![]() The subject of this project, a 1967 Elliott TRM 250 paper tape reader features photoelectric sensing and a transport system that enables reading speeds of up to 250 characters/sec. Paper tape reader transport mechanisms required a major rethink. In extreme cases the sprocket acted like a circular saw resulting in no tape movement at all. After only a few starts, the sprocket holes in the paper became oval, ruining the alignment accuracy and leading to read errors. The trouble was once again our old friend inertia, this time in the form of the tape’s resistance to move as the sprocket began to turn. Those readers used an electric motor driving a sprocket wheel whose teeth engaged with the sync holes in the tape. Unfortunately, the early motorised tape transport systems then became the blocking factor for speeds much above 100 characters/sec. Hence the hole-sensing system ceased to be a limitation on reading speed. The solution was to use a strong light source combined with a set of electronic photosensors instead. The inertia of the spring contacts rapidly becomes a big problem when the tape is pulled through at much more than 10 characters/sec - the contact doesn't have enough time to drop through a hole and make an electrical connection before the tape moves on. In practice, some sort of motorised tape transport is more convenient, especially when a big program or data file is to be loaded into computer memory. In theory, the tape can be pulled through by hand because the sync channel ensures the data channels are ‘sampled’ at the right time. It is possible to build a working reader based on a row of spring-metal strips that drop through the holes making electrical contact with a metal plate on the other side of the tape. ![]() In order to read the tape, all you need is a row of hole sensors across the tape path with the smaller sync hole being used to indicate when a data character is properly lined up. ![]() Later, the width of ‘standard’ paper tape was increased to 1 in. wide able to accommodate a 5-bit code character plus a sprocket or sync hole across its width. Early teleprinter communication systems based on Baudot code used a narrow tape, 11/16 in. The basic principle of punched paper tape storage of digital data is hardly sophisticated: a strip of paper is punched with holes representing a logic 1 on a 0.1 in. Paper tape was ideal for small systems like the 903 before the invention of high-capacity semiconductor RAM and the floppy disk drive. There was no room for what we would call an Operating System (OS) controlling a mass storage device such as a mag tape unit. This 18-bit computer only possessed up to 16 kwords of working memory or RAM and could only run one program at a time. For the small desk-size machines such as the Elliott 903, paper tape offered a much cheaper and simpler alternative. The first commercial computers of the 1950s were able to use magnetic tape for mass storage, but mag tape units were very expensive and complex to drive. Punched paper tape had been used to control looms in the 18 th century. That doesn’t just mean spare-part availability, it includes features to aid fault location and easy access to case internals – screwed not glued. Repairability: In order to provide a lifetime somewhat longer than a year, any equipment containing moving parts must be designed for service and repair.Read on to see an example of how they did it. Admittedly, old computer equipment seems to have been designed for a 100-year lifetime of brutal misuse, and that turned out to be excessive. Nowadays, our throw-away society and its impact on dwindling global resources is looking decidedly ‘unsustainable’. Design to last: More seriously, in those days the phrase ‘built-in obsolescence’ had no place in the lexicon of engineering terms.Nostalgia: I’ve used one of these things for real while working as a student engineer in the 1970s and want to relive the whole punched paper tape ‘experience’.I’ve never seen it work and that bugs me. Because it’s there: indeed it is, sat on a shelf for the last thirty years gathering dust. ![]() OK, so what’s the point of interfacing a piece of 55-year-old storage technology to a modern computer system? Well…. The gadget on the right is a hand punch for ‘manual’ tape editing. This machine is a tape reader, specifically a 250 character/sec TRM250 from Elliott Automation. Before floppy disks, punched paper tape was used for program and data storage on most ‘desk’ computers up until the late 1970s.
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