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Calculators: Handheld: Rockwell 31R (aka Slide Rule Memory) (type 2)

Size (approx): 75mm (max) x 155mm x 22mm (max)  (w,h,d)
Weight 132g excluding batteries.


9.0V DC, 1 x PP3 battery.  It accepts an adaptor/charger (undefined though centre negative) through a socket on the top side to the far left. There is an on/off sliding switch on the left hand side about half way up.  
Case: Sturdy stippled dark brown plastic case with light beige keyboard surround.  There is an oversized stippled red inset display filter with a smooth window. It has a raised Rockwell logo which along with the front edges is painted silver.  Lovely lozenge-shaped keys in caramel brown have a positive soft click and work well.  The keyboard surround is printed in dark brown for the key labels and a designer line.  The model number is printed on the lower front of the keyboard surround. 
Display: 8 digit LED with bubble lens with a ninth digit (to the far right) for negative and memory indicators.
Features: Four function plus percentages, square root, register exchange, reciprocal, squares, change sign and five function memory.
Age: 1975
Manufacturer: Rockwell International, Microelectronic Product Division.  Made in Great Britain, serial number 4B17234.


Typical Rockwell with a sturdy and unique design.  Trim often wears badly.  The logic is robust with good recovery and only let down by negative square roots and the pseudo fixed decimal bug. Original case is brown padded plastic with a flip over top flap that tucks in the front.  It is not embossed with the word Rockwell.  Similar to the Mexico built 31R type I.

Components: 1 x cpu: Rockwell A5502PB 7538 (week 38 of 1975), 42 pin staggered DIL, 0.6" width
1 x 9 digit bubble lens LED module
2 x capacitors
12 x resistors
1 x transformer: TDK Japan CD-42001 327R05
Boards: Keyboard assembly is attached to the main board (20167D58) by 9 protruding wires that slot into long sockets on the main board.  The transformer is so large that it sits in its own cut-out to reduce the thickness.  Now I know what the push-out hole is for you see on most boards not used.
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 whilst gently pushing a blunt object through the slots at the top of the rear.. Main assembly can be removed by slightly twisting away from the on/off switch.

Logic comments: (C) cancels an incorrect numerical entry, pressing a second time clears the calculator
Input overflow is suppressed, typing in nine digits ignores the ninth
There is automatic constant on all four functions
Negative numbers are shown by a minus in the far right (first) digit thereby allowing full eight digit negative numbers.
The change sign function can be used in mid number entry
Divide by zero results in "" and is recoverable by using (C)
Overflow shows the result with all decimal points alight and is recoverable using (C)
Higher functions are accessed by pressing the (F) key first.  Pressing (F) again will cancel the function mode but there is no indication
Memory store is indicated by a decimal point in the far right (first) digit
(X<>M) exchanges the contents of the memory with the display and register exchange (X<>Y) swaps the two numbers in an operation.
Overflow in memory is flagged and retains the original number
Negative square roots are allowed and result in a negative number
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 you need more digits or use multiply or divide

With the red display filter and keyboard assembly removed, you can see the main cpu board.

Note the two plastic connector blocks to the right and bottom that the keyboard plugs into.

The very large transformer is so wide that it sits in it's own cut-out to the left.  Some of the series did not use such a large component so you can see a removable panel in this position.