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Monroe 1665 Desktop Programmable Calculator

This calculator came to the museum through the generosity of Janet Harrison of Birmingham, Alabama. This machine was originally owned by her father, Thomas J. Harrison(1914-1996), who was an engineer that specialized in metallurgy specific to large-scale industrial and construction use of metals. Thomas received a degree in mechanical engineering from the University of Alabama in 1938, and ended up going to work as a sales engineer for Republic Steel, then later Fox Steel. Later, he started his own business serving as a manufacturers rep. for a number of steel producers. The Monroe 1665, along with a Wang 720C, were used by Mr. Harrison for performing the various complex mathematical functions relating to structural analysis, metals properties, and other aspects of his business. Mr. Harrison kept these machines as prized posessions, treating them with great care. After Thomas, and later, his wife, passed away, his family looked to find a place where these machines would be preserved with the same reverance and respect that Thomas gave them. Fortunately, Janet found the Old Calculator Web Museum, and after a family meeting, Janet and other members of her family decided that these machines would find a new and permanent home here in the museum.

The "Brag Tag" on the front panel of the Monroe 1665.

This great old machine is an example of Monroe's first-generation of printing desktop programmable calculators. Monroe debuted the 1600-series of calculators sometime in the early part of 1970. The 1665 may look similar to another machine in the museum, the Monroe 1860, but the 1860, while visually similar, is example of the second generation of calculators that were designed by Computer Design Corporation for Monroe. Computer Design Corporation had all of the electronic design expertise that Monroe needed when electronic circuits squeezed mechanical technology out of the calculating scene, making for a perfect partnership. Computer Design Corporation developed the electronics, and Monroe leveraged their large and experienced sales and service force to sell and maintain the machines.

The 1665 is the printing version of the Monroe 1655 Nixie-tube display calculator. Functionally, the two machines are almost identical, with the main exception being that the 1665 provides a thumbwheel setting for selection of decimal point position, versus floating decimal logic for the 1655. From a construction point of view, the 1665 uses a more modular design, taking up more desk space, but using a generalized backplane-based architecture than the more monolithic design of the 1655. Both the 1655 and 1665 were targeted toward the scientific and engineering user communities, with an extensive compliment of scientific functions built in.

Detail of the 1665's Keyboard

The 1665 is quite a capable machine for the early 1970's. The calculator boasts a wealth of mathematical functions including trig, logarithms, roots and powers, and factorials. Along with the higher-level math functions, the calculator is fully programmable, allowing rather complex programs to be written to perform just about any kind of mathematical calculation that can be expressed within the 128-step (or optionally, 256-step) program memory of the machine. As with most of the machines in the first generation of Compucorp/Monroe calculators, the 1665 has a total of ten memory registers, numbered zero through nine. Registers 7, 8, and 9 are general purpose registers used by some of the math functions of the machine, leaving registers 0 through 6 for more general user needs. Since the calculator uses IC-based Random Access Memory (RAM) that is volatile (the content of memory is lost when the machine is powered off) for program storage, the calculator has the ability to allow an optional external punched card reader (the CR-1 Card Reader) to be connected to allow for easy loading of programs into the memory of the calculator from punched cards.

Like the other first-generation of Compucorp-designed calculators, the Monroe 1665 uses Compucorp's "HTL" chipset. The HTL chipset is a comprehensive set of early Large Scale integrated circuits that are designed in a modular fashion to allow varying combinations of the parts to make up a wide range of calculators with different capabilities. The HTL chipset was really a wonder of the times, essentially a multi-chip implementation of a microcoded microprocessor, quite a number of years before the microprocessor became a reality. In the case of the 1665, a total of 28 of the AMI-made (AMI served as the fabricator for Computer Design Corporation's HTL chip designs) HTL chips combine forces to make up the logic of the calculator. Along with the HTL-labelled parts, there are eight other chips that are not part of the HTL set, that appear to be ROM (Read-Only Memory). These chips are also made by AMI, and appear to be mask-programmed ROM chips. These ROM's contain the microcode that implements the functionality of the calculator, and directs the operation of the HTL chipset.

