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Monroe 2880 Applications Calculator
Updated 2/6/2025
The Monroe 2880 is pretty unique among the Monroe electronic calculators that I've come across. First off, it is a customizable calculator. The 2880, while it can serve as a normal office calculator with a particularly nice display, can be customized with a plug-in ROM module that allows the calculator to assume varying application personalities. This particular machine came with a Payroll ROM module that provides complete payroll processing functions. If anyone reading this knows of other application ROM modules that were available for the 2880, or was perhaps involved in the development of the 2880 calculator, I'd be very interested in hearing from you. My guess is that Monroe made a number of different ROM packs that provided applications such as statistics, engineering/scientific, surveying, job costing for particular trades, and financial (e.g., banking, bond trading, loan origination, etc.)
Monroe 2880 with Top Cover Removed
The exhibited Monroe 2880 was built in late 1979
to early 1980, based on date codes on the IC's within the machine. The
2880 is a microprocessor-based system, utilizing Rockwell's second-generation
PPS-4 4-bit microprocessor. The PPS-4 microprocessor was one of the earlier
single-chip microprocessors on the market, and was initially designed specifically for
calculator applications. Despite the primary intention for the
microprocessor, the PPS-4 found its way into quite a diverse
number of applications, including microwave ovens, low-cost handheld game devices,
electronic thermostats, and numerous other applications.
The Monroe 2880 consists of the second-generation PPS-4 CPU [Part numbers
in parenthesis] (11660), three ROM chips (varying part numbers based
on ROM content) that contain the software that controls the machine,
two random-access memory chips (10932), and three General Purpose I/O (GPIO)
(General Purpose I/O) chips (10696) that interface the processor to the
display, keyboard, ROM pack, and printer. All of the LSI IC's in the machine
are made by Rockwell, and are all socketed "ZIP" style packages.
Three AMI(American Micro-Systems, Inc.)-made RAM (Random Access Memory) chips provide what is
designated on the circuit board as "USER RAM" storage, which appears to be RAM reserved
for use by the ROM-pack applications, while the other memory chips are for overhead
like subroutine stack space, temporary storage, calculator
register storage, etc. A number of Texas Instruments 74LS-series and Motorola
21000-series small and medium-scale glue devices tie all of the large scale
devices together.
The master clock for the microprocessor system is generated by a 3.579545 MHz crystal,
which is the frequency used in (pre-digital) US color televisions for the colorbust signal,
making the crystal a very common part that was quite inexpensive simply due to the volume of its use.
The maximum frequency for the second-generation PPS-4 is 4MHz, so this
frequency works well for providing near maximum speed for the microprocessor, while minimizing
cost.
The display is rather unique, with a very pleasant, highly readable presentation.
It is a 20-character vacuum-fluorescent integrated display module, using a 14-segment
"star-burst" arrangement to form each digit or character.
Closer view of Main Circuit Board Display Controller Circuit Board
The display subsystem is controlled by a fairly complex board that connects
to the main board via ribbon cable. The display controller board connects
to the display module with a number of ribbon cables. It appears
that custom driver IC's are used to drive the display. There
is no detectable manufacturer logo or date codes on the three identical
IC's that appear to provide this function. The star-burst format of the
display allows the display to show alpha characters as well as numerics.
Each digit position also has decimal point/comma segments. When operating
as a calculator(as opposed to when running an application program), the calculator
automatically positions commas every three
digits in front of the decimal point for easier reading.
Display in Calculator Mode Display Showing Alpha Prompt The 2880 provides the basic functionality
of a five function business calculator. It is a fixed-point machine, with
decimal point selections of 0 through 8 digits behind the decimal point. The
[SET DP] button on the keyboard is pressed, followed by a single digit
from 0 to 8 to select the decimal point position. The calculator comes
up by default with 2 digits behind the decimal point at power up.
Calculations are carried out to 12 digits, with overflow indicated by the
display resetting to zero. The calculator does not physically or logically lock out the keyboard
on overflow.
Addition and subtraction are done adding-machine style, with an
accumulator that acts almost like a memory register. The [+] key
acts like a [M+] key on a more conventional calculator, and the [-]
key acts like [M-]. The [◇] key recalls the current accumulated total
(akin to a [MR] key) , and the [*] key recalls the total and clears the
accumulator (like pressing [MR] followed by [MC] on a calculator
with memory). A LED on the keyboard panel lights to indicate when the
accumulator register has non-zero content. Multiplication and division work
algebraically, with the [=] key calculating the result. The [=+] and [=-] keys
calculate the result of a multiplication or division, and add/subtract the
result to/from the accumulator register. The [#] key forces (even if
the printer is shut off) the current number in the display to be printed
on the printer with a "#" in the left-most column for annotation or other
purposes. The [C/CE] key is a dual function key (though it seems it should have
been labeled "CE/C"), with one press clearing the
display, and the second consecutive press clearing the calculator (but not resetting
program functions). The [CHG SIGN] key toggles the sign of the current
number in the display. In addition to the basic four math functions, the
2880 provides a percentage function, useful for mark-up/mark-down calculations.
