John Wolff's Web Museum

Compucorp 155 "Surveyor" Desk Calculator

C155 External View Compucorp Model 155 "Surveyor", S/N 3555019
Functions: Scientific, programmable, special surveying functions
Programming: Keyboard or external 80-column card reader
Technology: MOS-LSI (AMI, 30 chips)
Display: 21-column impact printer, 8-bit register display
Dimensions: 380W x 405D x 175H, weight 10.9kg
Manufactured: Computer Design Corporation, Los Angeles, January 1972
Original Owners: John B White Pty Ltd, Sydney, Australia


Compucorp Label The Model 155 "Surveyor" from 1971-2 is one of Compucorp's first generation of programmable scientific desk calculators. All of the 100-series machines have a common computer-like architecture based on a set of 18 custom-designed MOS-LSI chips. The basic design was then customised for specific industries (science, statistics, surveying, etc) by changing the internal program ROMs and the keyboard layouts. The machines were available with either a numeric (Nixie tube) display or an internal printer, but not both. A similar range of machines customised for commercial applications was sold as the 200 series.

The 100-series machines all provided scientific notation, trig and log funtions, powers and roots, and multiple storage registers with register arithmetic. The specialised functions on this Model 155 "Surveyor" include calculations of bearings, traverses, and rectangular to polar conversions.

The machine described here was donated to the museum by John B White Pty Ltd, Consulting Surveyors, of Sydney, Australia. It was purchased new in 1972, and was used regularly for survey computations including closes, areas, adding & subtracting bearings, missing bearings and distances, calculating joins between co-ordinates, calculating curves, and various conversions. To do a property close it took around 8 punch cards! The Compucorp was retired in around 1979 in favour of the new HP-25C programmable pocket calculators.

This page gives a brief overview of the construction, circuitry, and operation of the Compucorp "Surveyor" Model 155.



Internal View Internal view

This view shows the internals of the machine with the cover removed.

The printer mechanism and paper roll tray occupy the left rear section of the machine. The power supply regulator board is in the centre rear, with seven logic boards mounted vertically at the right. The boards plug in to a horizontal backplane at the bottom and are secured by a clear acrylic retainer at the top. Three of the boards have additional edge connectors at the front for the printer, keyboard, and card reader interfaces. The keyboard assembly occupies the front section of the case.

Chassis and power supply Chassis and power supply

The machine is built on a substantial die-cast chassis which measures 355x380x12mm (approx).

The power transformer is mounted at the centre rear, with the line filter and storage capacitors at the left. A mains voltage selector plug is mounted above the transformer. Two MJE2955 regulator transistors are bolted directly to the chassis (in front of the capacitors). The power supply produces logic supplies of -15V and -30V DC, and a 200V DC (approx) supply for the neon indicators. The nameplate rating is 220V AC 60W.

The two plug-and-socket connectors at the front of the central vertical panel carry the mains switch and 200V DC connections to the keyboard assembly. The entire keyboard can be removed as a unit with no extraneous leads or off-board connections.

The printer assembly is mounted on the four rubber bushings on the left-hand side of the chassis, and the keyboard on the three pillars at the front.

Keyboard detail Keyboard construction

The main section of the keyboard is built using 43 individual key switch modules. Each module contains a glass reed switch mounted vertically along the central axis. The reed switch is surrounded by a sliding collar and an annular magnet, which are pressed down by the keystem above. The sliding collar is a fairly close fit in the outer casing, which makes the switches susceptible to sticking when dirt enters around the keystem. Three switches have previously been replaced in this machine, and two more needed attention during the overhaul.

The upper section of the keyboard contains the power and print enable switches (left, with provision for two more), and a row of push-buttons and indicators for use in programming. A rotary switch at the left front selects 0 to 9 decimal places.

The printer The EP-101 impact printer

The printing mechanism in the Compucorp "Surveyor" is historically significant in its own right, being an OEM version of the classic EP-101 impact printer - the device which launched the now-famous Epson Corporation.

The EP-101 evolved from a printing timer that was developed by the Seiko group for the Tokyo Olympic Games in 1964. Well over a million units were built by the Shinshu Seiki Co. since it first went on sale in 1968. The company adopted the "Epson" brand ("Son of EP") for a new model in 1975, and became the Epson Corporation in 1982.

