CasioMini CM602

CasioMini CM602 

Before Casio became one of the standard school suppliers with the well equipped and rugged pocket
calculators, there were actively supporting the spreading of pocket
calculators to the masses with low prices.
Since the technology these days was to be on the fledgling stages and every component added
costs, it led to a number of often quaint developments, like the "CasioMini" series, which
consisted of "pocket" calculators in landscape format and whose calculating power was
quite limited, let alone the reduced display capabilities compared with todays' devices.
The model CM602 described here is one of the early items of the "CasioMini" series and
has got a display tube with only six digits; additionally, this calculator can only work
with integer numbers or with numbers with two fixed decimals. To overcome the display
limit of one million (in integer mode), it has got an "arrow" key, which can show the
remaining digits.
Case and Keyboard

back view. The type plate is really upside down! 

When you hold the Casio CM602 in your hands, it reminds of a japanese transistor radio
because of its shape, its size and its weight. Those radios were equal in size in the
1960ies and 70ies, weight the same and often had got a similar plastic strap for carriage
on the wrist.
The very compact, very well worked up plastic case with its aluminium applications is
typical japanese quality: Nothing is loose, nothing rattles and everything gives a
very rugged impression.

side view 

The case consists of two halves, where the upper half with the light darkened, rather small
display windows can be seperated by losing two screws, which are driven into metal(!) counterparts.
The lower half contains the battery compartement, the 6V power jack and on the connected wires
the whole electronics, which is fixated only by the position of the two case parts and can
easily be removed.
The keyboard has got 18 keys with long stroke and without pressure points, but neverthess always
gives good contact. Only the age of 35 years took its toll and required a little cleaning
with a glass fiber key to remove the oxidation on the contacts. After that procedure, the
keys work again like a charm and you can easily imagine, what a pleasure it must have been
to work with that calculator.
The contacts are realized by spirally bent aluminium springs, which are pressed onto wire
jumpers. This allowes the keyboard to work without any external elements like springs. The
reason, why there are two spring plates, one for the ten digit keys and one for the rest
is unknown.

inside view of the keyboard 

The keys come in different colors (digits: white, operation keys: grey and delete key in
red) and can easily be differenciated by their functions. One special key catches the
eyes: The arrow key. It is used to show the remaining digits, if the result is too big for
the tiny display.
On top left of the keyboard, there's the decimal place key, with which you can set the
calculator to work either with integer numbers (Position "0") or with two fixed decimal
places (Position "2", a kind of financial mode). The keyboard module itself builds pretty
high with 1.1mm (not including the height of the keys) and indicates a rather antiquated
constriction.
The CM602 has got on the lower left its on/off switch, which has got a tiny but useful
detaul: If the calculator is powered on, a red dot appears left to the switch and reminds
the user about the enormous power consumption of the calculator.
Construction

inside view 

The CM602 uses two PCBs, which are connected by 2x6 wires.
The lower PCB generates the high voltage of 34V for the display tube, the upper
PCB contains the logics of the calculator with CPU, display driver, keyboard and
the connectors for the display tube.
The display tube, a NEC LD8088 is still in a round shape, has got an unusual large
diameter and shows six digits. These are already multiplexed driven and thefor the
tube has got only 16 connector wires, which are differently colored and are probably
soldered manually. In 1973 this was barely stateoftheart, however there were
already LED modules available, which allowed machine mounting. Flat VFD tubes, which were
also suitable for machine mounting came just some moths later on the marked.

there are two PCBs inside 

Due to the technical stateoftheart, a large number of external components had to
be used, capacitors, resistors, semiconductors, resistor arrays, the calculator contains plenty
of it.
Here you can again draw the analogy to a japanese transistor radio, since they also were
loaded with lots of components, too.
Display

display tube, front view 

The display is realized by a VFD tube of the type NEC LD8088 with a diameter of almoust 1.7cm and
a length of more than six centimeters. Since it could display only six digits, this gives a
really vintage impression, even more as the display height of 5mm is quite modest.

display tube, back view 

The decimal point on the display tube is unusual: Unlike on normal displays, there's no
display point after each digit, but only on the last but second place, which means, that
you can display only two decimal places this way. An unusual saving measure!
Like on other calculators of the same type, the zero
digit only uses the lower half of the display, which takes getting used to.
All sixteen connectors of the tube are lead through thin wires with color isolation. Connecting
them with the PCB is not uncritical, because if only one of them breaks, the whole display
is unusable.
On this tube, also a phenomenon can be observed which has never been seen or heared elsewhere:
On powering on, the heat wires make a clashing sound, which can be heared a little softener on
powering off, too. If it influences the lifetime of the tube cannot be determined.

displaying of a zeri 

CPU
In the Casio CM602, a Hitachi HD32154P is used as CPU, and a NEC µPD129C works as
periphery. Both chips are produced in 1973.