Monroe 2880 Keyboard Detail The 2880 has a printer that prints
22 columns on standard adding-machine paper. The printer is a line at a time
type, with a full line of characters/digits all printed at once in one cycle. Printing is
done by plastic wheels with digit and/or character reliefs on their periphery
which turn to the correct character, which is locked in the proper position through the action
of a tiny solenoid. When all of the wheels are positioned
to the correct digits/characters, the paper is driven toward
the wheels (with the ribbon between the wheels and the paper) to print the
characters on the paper, and then the paper is advanced to the next line. The ribbon is a two-color
ribbon in a cartridge, allowing the printer to print negative numbers in
red. The printer has limited alphabetic printing capability (only in the right-most
4 or 5 columns) to allow annotation of printed data, which appear to only be accessible through
programmed functions in the ROM cartridge. The printer
can be turned on or off independently of the calculator by a switch located
on the keyboard panel, though in the "off" setting, as mentioned above, the printer
may still print out error messages or overflow indication.
Close up of Monroe 2880 CPU Chip The ROM Module that provides application programs
2880 plugs into a slot on the top of the cabinet with a hinged, spring-loaded door
to keep out dust and debris when a ROM cartridge is not installed. If the ROM module is
not present, pressing any of the fourteen soft-keys that provide access to the
ROM functions causes the machine to 'TRAP', resulting in a diagnostic
code showing up on the display, and a cryptic printout of a few lines
on the printer (even if the printer is turned off). The ROM module
defines the function of two rows of seven soft-keys which, in the
case of the payroll module that came with the machine, allow entry of
items like hourly wage, tax withholding status, hours worked, overtime
hours, etc. The left side of the keyboard has a number of keys related
to the program module, including [F1], [F2], and [F3] function keys, a
key labeled [EXECUTE] that seems to start the payroll processing
program, a [RE-START] key that seems to be the master reset for
the calculator, as well as a key with a brick-red colored square on it which
is a second function key for the soft-keys, allowing each soft-key to perform
two differing functions, depending on whether or not the "red square" key is pressed
before a soft-key. A green LED located near the second function key
serves as a ready indicator for the calculator. When the machine is busy performing
calculations, this indicator goes out, and re-lights when the operation is complete, indicating
the machine is ready for further operation.
Some of the payroll calculating functions can take three to four seconds to complete, making the
ready indicator a key factor for knowing when the calculator is busy performing calculations.
When in calculator mode, the math operations take so little time that the extinguishing of the ready
LED is barely noticeable.
The Payroll Application ROM Cartridge
The 2880 is powered by 110V AC power only, and uses a conventional
transformer-based, IC and transistor-regulated linear power
supply to generate the working voltages needed to operate the machine's logic, display, and printer.
In calculator mode, which the machine comes up in by default when powered on, the 2880 is exceptionally fast,
with all operations taking approximately the same period of time to complete, typically on the order of roughly
60 to 80 milliseconds to complete. There seems to be no difference in the time that it takes for the 2880 to perform difficult
problems (such as all 9's divided by 1) and simple problems. This leads to the conclusion that the 2880 is
performing its math operations using binary floating point numeric representation rather than the Binary-Coded Decimal (BCD)
that is so common in the vast majority of electronic calculators prior to the advent of microprocessors as the brains of
calculators. In most cases, binary floating point operations take about the same amount of time to render the result
of basic math calculations regardless of the number of digits or the magnitude of the digits, whereas BCD representation generally
takes varying amounts of time calculation determined by the length and magnitude of digits in the numbers
being processed. Binary floating-point math operations were generally too complex to be practical to implement in the logic
of an electronic calculator without a microprocessor at its core. The use of a microprocessor allows the binary floating point
math routines to be coded as part of the calculator's "operating system" in ROM, while leveraging the capabilities of
the microprocessor to carry out the complex operations required to perform multi-byte binary floating point math.
Overflow and math errors (divide by zero, for example) result in the display clearing
to zero, and the printer spitting out a line of decimal points (even when
the printer is turned off). The display is blanked during calculation, which results
in a barely perceptible flicker when doing the basic math operations. The
payroll application can leave the display blanked for significant periods
of time while complex calculations are being performed. The ready indicator
is the only way the user can tell that the machine hasn't "gone off
the rails". The keyboard is buffered, so that when the printer
is running, the operator can be indexing keys up to eight keystrokes ahead of the
printer, and no keystrokes will be lost. If the operator gets too far ahead of
the printer, there is no lockout or other indication that keystrokes
have been discarded due to the keyboard buffer overflowing, but in most cases, the printer is fast enough such that
the eight keystroke buffer is plenty large enough to prevent the operator overrunning the machine..