The printer has an open "pancake" motor and transistorised speed controller under the left-hand cover. The motor operates at a nominal 17V DC, and drives a mainshaft across the rear of the machine at 3300 RPM. Gearing under the right-hand cover drives the print drum at the top of the machine at 200 RPM. The drum is about 25mm in diameter and 90mm long. It carries 336 raised numerals and symbols in 16 rows of 21 characters. The rotating mechanism runs continuously, but consumes only 2.5W and is almost silent when properly maintained. It has significant inertia, and takes nearly a minute to come to rest when the power is removed.

The 21 print hammers are located at the front of the rotating drum, under the cover at the top of the ribbon mechanism. The hammers and ribbon are in front of the paper, with the rotating print drum behind, so that the drum remains clean and free of ink. The lower section of the printer contains 21 small selector solenoids, arranged in three banks of seven. Energising a solenoid pushes a selector lever forward into the path of a flange on the rotating mainshaft. The flange flicks the lever down as it passes, operating the hammer via a short linkage. The hammer presses the ribbon and paper momentarily against the rotating print drum as the required symbol passes under.

In this OEM version of the printer, the selector solenoids are driven directly by the logic circuits in the calculator. Sensors on the mainshaft and the print drum provide feedback signals to synchronise the timing and sequencing. The calculator may have to wait up to a full revolution of the drum (300mS) until the required symbol reaches the printing position, and must then energise and release the corresponding solenoid within one revolution of the mainshaft (18mS). Two larger solenoids control the 2-colour ribbon and the paper feed mechanisms. The total cycle time is about 350mS, or 3 lines per second.

The printer measures about 165mm wide x 140 deep x 100 high, and weighs nearly 3kg. All power, solenoid, and feedback connections are made through a single cable and card edge connector. This unit is still in perfect working order after 35 years.

The card reader The card reader

The Compucorp R-8 card reader measures 150W x 230D x 100H (approx) and weighs 2kg. Standard 12-row 80-column cards are inserted one at a time through the slot at the front, and are drawn through and stacked in the tray at the rear.

The drive mechanism uses a small 115V AC induction motor with a die-cast and plastic reduction gearbox. One of the synthetic gears is made of a material which has not stood the test of time, so the reader is presently inoperable.

The reading mechanism uses two small incandescent lamps, an acrylic light guide, and a set of twelve phototransistors and discrete-component amplifiers. The top cover can be lifted off to change the lamps.

The reader attaches to the rear of the calculator through a short cable and a DB-25 connector. Warning: the cable and connector also carry the 115V mains supply for the drive motor.



Logic board The logic boards

The logic circuitry is built on a set of seven double-sided boards measuring 200mm wide x 140 high. The boards carry a total of 30 MOS-LSI devices in 40, 28, and 16-pin ceramic packages, from the AMI "HTL" chipset. The date codes in this machine range from May 1971 to the 3rd week of January 1972. There are also about 50 discrete transistors and 60 diodes, primarily in the keyboard and printer interfaces.

Power and logic connections are all made through the backplane, while I/O signals to the keyboard, card reader, and printer are taken through separate edge connectors on the fronts of the appropriate boards.

Details and photographs of all of the boards are provided on separate pages.



Unfortunately the Operator's Manual for this machine has been long discarded. The notes following have been prepared from personal observations, and from comparisons with the operation of the Compucorp 324. I would be grateful for any further information, especially in regard to the angle functions and the programming capabilities.

The keyboard The keyboard    Larger image (71kb)

At the top left of the keyboard are the power switch and the printer enable switch, with space for two more switches for use in other models. Then comes a power indicator lamp, a large "Resume" key, and a row of programming keys and indicators.

Below the first two switches are keys for "Print X" and "Print Angle". The printer runs continuously and normally prints all entries, operations, and intermediate results. The printer still continues to run when disabled, but only prints specific results when requested from the "Print" keys or the program.

The decimal point switch at the left-hand side selects from 0 to 9 decimal places. Settings 0 to 8 print 10 digits around a fixed decimal point, with leading zeros supressed. "Overflow" is printed when the available range is exceeded. When the switch is set to 9 the machine changes into "10+2" scientific notation, printing a full 10-digit mantissa with 9 decimal places and a 2-digit exponent.