Inside the Monroe 1665

The 1665 is built in a modular fashion, with four main components. The printer, power supply, keyboard assembly, and the logic. The logic of the machine consists seven logic boards that plug into a printed-circuit backplane. Each logic circuit board is made out of high-quality fiberglass board material, with plated through holes to connect the two sides of the board. Each board has edge fingers arcoss the bottom of the board that plug into a connector in the backplane. The function of each of the circuit boards is conveniently etched on each board. At the left end of the card cage, a small board contains voltage regulation circuitry for the power supply. To the right of this power supply board, the remaining seven boards make up the logic of the machine. The logic boards, left to right in the chassis as shown above, are: Printer Drive, Data Memory, Program Memory, Control, Compiler/Flags, LEMP, and (the optional) CLEMP.

The Printer Drive (left) and Data Memory (right) Circuit Boards

The LEMP (left) and CLEMP (right) Circuit Boards

Some of the boards' functions are fairly obvious from their names, but a few require some explanation. The Printer Drive board contains HTL03 and HTL14 chips, along with transistorized driver circuitry for the printer's solenoid-activated print hammers. The Data Memory board contains the RAM (Random Access Memory) where the memory and other working registers of the calculator reside. Along with the four HTL00 DRAM chips, the remaining IC's (HTL04, HTL05, HTL06 and HTL10) handle the addressing and refresh of the memory.

The LEMP board contains the functions that relate to the programmable capabilities of the calculator, including the RAM (made up of four HTL00 DRAM [Dynamic RAM] chips) for the base 128-step program memory, and additional logic (on HTL15, HTL16, and HTL17 chips) for the various programming functions, along with the calculator's keyboard interface. LEMP, by the way, is a Compucorp-coined acronym for "LEarn Mode Programmer". The optional CLEMP (with the C standing for "Card") board provides expansion memory for increasing the number of program steps from 128 to 256, as well as providing the interface for the external punched card reader. As with the LEMP board, the CLEMP board contains four HTL00 DRAM chips for program step storage.

The Program Memory (left) and Control (right) Circuit Boards

The Program Memory board's name is a bit misleading... it isn't where the program steps are stored (program steps are stored on the LEMP and CLEMP boards), but rather, it contains the ROM microcode 'program' for the calculator, and the addressing logic for the microcode ROM (HTL02 and HTL12 chips). The six ROM chips on the Program Memory board hold the low-level instructions that govern the operation of the machine.

The Compiler/Flags Circuit Board

The Control and Compiler/Flags boards combine to form the director of the whole operation. The Compiler/Flags board (containing HTL09, HTL11, and HTL13 chips) accepts keycodes and program instructions from various other sections of the calculator, and dispatches various functions to the Control board (with HTL07 and HTL08 chips) to cause the various operations to be executed in accordance with the microcode. Both the Compiler/Flags board and the Control board have their own small areas of ROM associated with them, as well as the main microprogram ROM on the Program Data board.

Moving into the other areas of the machine, the keyboard of the 1665 uses a modular approach, with sealed reed-switch modules providing the key mechanisms. The key modules are soldered into a circuit board, which contains some simple discrete component- based logic for encoding functions. The keyboard connects into the LEMP board via a wiring harness that terminates in an edge connector socket.

Printout from the Monroe 1665

The printer used in the 1665 appears to be the identical design as that used in the later Monroe 1860. The printer is a hammer and drum type impact printer that can print 21 columns (though it appears that only 19 columns are used on the 1665), and can print a 2 1/2 lines per second. The printer has a two-color ribbon that allows negative numbers to be printed in red. Numeric output is formatted with ten significant digits plus sign. When scientific notation numbers are printed, ten significant digits plus sign make up the mantissa, and two digits plus sign make up the exponent. The machine has a capacity of thirteen digits maximum, along with an extra guard digit, however only ten significant digits are printed. The printer has a number of alphanumeric characters that are used to indicate error and overflow conditions by spelling them out, i.e. printing ERROR when an invalid operation is performed, and OVERFLOW when the capacity of the machine is exceeded. As with all drum and hammer printers, this printer is rather noisy, with the hammer impacts and the ratcheting 'clunk' of the paper advance solenoid combining to make quite a racket.

