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Calculators: Handheld: Casio ST-1

Size (approx):

61-70mm x 133mm x 20-29mm (with cowl)  (w,h,d)
Weight 118g excluding batteries
Power: 1.5V DC, 1 x AA size batteries.  It accepts an adapter (DC 0.13W, type AD-1S) through socket on top side to the far right. 
Case: Strange shaped hand held so that your thumb can switch on/off and your index finger can trigger the left side start/stop stopwatch function (for your left hand, or thumb of your right hand).  The case is a strange glossy silver plastic, two-piece construction.  There is a ridged design on the back vertical edges to aid grip.  The neutral plastic display filter is inset and can be further shrouded by a black plastic clip-on cowl.  Below this is a less inset area with a black, red and green printed metallic sticker.  This labels the main function switch.  A smoked plastic door covers the main calculator keys with a small black printed metallic "open" label on the right side.  Another deeply inset metallic label is printed black at the bottom with the brand and model number.  The small keys are rubbery and not that easy to use.  There is a sturdy plastic tab for a (black woven) strap on the bottom edge.
Display: 8 digit blue VFD with no ninth digit
Features: Standard four functions with percentages, square root, three function memory and time (sexagesimal) conversion.  Four function stopwatch.
Age: 1977
Manufacturer: Casio Computer Co., Ltd., made in Japan, no detectable serial number on my example.  There is a sticker inside that has 1127239 on it.
Comments: The first combined calculator and stopwatch.  The calculator is rather basic and let down by poor recovery, negative square roots and negative zero bug, but the stopwatch functions are extensive.  The original cover is soft black plastic with CASIO printed in silver on the front.  It has a top flap with front popper closing and the calculator has to be inserted top down in order to use the carry strap.  The rear has a wallet for (presumably) the instruction manual.  For such a late date the component count is quite large.

Components: 1x cpu: NEC ?PD579C K71079, 28 pin DIL, 0.6" width black plastic
1 x IC: Toshiba TC4011P 7-A (date code January 1977), 14 pin DIL, 0.3" width, black plastic
1 x 8 digit VFD: single tube round front face
3 x transistors
11 x diodes
11 x capacitors
12 x resistors
1 x transformer: TDK CD-1255 4A1 (which would tend to be a date code of January 1974 - which is to early.  So either this is old stock or an incorrect interpretation of the date code.)
Boards: The main board (F8G-1A) floats lose on top of the keyboard assembly and is joined by 18 stiff copper wires.  There is a third piggy-back board with the power supply components.
Construction: Remove the single screw from the back.  This isn't much help as you then have to pop the internal lugs by pushing in the the front section (or levering out the back section).  Start at the stopwatch switch side and eventually the rear section can be lifted off.  Be very careful not to damage you calculator.

Logic comments: (C) is used to cancel the last number entered and (AC) to clear the whole calculator
There is selectable constant on all four functions by double hitting the operator: i.e. (3)(X)(X)(=) gives "9"
Input overflow is suppressed, inputting a ninth digit is ignored
Negative numbers are represented by a ?-? to the immediate left of the number but as there is no ninth digit you are limited to seven digit negative numbers
Overflow shows only ?E? in the right (first) and is not recoverable
Divide by zero shows only ?E? in the right (first) digit and is not recoverable
Square roots of negative numbers are allowed and result in a negative number
There is no indication of memory - you have to remember it and (AC) will clear the memory
(TM) can be used to calculate with sexagesimal (time) numbers: to input one hour, six minutes and five seconds plus ten minutes key in (1)(TM)(6)(TM)(5)(+)(0)(TM)(1)(0)(=) to give "1.2680556" then convert this to HMS by (TM) to give "1o16o05.0o"
There is a slide witch to chose between modes: COMP is plain calculator, whilst the other settings are various stopwatches.  These are controlled by the right hand side switch and the (TOTAL) key.  Standard is start/stop/clear start stop when the sum of all times is displayed with (TOTAL), TIME OUT allows you to count after stopping using the (TOTAL key), LAP-1 freezes the lap time and continues counting, whilst LAP-2  stops the counter between pauses and (TOTAL) accumulates.
In COMP mode the (TOTAL) key can be used as (=)
It suffers the negative zero bug: key in (1)(-)(2)(=) to give "-1" then (+)(1)(=) will give "-0"