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Calculators: Display Technology

Vacuum Fluorescent Displays were used from the late 1960s right up until the very early 1980s on calculators.  They are still in common use today in Hi-Fis, car dashes and microwaves.  Usually available in hues from strong blue, through blue-green to strong green and occasionally orange.  VFDs work just like a CRT TV; small areas of phosphorous glow when hit by electrons.  The cathode sits in front of the digits so if you look carefully you can see a grid or several very thin wires.  On ageing displays I have even seen the front wires glow a soft orange like a light filament.  Early units needed a lot of power so you will see an array of transistors and resistors to drive each digit.  Later ones used less power but still achieved a bright display that is typical of all VFDs.
Early 1970s single-digit VFD Tubes

The first VFD units were individual tubes for each digit.  This particular example has a metal mounting block that also served as the display border.  Underneath was a mass of wires as you can imagine.  Notice also the extra symbol tube with "E" for error, negative bar symbol and dot used for memory store indication.  It wasn't long before the inconvenience and cost of wiring this caused the development of a single tube display.

1973 Panaplex Displays
Easily recognised by the lovely high-brightness orange glow.  These displays are a bit of a hybrid idea: neon gas in a sealed glass sandwich.  Eats loads of power - but have proved to be very reliable.
1974 "Bath-tub" eight digit VFD
This odd display type is easily identified by the back glass section being shaped like a bath-tub (hence the name I have given it).  This example is an ISE DP89A made in Japan.  Notice how the flat glass front has its own black painted boarder.  It is from a Rockwell 51R dated late 1974.

As it is just an eight digit module they had to add an extra single tube VFD symbol cluster for a negative bar sign and a large dot (used in this case as an error indicator).

Mid 1970s multi-digit VFD Round Faced Tube
6 digit round VFD This particular example has six digits.  All the connectors are at one end, the other end of the tube being held in place by a plastic block.

Six digit displays were used in the early 1970s for handheld calculators to keep the battery consumption down.  In order to provide a useful calculator many used a "display shift" idea where a 12-digit result could be viewed.


This display has nine digits with connectors either end.  The pip-top can be clearly seen on the right hand side where the vacuum seal was made.  A variety of mounting techniques were used such as plastic brackets, front pushing clips and sometimes even the tension on the host of connecting wires.

Nine digit displays were more common; the extra digit being used to display overflow, negative and memory indicators.

Late 1970s Multi-digit VFD Flat Faced tube
This vacuum display is common of the late 1970s in that the single glass tube has a flat fronted face.  It has a more PCB-compliant mounting arrangement than the previous round faced tubes with a linear pin configuration.  However, you can still see the "pip-top" that was used to "vacuum-and-seal" the glass tube.

This particular example is from the Sharp EL-210 calculator of 1980, just about the last date these displays were used as LCDs (Liquid Crystal Displays) took over.

Gas Discharge Displays
Typified by their large orange digits this Gas Discharge Display unit is from the Prinztronic 80 of 1973.  It is a Flandipak CD802 by Matsushita and date coded 3D1 (April 1973).

These display is basically a neon light. The inert gas is excited by high voltages to give an orange glow.  However, a more modern version combines this idea with coloured phosphors to make plasma TVs. 

This particular display is eight digit so requires an extra LED indicator for negative number indication (it also has a similar hole to the right to allow for secondary function indication - like a memory).

Early 1970s single digit LED single module displays
From 1973 this display consists of eight separate seven segment display units.   It is from the Taiwan-made Teco TE8053 and came with a purple filter and linear magnifying lens to increase the readability.  

The board is labelled "Kuotiang".  Also notice it also has a separate single LED to the far left for negative indication.

Mid 1970s single digit LED single module displays
From 1975 this display consists of nine separate seven segment display units.  The lack of lens gives a flatter, wider angle of view and the red tinted packages add to the display contrast.  With this type, each segment appears as several dots in a line.

This particular example is labelled SM09I4A and is from a ER Memory Master calculator.  Notice it also has a separate single LED to the far left for memory indication.


Mid 1970s multi-digit LED multiple module displays
From 1974 this Casio red LED display is made up of two four-digit display modules with built in red plastic bubble lenses.   This is rather an odd configuration as most calculators were nine digit - to allow an extra digit for error, negative and memory indication.  To compensate for this problem, the board holds two isolated red LEDs.
Mid 1970s multi-digit LED single module displays
From 1974 this red LED display is a Bowmar Optostic M2 unit.  It consists of nine separate seven segment display units.  The lack of lens gives a flatter, wider angle of view and the red tinted packages add to the display contrast.

This Light Emitting Diode (LED) display has nine seven segment digits integrated into a single package.  Notice the plastic front which is a single clear moulding with bubble-type lenses for each digit.  These displays were multiplexed to save power and connections; they flash if waved before the eyes!

This is the most common form of 8+1 digit LED display.  Suffers from a very narrow angle of view and not very bright display.  Sometimes an extra linear magnifying front was placed on top of this unit to add to the size.