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Monroe 2880 Applications Calculator

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, though it can serve as a normal office calculator, 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 out there knows of other application ROM packs that could be had for this machine, I'd be interested in hearing from you. My guess is that Monroe made a number of different ROM packs that provided things like statistics, engineering/scientific, and financial (e.g., bond trading, mortgage calculations).

Monroe 2880 with Top Cover Removed

The Monroe 2880 was built in late 1979 to early 1980, based on date codes on the IC's within the machine. The machine is a microprocessor-based system, with a CPU [Part numbers in parenthesis] (11660), three ROM chips (varying part numbers, probably based more on ROM content), two RAM chips (10932), and three GPIO (General Purpose I/O) chips (10696). All of the LSI IC's in the machine are made by Rockwell, and are all socketed "ZIP" style packages. Three AMI-made 7931 RAM (Random Access Memory) chips provide USER RAM storage, which appears to be RAM reserved for use by the ROM-pack applications, while the CPU's RAM is for overhead like 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 is generated by a 3.579545 MHz crystal. The display is rather unique on this machine. It is a 20-character vacuum-fluorescent integrated display module, using a 14-segment "starburst" 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 itself with a number of ribbon cables. It appears that some custom driver IC's are used to drive the VF display, as there is no detectable manufacturer logo or date codes on the three identical IC's that appear to provide this function. The starburst 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, 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 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 [diamond] 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 press clearing the calculator (but not resetting program functions). A [CHG SIGN] key toggles the sign of the current number in the display. In addition to the basic four functions, the 2880 provides a percentage function.

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 line of characters/digits all printed at once. Printing is done by plastic wheels with digit and/or character reliefs on them, which turn to the correct character and then the paper is driven towards the wheels (with ribbon between the wheels and the paper) to print the characters on the paper. 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 (the right-most 4 or 5 columns, anyway) to allow annotation of printed data. The printer can be turned on or off by a switch located on the keyboard panel.

Close up of Monroe 2880 CPU Chip

The ROM Module that personalizes the 2880 plugs into a slot at the top of the cabinet. 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 dump of a couple of 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 things 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 machine, and 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. A green LED located near the second function key serves as a ready indicator. When the machine is busy performing calculations, this indicator goes out, and re-lights when the operation is complete. Some of the payroll calculating functions can churn for 3 to 4 seconds.

The Payroll Application ROM Cartridge

The 2880 runs off 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.

In calculator mode, which the machine comes up in by default when powered up, the machine is very fast, with all operations taking approximately the same period of time to complete. There seems to be no difference in the time that it takes for the machine to perform difficult problems (such as all 9's divided by 1) and simple problems. Overflow and math errors (divide by zero) result in the display clearing to zero, and the printer spitting out a line of decimal points (even when disabled). The display is blanked during calculation, which results in a barely perceptable flicker when doing regular math operations. The payroll application can leave the display blanked for significant periods of time while calculations are being performed. The ready indicator is the only way the user can tell that the machine hasn't "gone off into the weeds". The keyboard is buffered, so that when the printer is running, the operator can be up to 8 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.


Text and images Copyright ©1997-2011, Rick Bensene.