The Main Calculating Board (Top), Sequencer Card, and Keyboard Interface Card of the Burroughs C3660

The machine uses a 16-digit Nixie tube display, with each NEC CA-80 tube containing the digits zero through nine, a right-hand decimal point, and a 'tick' located to the left of each digit for indicating the location of 'commas' for easier reading of large numbers. The tubes are held in place by a metal frame with a rubber-like material that provides shock isolation and alignment for the tubes. To the right of the 16 Nixies, there are two special gas-discharge displays. The left-most tube of this group provides negative sign indication and error/overflow status for the calculator. The rightmost tube is in the form of a rather unusual segmented-type display (nine segments, arranged in the standard 7-segment form, with the two segments vertically through the center) digit that provides additional information relating to program instructions.

Burroughs C3660 Nixie Display in Operation (Note Unique 7-segment Digit at Right End of Display)

The C3660, programming functions aside, provides the basic four math functions and square root. Square roots are calculated by entering the number to calculate the square root of, pressing the [÷] key to enter the number, then pressing the [+=] key to generate the result. The calculator also boasts ten accumulator-style memory registers. There are four keys at the right-most side of the keyboard that control the memory registers. A given memory register can be recalled and cleared (with the [*] key), recalled (with the ['diamond'] key), subtracted from (with the [-] key), or added to (with the [+] key). When performing a memory function, the memory function key is pressed, and is immediately followed by a single digit from zero through nine to indicate which memory register the operation is to be performed upon. A special keystroke sequence, [*] followed by [.], clears all of the memory registers to zero.

Closer View of Burroughs C3660 Keyboard

The other functions of the machine are pretty straightforward. Addition and subtraction work adding machine-style, with the larger white [+=] key used for addition, and the smaller red [-] key for subtraction. Multiplication and division work as expected, with the [+=] key generating the answer. A push-on/push-off [K] key enables the constant function for multiplication and division when depressed. The [CD] key clears the display register, and the [RC] key recalls the last operand to the display, useful for swapping operands for division problems. The [CHG SGN] key toggles the sign of the number in the display.

The calculator can operate in fixed or floating decimal point modes, which are set by a couple of slide switches at the last of the keyboard panel. One two-position switch selects fixed or floating decimal mode for the computing unit of the calculator. Another eight-position switch selects the decimal point position of when the calculator is operating in fixed point mode (with selections of 0, 1, 2, 3, 4, 6, 7 digits behind the decimal), and an unusual setting "<=15" which provides for floating point storage of items in the calculator's memories evening when the computing unit decimal point is set to a fixed position. With this combination of switches, it is possible to have combinations of fixed point and floating point representations in the computing unit and memory unit of the calculator.

Detailed View of the C3660 Mag-Card Reader (Note read/write head with bundle of 8 wires (4 tracks) coming out of it)

Programs (but not memory register content) can be stored on mag-cards for future use. The program memory on the calculator is volatile, meaning that when the power is removed the content of the memory is lost (unlike machines such as the Hewlett Packard 9100B that use magnetic core memory for program/data storage, which does not lose its content when power is removed). The mag-card reader allows programs to be written onto the card, and later re-loaded into memory.

A Mag-Card for the Burroughs C3660

The card reader accepts a 4" x 2" mag-card that has two 'sides', A and B, for recording program steps. Each side can contain 144 program steps, for a total of 288 program steps per card. Notches which can be cut out of each end of the card cause that side of the card to be write-protected. Each mag card came with a protective sleeve that held the card and has areas on the outside to write information and instructions for use of the program contained on the card.

A Ten-Pack of Magnetic Cards (Burroughs Part #70-1001-711)
Sincere thanks to Mr. Tom Roughton for donation of a number of original un-opened C-3660 MagCard 10-Packs

The card itself has a writable surface on the front side with areas for recording the program number, date, title, and other information about the program on the card. The magnetic cards came in pacakges of 10 each (with sleeves) as Burroughs Part #70-1001-711.

MagCard Sleeve

There are three keyboard buttons that control the card reader, one for loading a program into memory ([ENTER]), one for storing a program on a card ([RECORD]), and one for comparing the content of a card with memory, for verification purposes ([VERIFY]). The card is placed in the slot on the top of the machine, and the appropriate function key pressed to initiate the action, The card is drawn into the reader, then pushed back out by a small motor as the operation is completed.

Mag-Card Head Cleaner Card

A special cleaning card was supplied with the machine. The card has a special material on it that would clean the read/write head of the mag-card reader when cycled through it. The cleaning operation involved placing the card in the reader, then pressing the [RECORD] button a number of times to cycle the cleaning card through the reader.

