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Calculators: Handheld: Rockwell "The 24MS"

Size (approx): 70mm (max) x 142mm x 12mm (max)  (w,h,d)
Weight 144g including batteries


Internal rechargeable batteries (three 1.5V cells).    Also accepts adaptor (undefined) through bottom end socket on the right. My model held its charge well - I used a 4.5V adaptor centre positive.
Case: Super slim solid calculator that was to be the future of calculator design principles for the forthcoming LCD models.  The blue plastic display filter set slightly raised but flat.  Three piece case; back and front of quite thick brushed aluminium, and central black plastic frame.  Subtle key colours and bluish display filter are lovely.  The keys are short travel with a loud click and wobble quite a bit.  Printed logo and key line has survived well on this model.
Display: 8 digit blue VFD with a ninth digit for minus sign and memory flags.
Features: Four function with percentages and four function memory. Change sign, square root and a key (the bi-directional arrow) for seeing the ninth digit of a number.
Age: 1976
Manufacturer: Rockwell International, Microelectronic Product Division, Anaheim California 92803.   Made in Taiwan, Republic of China. Serial number 169381-046D


Slim, smooth and shiny.  Difficult to find one in perfect condition as the vast expanses of brushed aluminium can damage easily.  Logically OK with useful extra precision for a basic calculator but let down by negative square roots and the pseudo fixed decimal bug. The original cover is a soft black plastic "wallet" construction.  When open the left side has an acetate lower pocket for the calculator with a pen holder and a pocket underneath for the manual.  The right side has a 5-leaf plastic credit card holder and a three-section full length paper notepad.  Appears to be a version of the Rockwell 24K.

Components: 1 x cpu: Rockwell A5901CA 7615 (week 15 of 1976) 42 pin staggers DIL , 0.6" width
1 x 9 digit single tube VFD display Itron FG95A Japan 
0 x transistors
2 x diodes
2 x resistors
1 x capacitors
1 x transformer Best Choice DCV 50560D 7610
2 x SIL resistor arrays
Boards: Main cpu board (ST-222-25B) is attached to the keyboard assembly via a 12-way clear coated ribbon cable.  It is held in place by four plastic lugs.  The keyboard assembly is about the same size as the cpu board.
Construction: Remove the two screws on the back, one at the top, the second hidden under the serial number sticker.  The rear then comes away easily.  The metal rear panel is coated inside with plastic as it comes into contact with the main board.

Logic comments: (CE/C) cancels an incorrect numerical entry, pressing a second time clears the calculator
It has automatic constant on all functions
Overflows show  the answer and all eight decimal points alight and is recoverable by use of (CE/C). 
Divide by zero shows "" and is recoverable by use of (CE/C). 
Minus sign is shown on the far left (ninth) digit thereby allowing full eight digit negative numbers
Input overflow is suppressed, typing in nine digits ignores the last one
The sign change function can be used in mid number entry
Memory storage is flagged by the decimal point in the far left (ninth) digit
Negative square root are allowed and result in a negative number
The arrow key is used to see the ninth (least significant digit of an answer; i.e. 22/7 is 3.1428571, pressing (<->) displays "7", the eight digit after the decimal point.  Pressing the key again goes back to the normal display.  This does not operate for square roots (zero is shown).
It suffers the pseudo fixed decimal bug: key in (1)(+)(.)(0)(0)(0)(=) to give "1.000" which remains a fixed three digit number until more digits are needed, the result is zero or you multiply / divide.

The left image shows the main cpu board in place, covering the keyboard. At the top you can see the back of the display unit, soldered to the top of the board.

Notice that the blue capacitor about midway up on the left sits in its own board cut-out; in order that the calculator can be so slim!

The lower section is taken up by the large rechargeable button cells, (removed on this example) the square block of the transformer and the adapter socket.

The display filter (under the display in this image) is fixed to the calculator by melting it - looks like a soldering iron with a circular tip has been pressed into the edge.

This image shows the front of the main cpu board.

Notice the sparse component count to minimise the costs in a now very aggressive calculator market.  The holes show that the board was designed to take more components, perhaps an earlier version or a different model.

Rockwell main chips are easily recognised from their staggered pin configuration.