|
POCKET CALCULATOR KIT REVIEW
Supacal Calculator
Obtainable from S.C.S. Components Ltd., P.O Box 26. Wembley, Middx HAO 1YY.
Price £39 (+ V.A.T.) with special £4 discount to P.E. readers.
Immediately upon opening the box containing the SUPACAL calculator kit the decision should be made as to whether one is sufficiently equipped in terms of both tools and experience to undertake the construction of the
calculator.
Should there be any doubt in this respect, it is strongly recommended that the services of a more experienced friend or colleague be recruited for this work. The minimum tools required for building the kit are a good soldering iron of 10 to 25 watts with a bit preferably no more than a sixteenth of an inch diameter, also a pair of side cutters, a pair of long nosed pliers, and a good light.
Many solder joints have to be made in a small area and a certain degree of skill is needed to avoid short circuits and dry joints. Fine solder is provided with the kit and this greatly
simplifies the construction.
COMPONENT PACKING
All components come in plastic bags marked with the value of the components contained, but it is as well to check these values against those required as it is always better to
be safe than sorry. Because the printed circuit board has tracks both sides many solder joints have to be made on both sides of the board; this is tedious but it is essential to make good connections.
The display board and the inverter board for the 25V i.c. supply come as ready made units and this eases the constructor's task enormously. The keyboard is already fitted into the lop half of the case
and wiring to the main circuit board is simple, though here again solder joints have to be made top and bottom of the board.
HANDLING PRECAUTIONS
The calculator i.c. comes in a piece of protective conductive foam and the handling precautions in the instructions should be carefully followed. A socket is provided for
the i.c. so the usual problems of soldering to an m.o.s. integrated circuit are overcome.
Instructions for the insertion of all components are given and these are easy to follow. A circuit diagram
and point-to-point wiring diagram are also supplied. Total construction time for the SUPACAL was three and a half hours in this reviewer's case. Any difficulties will probably be caused by poor soldering and a good
light is useful for checking.
The kit suppliers offer a back-up service in the form of a fixed price repair facility and this ensures a fully working calculator no matter how much of a mess you made
of the construction. Once the calculator is working then all parts are guaranteed for one year.
CALCULATOR CONSTRUCTION
Now something about the calculator itself. It is housed in a strong plastics case and uses two rechargeable batteries for its power source. A ready built charger is
provided with the kit; fully charged batteries give about eight hours working. There is an automatic fade-out facility which switches off the display (which consumes about half the total power) approximately ten
seconds after a key has been pressed.
The keyboard uses conductive rubber contacts which give the machine an extremely light touch; in fact the switches are almost too sensitive, some care being
needed not to enter a digit unintentionally.
CAPABILITY
The calculator can handle all four arithmetic operations with all calculations being in floating point, with eight digits being displayed. There is a stored constant facility which,
unlike most other calculators, can be used with all four functions. Another advantage of the calculator is that it will not overflow. Other calculators will ignore any entries over eight digits long; the SUPACAL
stores not only the eight digits entered but also stores an exponent up to 1079; the true decimal point position is found by dividing by powers of ten until the point appears on the
display. This enables more accurate answers to be found.
INSIDE THE POCKET CALCULATOR
How does the pocket calculator work? The "heart" is the second generation single chip MOS LSI circuit type C500 manufactured by General Instruments
Microelectronics. It contains within its 4000 active devices all the logic necessary for performing eight-digit, four function plus constant and floating decimal point calculations.
The chip contains
a read-only-memory (ROM) of 2,000 bits, a random-access-memory (RAM) of 100 bits and associated control logic, dividers, and timing circuits for for clocking and multiplexing. The chip simply requires a 25V supply
and a clock input of 50 to 100kHz, a keyboard, display, and circuits to drive the display and it becomes a fully operational calculator.
The display interface is by means of discrete buffer/driver
transistors for segment and digit driving. The display is multiplexed, i.e. each digit is only driven for one-ninth of the time, there being nine l.e.d. digits with seven segment format.
|