Like all of the Compucorp-designed calculating machines from this era, the 1665 uses the rather unusual "2ND FUNC" key, which provides the result for the secondary function on keys with two functions listed when pressed after the function key is pressed. The "2ND FUNC" key really only recalls the content of memory register 9. When one of the dual-function keys is pressed, both functions on the key are calculated, with the primary function's result printed, and the secondary function's result stored in memory register 9, immediately recallable by pressing the "2ND FUNC" key. So, for example, to calculate the cosine of 60 degrees, one would key in "60", then press the dual-function key labelled "SIN/COS" (resulting in the sine of 60 degrees being printed), followed by the "2ND FUNC" key, which would print the result of the cosine calculation.

The 1665 is very much a general-purpose calculator. The selection of math functions, combined with its programmability makes it equally applicable for scientific, statistical, financial, and engineering use, although Monroe's marketing literature targets the machine specifically for engineering use. Included in the repertoire of functions available from the keyboard are: Trigonometric functions (sin, cosine, arcsine, arccosine); factorial; square root; power function (ax); natural and base 10 logarithm; radians to degree conversion; reciprocal; rectangular to polar conversion, and one key recall of the constants e and Pi. Additionally, a summing function key provides automatic accumulation of item count, sum of numbers, and sum of squares for statistical calculations.

There are a number of keys that control access to the calculator's ten memory registers. The "uparrow []" key recalls the content of the memory register specified by the following keypress (0-9). The "downarrow []" key stores the current content of the working register into the memory register specified by the following digit keypress. The "up/downarrow []" key exchanges the working register with the memory register specified by the following numeric keypress. The "=sigma 7" key adds the content of the working register to register 7, and likewise, the "=sigma 8" key does the same thing, but to register 8.

Rounding out the compliment of functions available from the keyboard are basic operational keys, including the "RESET" key, which serves as a "clear all" function. The "CHG SIGN" key toggles the sign of the number being entered. The "EXP" key shifts the calculator into exponential mode, allowing entry of the exponential part of a number in scientific notation. The "CLR x" key clears the content of the numeric entry register, allowing correction of erroneously entered data. The "PRINT x" key prints the current content of the working register, mostly used in programs to print out results of calculations. The "PRINT ALL" key causes the content of the working register, followed by the content of each of the memory registers, to be printed. Pressing this key results in a machine gun-like rattling of the printer as the numbers spew forth on the printout. A group of three paddle switches control the power, printing format (standard or forced scientific notation), and whether the calculator printer prints all transactions, or only results.

Unlike it's Nixie-display counterpart the 1665 is a fixed decimal point machine. A thumbwheel at the left hand side of the keyboard selects the number of digits in front of the decimal point. This is rather unusual, as most fixed-decimal point calculators allow the user to specify the number of digits behind the decimal point rather than those in front of the decimal point. What this curious method allows, however, is the calculator to always try to give the most precise answer possible, maximizing the number of digits behind the decimal point, while preserving the most significant digits that the user selects.

