+Home     Museum     Wanted     Specs     Previous     Next  

Royal Digital VIII-K Calculator

The Royal VIII-K is a great example of the early, low-cost calculators based on large-scale single-chip integrated circuit devices. The machine is functionally identical to another calculator in the museum, the Commodore US*1, of similar vintage. Both machines are based on the General Instrument C-500 single-chip calculator large-scale integrated circuit, and share their origins in the early part of 1973. The calculator was made by Royal Typewriter Company, which by that time had been acquired by Litton Industries, which made a point of acquiring a number of business machine companies in the late 1960's and early 1970's.

Given that both the VIII-K and the Commodore US*1 share the same calculator chip, it would stand that both machines operate identically. What that assumption holds true in most cases, there are differences based on the implementation of the support circuitry around the calculator chip. Both machines provide floating decimal, algebraic logic, the four basic math functions, and a constant that operates in all four functions.

The Keyboard of the Royal VIII-K

The keyboard of the Royal VIII-K is a sealed unit that uses spring-type contacts. The keyboard assembly connects to the main circuit board via wire jumper wires. The keyboard uses nice large keycaps with molded in nomenclature.

Display in Operation

The Royal VIII-K uses a very unusual planar gas-discharge display. The display module consists of a glass sandwich, with wires coming out of it which are soldered to a circuit board which connects the display elements together for multiplexed operation. The circuit board connects to the main circuit board by a series of wire jumpers.

Detail of the Display Module

The display is driven by discrete transistor circuitry. The VIII-K does not provide leading zero suppression. The display uses the standard seven-segment digit arrangement, with a right-hand decimal point included with each digit. The display module also includes a 'tick' at the upper-left of each digit, however, this feature is not used.

The General Instrument C-500 LSI and Supporting Circuitry

The VIII-K seems to be a bit faster than the US*1, giving virtually instant results to any calculation. Given that both machines share the same IC brain, all one can conclude is that the VIII-K provides a faster main clock frequency to the calculator chip than that of the US*1.

The Royal VIII-K provides a constant function on all four math functions. The [K] key is used to set the constant and the type of operation to be performed. For example, constant multiplication by two would be done by pressing [2], then [X], followed by the [K] key. Once constant operation is set, constant mode is cleared by pressing the [C] key, which clears constant mode.

Overview of the Guts of the Royal VIII-K

The Royal VIII-K uses a simple linear power supply, with a small transformer stepping down the line voltage to lower AC-power, which is rectified, smoothed, and regulated by circuitry on the main circuit board. The calculator uses a removable standard NEMA-style instrument power cord. An unusual feature of the VIII-K is that the power supply of the VIII-K is fuse-protected, using a standard fuse holder located inside the case of the calculator. Most calculators in this cost category have soldered-in fuses, requiring unsoldering of a blown fuse, and soldering in a replacement should the fuse fail. Not so with the VIII-K, the fuse holder cap is simply unscrewed, the bad fuse removed, a replacement fuse installed, and the fuse holder cap screwed back on. In order to access the fuse holder, the cabinet of the calculator must be taken off, which requires the removal of four screws which hold the halves of the cabinet together.

Detailed View of Insides of the Royal VIII-K with Keyboard Tilted Away

Like the Commodore US*1, the Royal VIII-K does not provide any form of overflow or error detection. Results in excess of the capacity of the machine are simply discarded. The calculator does keep track of decimal point locations up to an additional eight digits beyond the end of the display. For example, performing 99999999 X 99999999 results in "99999998", with no decimal point lit. Following this calculation with a division by 10000000 results in "999999998." (note decimal point). Numeric input in excess of eight digits is ignored.

The VIII-K also exhibits the same odd counting behavior as the Commodore US*1 when commanded to divide by zero, clearly being an artifact of the General Instrument C-500 calculator chip. Pressing the [C] key will clear the confusion, returning the calculator to normal operation.

Text and images Copyright ©1997-2021, Rick Bensene.