Casio Personal M-1

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Calculators: Handheld: Casio Personal M-1 (aka H-813)

	Size (approx):	75mm x 125mm x 20mm (max) (w,h,d) Weight 98g excluding batteries.
	Power:	3.0V DC with 2xAA size, 0.2W. Also accepts adapter (AD-2S) through top side socket on the far left. The on/off switch is on the left side just above the Casio logo.
	Case:	Two -piece case in white and black plastic of a square design with rounded edges. The neutral display filter is slightly tilted and gives a clear bright image. Above the display a black and blue printed brushed aluminium sits in its own recess. The keys are typical Casio, long travel and squishy but work well on all my examples.
	Display:	8 digit blue VFD fluorescent display with no ninth digit
	Features:	Four function with square root, percentages and one function memory
	Age:	1977
	Manufacturer:	Casio Computer Co. Ltd. Made in Japan. Serial number on sticker in the battery compartment missing.
	Comments:	Compact little calculator with basic functions and and auto-accumulating memory. This model was revised to the Casio H-813B and then later to the H-813C. The "B" revision must have been a change in IC relationship as there are actually more components (i.e. expense) inside. The logic is sound enough but the lack of ninth digit is annoying, the negative zero bug stupid, and the memory too basic to make much use of.

Components:	1 x processor Hitachi HD3780 7F13 (date code 13 [or batch 13] June 1977?), 28 pin DIL, 0.6" width 1 x display GCMK-19EHB E6545S (bowed) 2 x resistors 6 x capacitors 4 x diodes 1 x transistor 1 x TC-01 (TDK) 4K2 transformer
Boards:	One main board (E8X-1A) with few components and second keyboard board joined by 13 strong copper connectors which can be hinged out. The latter is held to the front with six screws.
Construction:	Remove the two screws inside the battery compartment and the rear lifts of easily. Adaptor socket tends to come lose which makes reassembly a little more difficult.

Logic comments:	The Clear key (C) is used to clear an input number, the all-clear (AC) to completely reset the calculator.
	Overflow on the input of a number is suppressed, typing in a ninth digit is ignored
	The constant function is invoked on all four function by double pressing the operator, i.e. (3)(+)(+)(=) gives "6" (=) gives "9" etc.
	There is no indication of memory use - you have to remember it.
	The negative sign is to the immediate left of a number, but as there is no ninth digit to travel into you are limited to seven-digit negative numbers
	On overflow the display just shows "E" in the far right (first) digit and is not recoverable.
	On divide by zero the display just shows "E" in the far right (first) digit and is not recoverable.
	Negative square roots are allowed and result in a negative number
	This calculator has the "negative zero bug", try (1)(-)(2)(=) to give "-1" then (+)(1) (=) gives "-0"
	The memory automatically accumulates all numbers that result when the (=) key is pressed. (AC) also clears the memory.