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Calculators: Handheld: Rockwell "the 24K"

Size (approx): 70mm (max) x 142mm x 12mm (max)  (w,h,d)
Weight 140g 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 No.554662-056D


I think this and some of the contemporary Sinclair models pointed to the direction that calculators would go.  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.  There appears to be a replica revision called the Rockwell 24MS.

Components: 1 x cpu: Rockwell A5901CA 7615 (week 15 of 1976) 42 pin DIL 
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 NEC DCV 50560P
2 x SIL resistor arrays
Boards: Main cpu board (ST-222-25B) holds the display and attached to the keyboard board via a 12-way clear coated ribbon cable.  The latter board is about the same size as the cpu board with ref number (5901) 222-25.
Construction: Remove the two screws on the back, one at the top, the second hidden under the serial number sticker.  The case then falls apart.  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
Input overflow is suppressed, keying a ninth digit is ignored
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). 
Negative numbers are shown with a minus sign in the far left (ninth) digit thereby allowing full eight digit negative numbers
Negative square root are allowed and result in a negative number
The sign change function can be used before, during and after a number entry
Memory storage is flagged by the decimal point in the far left (ninth) digit
Overflow in memory is flagged and retains the original 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 this slim!

The lower section is taken up by the large rechargeable button cells, 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.