The numeric and ASMD keys in the centre of the keyboard operate as expected. There is a "Change Sign" key, a "Clear X" key, and an "EXP" key for entering exponential notation. It is necessary to enter both digits of the exponent, as 2 EXP 3 means 2^30 rather than 2^03. The EXP key produces an immediate overflow if the decimal switch is set to anything other than 9.

The four black keys to the left of the keyboard operate on the ten internal storage registers. The keys are labelled STOn, RCLn, EXCHn, and +n, and function as expected. To the left of the register keys is a "Reset" key and three angle keys. One key converts degrees, minutes, and seconds to decimal degrees (and back), but the functions of the "Bearing" and "Φn" keys are not immediately obvious.

At the top right of the keyboard are keys for square roots, powers, and reciprocals, which operate as expected.

The remainder of the yellow keys are marked with dual functions, and operate in conjunction with the "2nd Func" key at the top right. The operation is unusual in that pressing the labelled key calculates both functions, displays the first in X, and stores the second in a separate internal register. Pressing the "2nd Func" key switches between the two registers. For example, "30 Sin/Cos" calculates both sin 30 and cos 30 and displays 0.5. If you wanted cos 30, press "2nd Func" to retrieve 0.866. "2nd Func" can be used repeatedly to switch between the two results.

Dual-function keys are provided for Sin/Cos, inverse Sin/Cos, common and natural logs, degrees to radians and inverse, Integer/Fractional parts, and the constants π and e. There is no Tan key - tangents can be calculated from the sequence "X, Sin/Cos, Divide, 2nd Func, Equals".

The remaining four black keys at the lower right are basically rectangular to polar conversions, customised to assist in calculating a survey traverse. The first of the keys converts azimuth and length ((Az,l) or (&Theta,r)) to latitude and departure ((y,x) or north and east coordinates). The second key calculates the inverse.
For example, "30 (Az,l) 2 Equals" displays the northing (1.732 or root 3), and pressing "2nd Func" displays the easting (1.0). (Surveyors measure azimuth clockwise from North). Pressing the FWD or INV keys instead of Equals accumulates the north and east increments from each step of a traverse into storage registers 0 and 1. The coordinates can be retreived at any stage of the traverse, and should total to zero on returning to the starting point.

The programming keys Programming

Programming is controlled by a set of push-buttons at the upper right of the keyboard, along with the large "Resume" key near the top centre. The eight neon lamps display the value of the program counter or the instruction register, as selected by the "P" and "I" buttons. The lamps are arranged in octal groupings, with the lenses labelled in 4-2-1 sequence to assist in reading.

The program memory appears to be organised as 256 contiguous locations, each holding one instruction or numeric value. Every sixteenth location (20 octal) is directly addressable with the "TO[ ]" (as in "go to") button, and can be used as a program entry point. The first ten entry points at (octal) 0,20,40...220 can be addressed from the keyboard with TO[0] to TO[9]. The remaining six at 240 to 360 are probably intended to be accessed through program cards, but can also be obtained with non-numeric keys. For example, TO[CHS] goes to address 260. The addresses are displayed in the lamps with the "P" button down.

With the "I" button down, the lamps will display the instruction opcode as each key is pressed. The keyboard opcodes are detailed in a separate table. The opcodes appear to be 6-bit (octal 000 to 077), with the keyboard using 43 of the possible 64 combinations. Some of the keyboard opcodes appear to be additive - for example, pressing 1/x (opcode 54) and numeral 2 (opcode 2) simultaneously registers as opcode 56 and calculates square root rather than reciprocal. It is very likely that further opcodes and addressing modes exist, for access only when programming via the card reader.

To enter a program, the program counter is set to the desired entry point with the TO[ ] key, the "LOAD" key is latched down, and the program is entered from the keyboard or card reader. The program counter advances as each instruction is registered. The "LOAD" key is released on completion. To run the program, the entry point is set again with the TO[ ] key and the "Resume" key is pressed. The "HALT" key can be used as a program instruction to halt execution to allow data entry from the keyboard. Execution is continued with the "Resume" key. Several simple programs have been loaded and run successfully from the keyboard. The "STEP", "SENSE", "RCLP", and "STOP" keys have not been investigated in detail.

Resources for further information

Original text and images Copyright © John Wolff 2006-09.
Use at own risk; beware of errors; suggestions for improvement welcome.
Last Updated: 15 July 2009

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