Drum Printers

Computers are fast. Once the idea of using valves or semiconductors to perform arithmetic had been conceived there was no great difficulty making machines that would perform thousands of arithmetic operations per second. There was a bit of a leap to get to the 75,000 operations per second of machines like the SAGE AN/FSQ-7 but by the 1960s that sort of speed was becoming commonplace.

The SAGE computers were doing fairly complicated operations, transforming radar data into visual display information. Some computer operations like calculating a view of an object from coordinates are sufficiently complicated that it still stretches even the most powerful machines. Others like sorting a long list of words are moderately stretching. So there has always been a demand for more computer power. However a lot of computer operations, adding a margin to a pricelist, deducting shipments from stock impose very little burden on that expensive CPU, they hit limits because the computer can't get information out quickly enough. This sort of process is sometimes said to be I/O bound.

In the days before video terminals, PCs and little local printers there was one way to get information out of a computer and that was to print it.   Computers could have teleprinters and modified typewriters attached to them, but the fastest devices such as the IBM Selectric typewriter produced a mere 13 characters per second. That wouldn't be much good if you wanted to process the weeks stock levels for a supermarket or the billing run for an electricity supplier.

Spoolers are one way to deal with the problem. The original idea of a spooler was that the data stream out of the computer wasn't sent to the printer, it was sent to a fast peripheral like a tape drive. Then the tape drive was switched to drive the printer, so only a relatively inexpensive tape drive was tried up waiting for the printer.

Drum printers were one way to get fairly fast print - at a fairly high cost.

Drum printers were presumably an obvious product.   Various kinds of number-wheel printers had been around for years. The teleprinter type cylinder and Selectric golf-ball also have similarities. However getting the technology right would have been difficult. The drum itself was often a big casting. The hammers need to work very rapidly to give an unsmudged imprint on the paper.

Drum

Drum printers contain a metal drum with the character set embossed round its circumference in each of 132 character positions. The drum rotates at moderate speed on one side of the paper. On the other side of the page a hammer bank with a hammer in each of 132 1/10th inch wide character positions waits for the required character. The hammer activates just as the character comes into position. If the drum rotates at 1200 rpm it will take a 20th of a second to print a line and since a typical page has about 50 lines it will take about 3 seconds to cover it with print. So fast printers have been around for a long time.

Typical computer forms like invoices don't have print on every horizontal row, in fact whilst an invoice might have 50 lines most are fewer than ten lines long. The printer might be able to take advantage of non-print areas by accelerating the paper feed in non-print areas.

A typical drum would have 64 characters around it, upper case, numbers and some punctuation. this would require a diameter of about 10 inches (characters are 1/6th of an inch high) A metal drum 10.6 inches in circumference and 14 inches long with 132 rings of characters around it - 8448 characters - is rather difficult to make.

Drums could pose a problem; if every column of letter around the drum were in the same position then a horizontal line of hyphens or asterisks would mean all the hammers firing at once which would be a huge drain on the power supply. A drum might therefore have its characters offset a position or so in each column. It may need to avoid the pattern ABCDEFG etc as well because ASCII swirl is a common test pattern.

Hammer Bank

When drum printers were common the 132 hammer drive circuits were quite expensive as well. Each hammer-drive is basically a voice-coil mechanism so that it responds quickly and that means the a printer contains 132 amplifiers - in some ways challenging rock-bands for amplification power. (OK its different). Printer were often available with half the number of hammers arranged on a shuttle so that the hammers would print every odd position then move and print every even position, halving the number of hammers and shuttling them resulted in a printer that worked at half speed so there were 300, 600 and 1200 line per minute versions.

The electronics for a drum printer was expensive at the time but was basically a one-line memory, a character position counter on the drum and a set of comparators driving the hammer-bank.

Ribbon

Drum printers typically used ribbons not unlike those used for a typewriter, cloth impregnated with sticky ink. They were often similar except in one respect - they were 13.5 inches wide - more like a roller towel than a ribbon. A couple of motors either side of the hammer-bank wound the ribbon back and forth. When print faded unacceptably the operator changed it. The ribbon's size wasn't essential to the design, a narrow ribbon would have worked. But these printers were intended to produce hundreds of thousands of pages per month.

Drum printers appeared in the late 1950s and were commonplace in computer installations in the 1960s and early '70s. The main manufacturer was Dataproducts.

Dataproducts ended drum printer production in the late 1970s in favour of band printers. The printers were still in use into the 1990s but spares prices were at ridiculous levels. So far as is known there are no drum printers working outside of museums.

The problem with drum printers seems to be the manufacturing cost for the drum, which is a big piece of precision work. Most drums were upper-case only. Getting the full ASCII character set would have needed a 16 inch diameter drum and the machine would still have one font and no graphics. The cost of making a drum was high; one accident and a drum costing thousands of pounds would be damaged.

Print quality could be good, within the limitations that it's a fixed space font with no lower case. However very precise hammer timing is needed to get good print quality and that means an engineer periodically working on each print position to remove dust and ensure precise timing - something that is also needed with band printers. Print quality often wasn't all that great.

Band printers do a similar job to drum printers. The band printer has a very similar hammer bank. The print element is just a steel band, making it smaller, lighter and easier both to make and replace  .