The Programming Control Panel for the Monroe 1665

A comprehensive programming environment allows the 1665 to perform a wide range of calculations. As with most programmable calculators of the time, the calculator is 'learn mode' programmable, meaning that the calculator is put into a learning mode, where keyboard presses are converted into program codes and stored into program memory, step at a time. A special set of keys and indicators located above the main keyboard provide access to the programming functions of the machine. Depressing the push-on/push-off "LOAD" key activates learn mode, causing keyboard keypresses to be stored into program memory. The "P" and "I" keys select the function of the 8-bit neon-lamp display for indicating the content of the program counter (P) or instruction code (I). When the "P" key is depressed, the display shows the current content of the program counter, which advances one step for each keypress stored into program memory. When the "I" key is depressed, the display instead shows the 8-bit keycode of the key that is stored into memory. The "TO()" key is used to enter jump instructions...pressing the key followed by a single keypress selects a destination address for a jump. The "HALT" key enters a halt instruction into memory, used to stop the program to allow the user to enter data manually. The "RESUME" key begins or restarts execution of a program at the current position of the program counter when pressed. The push-on/push-off "STEP" switch puts the calculator into single-step mode, causing execution of one instruction each time the "RESUME" switch is pressed. The "SENSE" switch is a push-on/push-off key that a program can test the status of, allowing the program to take different action depending on the state of the switch. Lastly, the "RCL P" switch enters an instruction that jumps to the content of the program counter just after execution of the last TO() instruction. This function is used for providing a simple subroutine capability. Base program memory on the 1665 is 128 steps, but the calculator (with addition of the CLEMP board) is expandable to 256 steps of program memory. One weakness of this series of calculators is the inability to make a record of a program in memory. Later Compucorp-designed calculators using the "ACL" chipset (such as the Monroe 1860) provided a means to print a listing of the program out on the printer.

An Unused Monroe Punched Card for use with the CR-1 Card Reader

Along with its learn-mode programming capabilities, the 1665 can be connected (via a DB-25 connector on the rear panel) to the optional CR-1 card reader to allow programs to be loaded from punched cards. The punched cards can each hold up to 40 steps (but Monroe recommends that only 32 steps be coded per card), requiring that larger programs be stored on multiple cards. The CR-1 card reader has a motor that draws the cards through the reader, with an optical system that reads each column of punches as 9-bit instructions that are stored in the calculator's program memory. With punched card programming, additional instructions are available that aren't accessable through the keyboard.

The CR-1 Punched Card Reader

Nine rows are used to represent the 9-bit operation code. One row is used as a 'validation' row -- this row must have a punch in it to make the operation code in the column valid. The other row is used as a 'cancel' punch -- if this bit is punched, the column is ignored by the card reader. This allows incorrectly punched columns to be ignored.

The Monroe "Port-A-Punch" Fixture for Punching the Cards

Cards can be punched with a pencil point or other pointed object, or, via the use of the "Port-A-Punch", an accessory fixture into which a card is placed, making it much easier to punch cards, while catching the 'punches' so they don't end up all over the place. The Port-A-Punch comes with a stylus to allow for easier punching of the cards. A transparent plastic overlay with small holes for the stylus to poke through is removed, a card slid into the Port-A-Punch, and then the plastic overlay slid in on top of the card. The card is then punched, and when complete, the card is slid out, ready to place into the CR-1 to be read into the calculator. The "Port-A-Punch" card punching fixture was actually invented by IBM and was licensed by a number of manufacturers including Wang and Compucorp/Monroe.

Via punched card programming, it is possible to access additional math capabilities of the calculator. Included are functions such as ex, integerize, direct branching, special conditional operations, shift operations, as well as access to special routines in the calculators microcode that allow low-level manipulation of the calculator's hardware.

The 1665 calculates at a rate in line with the other calculators based on the HTL chipset, with most operations completing within a couple tens of milliseconds. Some of the more complex operations, such as the ax and x! (factorial) functions can take up to three seconds to generate a result. Progrmaming functions aren't terribly fast, with 256 steps of "CLEAR x" instructions taking six seconds (with the printer turned off), or about 42 steps per second. A simple program that loops, adding 1 to the working register, runs at about 2.3 loops per second.


Sincere thanks to Janet Harrison for donation of this wonderful artifact to the Old Calculator Web Museum
This exhibit is dedicated to the memory of Thomas J. Harrison (3/13/1914 - 8/11/1996)

Text and images Copyright ©1997-2014, Rick Bensene.