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Calculators: Handheld: Rockwell 21R Memory

Size (approx):

75mm (max) x 155mm x 25mm (max)  (w,h,d)
Weight 140g excluding batteries.
Power: 9V DC, 1 x PP3 size battery.  Also accepts adapter/charger (undefined) through a top side socket on the left.  The on/off switch is on the left hand side. midway up.
Case: Brick-shaped calculator with brown matt plastic base and red coloured top dominated by smooth cream plastic keyboard surround.  Raised Logo on the front is painted silver (slightly worn on this example) I'm not sure if the two rails were ever painted as in other Rockwell's.  Small red display filter shows the digits clearly. Squashy, lozenge shaped keys are very positive to use and brown orange colour scheme quite pleasant.  
Display: 8 digit red LED display.  A ninth digit (on the far right) is reserved for the minus sign and memory store flag.
Features: Four function with  percentages and six-function memory.
Age: 1975
Manufacturer: Rockwell International, Microelectronic Product Division. Made in Great Britain.  Serial No. 3B10799
Comments: Lovely typical Rockwell calculator with sound logic and quirky right hand minus sign.  Both the silver trim and any lettering on the keyboard surround have a tendency to rub off so finding good examples is not easy.  Built to last - and they have.  Original case is padded brown plastic with flip top cover and tuck-in tab at the front. It is embossed with the word Rockwell.  Unusually, made in Great Britain.  Nicely designed with few additional components compared to those of a similar age.

Components: 1 x cpu: A5300PD 7528 (week 28 of 1975) 42 pin DIL 
1 x single unit 9-digit bubble lens red LED
0 x transistors
0 x diodes
12 x resistors
2 x capacitors
1 x smoothing choke assembly (CD-42001 327R05 TDK Japan)
Boards: Keyboard assembly is attached to the main board by the back screws.  Nine pins self located into sockets on the main board.  Main CPU board ref 20167D57
Construction: Remove the four screws on the back.  Gently pull out the front keyboard which hinges up from the display surround.  Pop off the display surround by pulling it upwards. Main assembly can be removed by slightly twisting away from the on/off switch.

Logic comments: The Clear key (CE/C) is used once to clear an input number a second time to completely reset the calculator
Overflow on the input of a number is suppressed
The constant function is automatic on all four main operators
Memory use is by two keys i.e. to add to a memory press "M" "M+".  Sophisticated memory functions with "M divide" and "M Multiply" not normally seen.
Memory storage is flagged by the ninth (far right) digit decimal point alight
The minus sign is shown in the ninth (far right digit) thereby allowing eight digit negative numbers.
A divide by zero shows "0.E". Overflow error is shown by "E" in the ninth digit and is recoverable by using (CE)