The following is "abstracted" (ripped off) from "IBM's Early Computers" chapter 12 pages 490-495, MIT Press. Bashe, Pugh, et al - An excellent IBMcentric computer history of the era.

This is a background to the 1403 printer -

I have removed about 50% of the text and all images to seem less like a thief. My only excuse for this offence is an attempt to make the world a better place through history.

I hope MIT Press is tolerant and lots of folks buy the book. Amazon has 'em 2nd hand

Chapter 12


Significant in the report of the committee on intermediate-speed printing is its recognition of certain basic advantages in the horizontally moving chain as a carrier of printing type slugs. Competitive printers that had appeared in the mid-1950s for use with computers were based on a drum or stack of type wheels, much like those of the 407 accounting machine, the drum with its axis horizontal and rotating at constant speed.

Companies that introduced such printers included Anelex, Shepard Laboratories, Potter Instrument, and Sperry Rand. Capable of speeds up to 600 lpm, they represented an embarrassment to IBM, which could offer only a choice between its 150 lpm 407-derived printer and its still-unsuccessful wire printer. Drum printers were used in computer systems of Honeywell, Burroughs, General Electric, Bendix, Philco, Datamatic, and Ramo-Wooldridge, as well as those of Sperry Rand. Like IBM's proposed chain device, they were all back-printing, "on-the-fly" printers (printers in which the type-carrying elements were in constant motion).

On-the-fly drum printers presented several problems. One of the most serious was the difficulty of maintaining horizontal alignment of printed characters produced from type moving at high speed in a direction perpendicular to the print line. It was necessary to move the type wheels or drum at high speed in order to present the full set of characters to the paper within the time available-for example, considerably less then one-tenth of a second for 600 lpm printing (allowing also for line-spacing the paper). But the smallest variations in hammer-flight time caused the hammer to strike at an instant when the character was not in precisely the desired position.

The eye is particularly sensitive to vertical misalignment of characters on a horizontal line (waviness), and the timing problem increased noticeably as attempts were made to obtain operation at speeds over 500 lpm or so. Other disadvantages of drum printers related directly to the need to provide a full font of characters for each column to be printed. At a spacing of ten characters per inch along the line, "shadow printing" resulted from the proximity of adjacent-column characters to the one being printed. Moreover, since the tooling for type was quite expensive, the cost of the machine was greatly increased by the need for 120 sets of type to print a page of 120 columns.

The chain was clearly worth pursuing, since it was much less affected by any of these problems. The type slugs (fastened to a steel band, forming the "chain") moved horizontally; imprecision of hammer flight resulted in variations of character spacing along the line, a defect to which the human eye seems relatively tolerant. Shadow printing could, for all practical purposes, be eliminated by the adequate horizontal spacing of individual characters of the type. (Since each character would at some instant be presented to any given column of the page, there was no need for a type character on the chain for each column.) The cost of type was thus radically reduced; in theory, a single set of characters would serve for all columns. For reasons of timing and to keep the speed of the chain within desired limits, the full font was repeated for a total of five appearances on the chain.

Chain printing offered certain advantages beyond those suggested by the problems inherent in drum-type printers. The one most immediately apparent was the ability to incorporate a larger character set, at reduced printing speed, with fewer than five copies of the font on the chain; or to print faster by including more than five copies of a smaller (for example, purely numeric) font thereon.

Even the uniqueness of the chain among competitive products was an attractive aspect of the device. There was also interest fairly early in the project in taking advantage of the lower cost, lower weight, and greater simplicity of the chain to design operator-interchangeable print chain "cartridges."

By July 1957, Dayger had made the decision to gamble on the chain. As a new, high-speed, fairly complex mechanical device, the printer posed many serious problems on its way to announcement in 1959. One was breakage of the type-bearing chain, under the stress of frequent hammer blows, at its high speed of 90 inches per second. Another problem was simply print quality. Although better than the drum, the chain was still an on-the-fly printer, and with rapidly moving type it proved difficult to approach the standard established by the 407 for quality of printing. These problems were solved through the persistent efforts of the engineering team, and at least one alternative to the chain - a train, with type slugs pushed along a track - which had been conceived to solve the chain breakage problem, was used instead in a subsequent, faster printer.

By the end of 1957, it had been decided to energize the hammers by electromagnets rather than by electrostatic clutches, since the latter had not proved reliable. The hydraulically operated paper carnage, developed for the wire printer, was substituted for an electrical stepping motor on which some effort had been expended. During 1958, a rather major change in the program took place when the chain printer was adopted as a unit of the emerging 1401 system. Shortly after Branscomb had agreed to add magnetic tape to the 1401, he raised the question of whether the chain printer needed a control unit, which (as in the case of the wire printer) had been planned for tape-to-printer operation. As it turned out, the 1401 was so well suited to (and in such large part designed for) the editing of data and the control of printing that it was itself the most economical and generally desirable "control unit" that could be devised for the printer. Thus, it was in the 1401 processor that IBM introduced its first "intelligent" (programmable) control unit.

The chain printer was announced in October 1959 as a unit of the IBM 1401 data processing system and was given the type designation 1403 - available in Models 1 and 2 with 100 and 132 columns, respectively. ... The printer had a maximum speed of 600 lpm and printed at 10 characters per inch horizontally from a font of 48 characters. The machine could be set by the operator to print either 6 or 8 lines per inch, and it was able to skip groups of 8 or more lines at more than twice normal line-spacing speed. The 240 characters, representing 5 sets of the font around the chain, required 120 type elements each carrying the raised type for two characters. (See figure 12.3.)

Regions of the core memory of the 1401 processor were designated for use as input-output buffer memory, with two regions of 80 characters each reserved for the card reader-punch and one of 132 characters for the printer. (These regions were usable as ordinary main memory when input-output instructions were not being executed.) As cash character of type came into alignment with a column, the buffer memory location corresponding to that column had to be interrogated.

If the character stored there agreed with the character on the chain (as indicated by a "print compare counter" in the 1401 processing unit), the hammer was actuated by current in its electromagnet from the transistor drive circuit for that column. Thus, the entire buffer memory had to be scanned once for every passage of the time interval required for a single character alignment in each column; the total number of such scans was 48, the number of characters in the font. (See figure 12.4.)

Because of the high speed of the chain (90 inches per second), good print quality required that the hammer flight time be held to within a few percent of a constant value. The requirement was met, and the chain printer became an unqualified success. Uniquely an IBM product for many years, it set a standard for print quality in its speed class and added immeasurably to the popularity of the 1401 system. Early in 1961, the 1403 was enhanced by the announcement of two new features: an operator-interchangeable chain "cartridge," to permit convenient selection of alternate fonts; and the capability of higher- speed printing-up to 1285 lpm-in purely numeric mode.

... (Discussion of other types of current printers, some not successful)

Meanwhile, there were many improvements on the original chain printer. Chief among these was the train printer, designated 1403 Model 3, which made use of the principle considered earlier as a remedy for chain breakage. The chain was eliminated, and the type elements were guided around a track-their rear surfaces toothed to engage driving gears and thus push each other around the track. Announced in 1963, this printer was capable of printing alphanumeric data at 1100 lpm; later enhancements provided for the printing at still higher speeds of characters in a "preferred" set. The 1403-3 was still a "back printer," but the chain-train family was to continue for many more years, ultimately producing a front-printing machine that operated at 2000 lpm." After a few years, the train printer design was adopted by numerous other manufacturers and became by far the most widely used high-speed printer in the data processing industry.