost intermediate.
	6. Photoelectric system using one photocell and a mechanically driven scanning disc to give a serial output from the tape. The scanning disc replaces electronic equipment otherwise required to obtain serial output for single line transmission of the tape information.
	7. A silvered prism behind the punched tape reflects light from the exciter lamp back out through the same hole, to fall normally on the phototransistors. This same arrangement will also read opaque tape with printed dots by reflected light (the prism being inoperative). Approximately the same ranges and limitations on reading speeds and channel spacing as in the more common arrangement in Figure 8.
	8. An optional method for high-speed reading of 6, 7 or 8 channels (plus sprocket) using small phototransistors (about 0.080 inch diameter by 9/16 inch long) mounted in phenolic block directly over the center of the 1/10th inch spaced information channels. Reading speeds from about 7 to 1,000 characters per second, depending on tape drive mechanism. (Present phototransistors have a maximum limit of about 10 kc). Cost relatively expensive.