The operating mode of the calculator is set with a slide switch on the keyboard panel that has five different settings. "MAN" (Manual) mode is selected to use the machine as a normal calculator, with program execution disabled. "AUTO" mode changes the function of the [+=] key to act as a 'start' key to cause program execution to begin (or continue). In "LRN" (Learn) mode, the calculator stores keypresses as program steps. As each key is pressed, a code representing the key is stored in program memory. "DBG" mode is used to step through programs step at a time, with the panel display indicating the step being performed, and the calculator performing each operation step at a time each time the [+=] key is pressed. Lastly, there is "CHK" (Check) mode, which works similar to Debug mode, except program steps are not executed as the user steps through the program. Check mode is used to verify correct entry of the steps of a program.

A bank of 16 round indicators on the front panel of the machine below the display panel, and the 7-segment display tube in the display are used for indicating program steps. The round indicators have legends behind them which, when the indicator lights, shows the key programmed into that location. The digit keys zero through nine and the decimal point key are indicated by the 7-segment display tube, with the rest of the keys showing on the indicator panel. The indicator panel has the following legends on each indicator:

C

CD

X

÷

+=

-

RC

*

<>

M-

M+

CS

JP

EJ

EP

H

C	C	Clear Machine
CD	CD	Clear Display
X	X	Multiply
÷	÷	Divide
+=	+=	Add
-	-	Subtract
RC	RC	Recall Last Operand
*	*	Recall Memory Register X and Clear
<>	'Diamond'	Recall Memory Register X
M-	-	Subtract from Memory Register X
M+	+	Add to Memory Register X
CS	CHG SIGN	Change Sign
JP	JMP	Jump to Tag
EJ	END JMP	End of Jump Tag
EP	END PRG	End of Program Marker
H	HLT	Halt Instruction

As each keypress is loaded into memory, the appropriate light in this panel lights up to confirm the keypress. This panel of lights is also used for verification of program steps, debugging, and listing out programs.

The machine has the capability of eight different conditional branches, and six unconditional branches within any given program. The calculator uses keypress tags to mark locations in programs for branching operations to transfer to. Branching functions are designated by the [JMP] (Jump) and [EJ] (End Jump) instructions. The tags consist of the "JMP" or "EJ" instruction, followed by a single keypress that indicates the tag. The keys 0 through 7 and [HLT] are tags for conditional jumps, and the 8, 9, 0, decimal point, [CHG SIGN], and [C] keys are the tags for unconditional jumps. When a branch is needed in a program, the [JMP] key is pressed, followed by the tag key identifying the type and destination of the branch. When executing a [JMP] instruction, the calculator searches through memory for an [EJ] instruction followed by the same tag key, and begins execution at that point. In the case of a conditional branch, the display is checked for zero content, and if it is zero, the branch is taken to the specified tag, else the tag key is skipped, and execution continues with the next step in sequence. While searching for the matching [EJ] for a given [JMP], if the end of memory is reached, searching rolls over to the beginning of memory, making backwards jumps possible. This method of branching is a primitive version of Label-type branching used in later programmable calculators by Hewlett Packard, Texas Instruments, Tektronix, and others.

The [HLT] key stops program execution at the end of a program, or when data input by the operator is required. The machine has the capacity for two programs to be loaded into memory at once, with an [ALT PGM] key designating which program is to be executed. When the calculator is in "AUTO" mode, program execution begins when the [+=] key is pressed. If the [ALT PGM] key is pressed before starting a program with [+=], the machine searches through memory looking for two consecutive [END PGM] instructions, and sets up for program execution to begin after the last of the two [END PGM] instructions. This function is useful for programs which have inverse functions. For example, a program that calculates the sine of a given angle could have the [ALT PGM] set up to calculate the arcsin when invoked.

Program Library Storage Binder

As an incentive to buyers, Burroughs offered a printed library of many different math, science, and business functions as a free bonus with purchase of a calculator. The user could type these programs into the machine, then record them on magnetic cards for future use. Optionally, the entire library of programs could be purchased already recorded on magnetic cards. The program library includes programs to perform trigonometric functions, exponential and logarithmic functions, statistics, financial functions (e.g., payment/interest calculations and depreciation), and, peculiarly, a set of programs for "Ford Motor Credit Corporation" accounting functions.

This particular Burroughs C3660 was built in the mid-1971 timeframe, based on date codes on the IC devices in the machine. The 3660 originally sold for $1795 when it was introduced. An upgraded version of the 3660 was available, the model C3661, which was four times faster than the C3660, provided two additional memory registers, had double the amount of program storage space (288 steps), included a subroutine capability, additional conditional branching instructions, and the ability to store the content of the memory registers onto magnetic cards.