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Sharp EL-804 Desktop Calculator
The Sharp EL-804 seems to be
extremely similar to the Sharp EL-803
in the museum. In fact, they use the same single-chip calculator
IC, and operate identically. The case styling is similar
between the two machines, with the EL-804 being slightly narrower, but
slightly deeper than the EL-803. The only real difference that I can see
is that the display drive IC's are more integrated in the EL-804 (two
chips versus three chips in the EL-803). The EL-804 appears to be a slightly
later vintage than the EL-803, which could account for more efficient
display driver technology. The EL-804 appears also to have minor
design changes which would result in a lower cost to manufacture, which
translated to a lower price for the consumer. At the time these
machines were made, the price wars that began the shakeout in the
calculator business were beginning to occur, and the resulting price pressure
was a fact of life, forcing calculator makers to figure out ways to
continually reduce manufacturing costs.
Inside View of Sharp EL-804 The EL-804 is a classic four-function
calculator. It provides eight digits of capacity, with floating
decimal operation. A slide switch selects whether a constant multiplier
or divisor can be set for multiplication and division.
The display is made up of nine individual vacuum-fluorescent
display tubes, using a slightly-modified seven-segment digit rendition.
Each tube has a right-hand decimal point. The right-most tube is used
only for sign(-) and error(E) indication. The light-blue
glow of the vacuum-fluorescent display elements is converted to a bright
green by a green filter window positioned between the tubes and
the user.. The calculator logic provides leading zero
suppression, but does not perform trailing zero suppression. For example
performing 12 divided by 3 results in 4.0000000 being displayed.
Close up of EL-804 Circuitry
The EL-804 is based on a Hitachi HD3276 Large Scale calculator chip. A
Hitachi HD3253 chip appears to be used for generating the master clock
that the LSI chip uses to create its internal timing events.
Two display driver chips (HD4350) combine forces to drive the vacuum-
fluorescent display tubes. Lastly, a Toshiba-made TD2000TP chip
rounds out the group of five integrated circuit devices in the
machine. Based on date codes on the integrated circuits, the machine was
built in the mid-part of 1973, somewhat later than the EL-803, which
further substantiates that this machine was a 'lower-cost' redesign of
the earlier EL-803. The circuitry of the machine, except the keyboard
assemblies, sits on a single-sided phenolic circuit board. The power supply
resides on the main circuit board, and is of a simple zener diode and
transistor-regulated linear design.
The EL-804's Display The keyboard construction and
layout of the EL-804 is identical to that of the EL-803. One keyboard module
contains the numeric keypad (zero through nine and decimal point), and the other
module contains the function keys. This modular design allowed Sharp
to use the same keyboard modules in a number of different calculator
models, saving on tooling and production costs. The EL-804 function key
module contains two 'extra' positions for keycaps, but these extra positions
are not wired up to anything. My guess is that these extra positions are
populated with memory function keys in higher-end models. The EL-804 forsakes
the more modular construction of the EL-803, getting rid of edge-connector
wiring for the keyboard modules and the constant slide switch, in favor
of directly wired connections. This probably saved both on component and
labor costs, at the expense of ease of service. However, by this time in
calculator history, the machines were becoming inexpensive enough to simply
discard and replace when a failure occurred.
The EL-804 seems a little faster
than the EL-803, with 99999999 divided by 1 taking around 1/2 second.
9999.9999 times itself takes about 3/4 second. The display is not blanked
while calculations are taking place. This difference in operating speed
between the two machines could simply be the result of a slightly faster
main clock frequency on the EL-804. Overflow or invalid operations
(divide by zero) are indicated by clearing the display to '0', and lighting
an "E" in the right-most display tube. A single press of the "C" key clears
the error. The "CE" key allows correction of incorrect numeric entry
by clearing only the entry made thus far. It is interesting to note
that when first powered up, the EL-804 comes up in "ERROR" mode, with
0.E on the display and the keyboard locked out. Pressing the "C" key
clears the machine and readies it for normal operation.
