Radio Shack EC-220

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Calculators: Handheld: Radio Shack EC-220


		Size (approx):	70mm x 148mm x 20mm (w,h,d) Weight 130g excluding batteries
		Power:	9V DC using 1 x PP3size battery. It accepts an adapter (catalogue no. 65-702) through a socket on the top side to the far left. The on/off switch is on the left hand side, mid way up.
		Case:	It is made from a two-piece matt beige plastic case which is brick-shaped but with all corners and edges rounded. The top panel houses a brown plastic branding label which is printed white and sits in its own recess. A red display filter sits flat and slightly recessed and gives a passable image with limited angle of view. The large keyboard area is surrounded by a matt brown plastic panel that is printed white and orange with the key labels. The lozenge shaped keys are rocking action with a soft click – just like Rockwell keys.
		Display:	8 digit red LED with bubble lens but no ninth
		Features:	Standard four functions with percentages
		Age:	1975/6
		Manufacturer:	Radio Shack, a Tandy Corporation Company, Fort Worth, Texas 76107. Assembled in Mexico. Serial number 030275.
		Comments:	This is obviously a Rockwell 8R. The keyboard is the same mechanism (but white not brown), the IC and board is Rockwell and the place of manufacture is Mexico – where many Rockwell calculators were assembled. A solid feeling calculator that is a pleasure to use. I think the keyboard surround is especially nice with quality colouring and chic 70s colours. The only bad thing being the stupid position of the equals key. Logic OK but lack of a ninth digit is annoying.

Components:	1 x cpu; RockwellB5000CC 7608 (week 8 of 1976) 42 pin staggered DIL, 0.6" width 1 x 8 digit red LED unit with individual bubble lens: National Semiconductor NSA1188A 604 0 x transistor 0 x diodes 2 x capacitors 0 x resistors 0 x transformer
Boards:	The main cpu board (Rockwell 20167D67-03) sits on top of the keyboard assembly and is connected by a side and bottom connecting socket. Four plastic pillars also guide it into position. The keyboard assembly is secured to the front with four screws.
Construction:	Remove the four screws from the rear and the back will lift off easily.

Logic comments:	(CE/C) clears the last number entered whilst a second press clears the whole calculator
	Input overflow is suppressed, inputting a ninth digit is ignored
	Negative numbers are shown with a minus in the immediate left hand digit which has no ninth digit to travel into so you are limited to seven digit negative numbers
	There is automatic constant on all four functions
	Divide by zero shows "0.0.0.0.0.0.0.0.” and is recoverable using (CE/C)
	Overflow shows the result and all decimal points alight and is recoverable using (CE/C)

	This scan if the main board clearly shows it's a familiar general Rockwell board. Notice the left hand area that can be punched out to house a display transformer, and numerous empty holes for other models to use. Also you can clearly see the right hand side and bottom connector rails that join to the keyboard assembly. On later models these connectors were soldered wires in order to reduce cost. One Rockwell IC and two capacitors - can't get much more basic than that. Due to the large size I suspect the capacitors are for "Switch on reset" and power supply smoothing.