Casio-Mini CM-602

Casio-Mini CM-602
Casio-Mini CM-602

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 "Casio-Mini" 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 CM-602 described here is one of the early items of the "Casio-Mini" 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!
back view. The type plate is really upside down!

When you hold the Casio CM-602 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
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
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 CM-602 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
inside view

The CM-602 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 state-of-the-art, 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
there are two PCBs inside

Due to the technical state-of-the-art, 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
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
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
displaying of a zeri

CPU

In the Casio CM-602, a Hitachi HD32154P is used as CPU, and a NEC µPD129C works as periphery. Both chips are produced in 1973.


CPU and perpihery
CPU and perpihery

Calculating Capabilities

As on all compareable simple and display reduced pocket calculators, the calculation capabilities of the Casio CM-602 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 CM-602 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, first part


integer mode: 10/7, second 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 re-multiplication 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, first part


10/7, second 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
error indication

Summary

The Casio Mini CM-602 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

ManufacturerCasio
ModelCM-602
Year1973
CPUHitachi HD32154P
Display6 digit VFD
Number of keys 18
Functions+ - * /
Entry logic (classificationALG (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
Size7.8 x 14.9 x 3.5 cm
Serial Number781872
ConditionA-
Factsheet


Links
Casio Mini series (en) (A japanese page about the Casio Mini calculators with many photos)
Vintage Technology: Casio Mini CM-602 (en) (Very interesting page)