ASIC and FPGA designer Sven Anderson continues his "Retrospective" series with reflections on the (r)evolution he's seen with regard to computer memories.
(Editor's note: following a "retrospective" in which he described how he entered electronics, blogger Sven Anderson next described his first job as an electrical engineer. Now, Sven continues his saga with reflections on the (r)evolution he's seen with regard to computer memories...)
In almost every kind of electronic equipment we buy today, there is memory in the form of SRAM and/or flash memory. Following Moore's law, memories have doubled in size every second year. When Intel introduced the 1103 1Kbit dynamic RAM in 1971, it cost $20. Today, we can buy a 4Gbit SDRAM for the same price.
This article will show why memory is so important in today's electronic products, and how I got involved in testing memories in 1976. But before we go there, let's look back and see where it all started. We will take the telephone switch as an example (I have been working for Ericsson, the telecommunications company, for more than 30 years), but we could have chosen almost any other electronic product.
The telephone switch
The first electro-mechanical telephone exchanges didn't need any memory. The number called directly controlled the switch movements and nothing had to be remembered. When the call was finished, the connection was terminated. One of the first electro-mechanical switches was called the Strowger switch after the inventor Almon Strowger. This was patented as early as 1892 and looked like the following:
The Ericsson 500-switch.
The first telephone exchange to use the 500-switching system in regular operation was opened in 1923. Eight years later, Ericsson had delivered about 100 stations with a total of more than 350,000 lines. By 1974, 4.8 million lines using this system were in operation in public telephone stations.
As its name implies, the main characteristic of an automatic telephone system based on the 500-switch is that the mechanical switches have a capacity of 500 lines. Lines enter the switch in 25 multiple frames with wires that are arranged vertically in the switch rack in groups of 20 lines. The selectors are driven by electric motors, and the wiper arm can move in both a circular movement to select the correct contact field and a radial movement to find the correct contact wire within that field.
With 25 multiple frames and 20 groups of wires in each switch, a selection can thus be made among 500 lines. The internal workings are based on a "register" that stores the selected numbers (current pulses), which are then used to control the operation of the switch. This means that this type of switch did not require very much memory at all.
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