CPU and perpihery 

Calculating Capabilities
As on all compareable simple and display reduced pocket calculators, the calculation
capabilities of the Casio CM602 are rather limited, too.
It is only capable of the four standard calculation methods, which are executed
direcly after entering the second number. An intermediate or real memory is
not available.
The CM602 offers two different ways of showing the result: Without or with two decimal places
after the comma. Depending on the choice, the results and the further processing of them
differ:
Without default decimal places, pressing the arrow key can show up to six remaining decimal places
on the division. Using them for further calculations is not possible, they are simply ignored (as
well as any use of the decimal dot key on the entry). If you calculate e.g. "100 / 7 =", the result "14"
is shown and the arrow key shows the remaining fractional digits "285714". But if you multiply this
result again by 7, the result is "98" and the arrow key shows no fractional digits at all (which
is a bit weird, because normally, "000000" would have been shown), which means, that internally,
only "14 * 7" was calculated and not "14.285714*7".
On an addition, which overruns the display range of six digits, you always get zero as result, and
any calculation with negative numbers uses the negative sign on calculations only, when it is
shown in the display. This is only the case, until you press the equal sign, and so you will
be surprised, when you calculate "(9) 3 =", get "12" as result and the further addition of "3"
leads to the completely wrong result "15". If you had instead calculated "(9) 3 +3" without
intermitted result, you had got the correct result of "9".
But the calculation weirdness can still be increased: "(99999) 3 +3" gives the surprising intermediate
result of "100002" just when you key in the plus sign, since there's no space for the minus sign
on the display. The final result of "99999" is correct, though.
If you had calculated "(99999) 3 =" instead, the result would have been "100002" and for nothing
in the world you would have known that the result should be a negative number instead.

integer mode: 10/7, first part 


integer mode: 10/7, second part 

On switching to the two decimal places mode, the calculation precision on the division is reduced.
Using the arrow key, "100 / 7 =" results in "14.28571", which means, that you have one decimal place
less than on the integer mode. But the later remultiplication of seven leads now to the much
more correct result of "99.96". This result is still quite far from 100 and shows, that internally,
again only those numbers, which showed on the default display, were taken into the calculation,
this means, that "14.28 * 7" was calculated because we are in the two digit mode.
Pretty fatal is, that in this mode, the entry range ends on 9999.99. "9999 + 1234 =" results in the
error value "0". Why you need a pocket calculator for financial calculations below ten thousand is a
miracle.
The "weird" minus sign causes problems in this mode even from "999.99" on, but that was not to be
expected differently.
A further oddity is, that in this mode, the calculator is locked, when you try to enter a fifth digit
before the decimal point, since this would lead to an overflow error. This requires attention on the
user and is not very much praticable.

10/7, first part 


10/7, second part 

The division by zero is correcly indicated by an error. In this case, the zero digit
is displayed in all digits:
A division by zero is properly indicaded by an error, in this case displaying
six times a zero:

error indication 

Summary
The Casio Mini CM602 is an evolutionary very interesting device, which shows, what kind of really
weird solutions Casio made before they conquered the mass marked. They all were made with regard
to lowering the costs of the pocket calculators, thus making them available for everybody.
Technical data
Manufacturer  Casio 
Model  CM602 
Year  1973 
CPU  Hitachi HD32154P 
Display  6 digit VFD 
Number of keys  18 
Functions  +  * / 
Entry logic (classification)  ALG (DCA) 
 1 + 0.000 =  1 (the correct value would be 1.) 
0 / 0 =  Error 
(1)+1 =  0 
 Power supply  4xAA 
Power consumption  0.5 W 
Size  7.8 x 14.9 x 3.5 cm 
Serial Number  781872 
Condition  A 

Factsheet 