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Calculators: Handheld: Rockwell Memory 20R

Rockwell 20R


Size (approx): 77mm (max) x 155mm x 22mm (max)  (w,h,d)
Weight 134g excluding batteries


9.0V DC, 1 x PP3 size batteries.   Also accepts an adaptor (Part No. 331 R03-001 for the US 110V-120V) through a top end socket on the left. I found a 4.5V centre positive worked ok.
Case: Sturdy stippled brown plastic case with light beige keyboard surround.  This surround is printed with dark brown text and lines, which carries on to the bottom front edge for the model name.  Red display cover shows a clear view of the numbers.  Squashy but positive lozenge-shaped light brown keys work well with a rocking action and soft click.  The raised Rockwell logo and two vertical edges are painted silver and have survived very well on this example.  The on/off sliding switch is on the right hand side midway up.
Display: 8 digit red LED with bubble lens and ninth digit for minus and memory indication 
Features: Four function with percentages and six function memory
Age: 1974-6
Manufacturer: Rockwell International, Microelectronic Product Division.   Made in Great Britain. Serial No. 8A20662.  Also, Assembled in Mexico, US and foreign parts. Serial No. 164287 - original price sticker $39.95..


Lovely typical USA Rockwell calculator with sound logic and clear display.   Both the silver trim and any lettering on the keyboard surround have a tendency to rub off so finding good examples is not easy.   Original case is padded brown plastic with flip top cover and tuck-in tab at the front.  The logic is good but let down by the pseudo fixed decimal bug and odd right-hand minus sign.

Components: 1 x cpu: Rockwell A5300PA 7517 (week 17 of 1975) 42 pin staggered DIL, metal capped
1 x 8+1 digit LED unit with bubble lens
0 x transistors
0 x diodes
10 x resistors
2 x capacitors
1 x voltage converter Astec AA1225A
Boards: Keyboard unit is attached to the main cpu board via eight pins into sockets on the bottom and side of the main board.  Later models solders these connections as it reduced manufacturing costs.
Construction: Remove the four screws on the back.  Gently push out the front keyboard which hinges up from the display surround.  Pop off the display surround by using gently pushing a blunt object through the three slots on the rear.  The area above the display has a piece of matt grey card so that light does not show through the upper part of the red plastic cover.

Logic comments: (C) cancels an incorrect numerical entry, pressing a second time (C) clears the calculator
Divide by zero shows "0.E" and is recoverable using (C)
Negative numbers are shown with a minus sign on the far right (ninth) digit
Input overflow is suppressed, typing in nine digits ignores the last one
Overflow shows the result shifted by 10^8 with "E" in the far right (ninth) digit and is recoverable using (C).  Using "E" means you cannot tell if the overflow is positive or negative until (C) is pressed.
A change sign function can be invoked by a double (-)(-) entry
Memory functions are called up by two button presses; i.e. (M) (+) will add the main register contents to the memory
Memory storage is flagged by the far right (ninth) digit's decimal point
Overflow in memory is recoverable using (C)
It has automatic constant on all functions
It suffers the pseudo fixed decimal bug: key in (6)(.)(0)(0)(+)(1) to give and answer of "7.00" which remains a fixed two decimal answer until more are required or you use multiply or divide

Rockwell 20R Manual

Small but thick manual measuring 128mm x 68mm has colour cover (including inside diagram) and 48 additional one colour black printed pages.  Back states "Rockwell International ... where science gets down to business".  Ref 2520-D-59-R2-407, Litho in the USA.  Stated copyright of 1974.

Excellent in depth manual with advert in the middle for the whole range that year; 10R, 20R, 30R, 51R, 61R and 80R.

My sample has a retailer's stamp on the back; Boots the Chemist Ltd, 49-53 High Street, Dundee.  It also has a hand-written date of 15/1/76 which may mean nothing.

Rockwell 20R Box

Rockwell 20R Box side

The box measures approx. 202mm x 87mm x 35mm and has a small inner former that looks like it was supplied with a 9V battery.  It is three colour: black and reddish brown with a beige background.

The front design shows a businessman, student, (token) woman and (also token) coloured doctor.  The reverse has a similar design but with a construction worker.  The sides boldly proclaim "The Answer".  Funny, I thought is was 42.

"Rockwell International, Microelectronic Product Division, Made in Great Britain. (c) Rockwell International Corporation 1974."

and another version:

"Rockwell International, Microelectronic Division, Anaheim, California 92803.  Assembled in Mexico.  Of U.S. and Foreign Parts. (c) Rockwell International Corporation 1974. Printed in U.S.A."