Compucorp 325 Scientist

The Compucorp 325 Scientist could be called the best engineered calculator of its time. Introduced in the year 1972 the 300-line from Computer Design Corporation (CDC) were far ahead of their time. CDC, Los Angeles designed an universal data-processor with external ROM's for the operating system and mathematical functions and external RAM for data- and program-storage. 
This rare 325 Scientist is unique in the 300-line sporting a huge drum printer and a desktop style housing. An external tape recorder provides a storage capacity of about 75,000 Bytes. The 325 Scientist was replaced soon with the 327 Scientist with four times of internal memory.

Much more machines appeared in the smaller 300-line housing, like the Scientist models 320G, 322G and the 324G:

Don't miss the wonderful web-sites of Rick Bensene and Viktor T. Toth to get an impression of the remaining models for financial or statistical operations. 

The integrates circuits of the 300-line were originally manufactured by AMI, later TI was chosen as a reliable second source. Please use the sixteen thumbnails below to get a deeper look inside the wonderful Compucorp 325 Scientist.

Compared with the sleek Compucorp 324G this 325 Scientist looks like the big brother. We feel familiar with the keyboard layout and the display, only the big drum printer looks a little bit out-of-place to us.

Dismantling the Compucorp 325 Scientist takes some time. After removing the upper shell of the housing with the keyboard and display attached we get a bottom housing dominated by the huge drum printer. On the left of the printer we locate the power supply and opposite the four stacked printed circuit boards (PCBs) forming the calculator brain.

Removing the printer and the four PCBs reveals additional details: Backplane connectors known from the Compucorp 324G, lot of cable wiring instead PCBs and a huge cooling area for the power supply.

Final step of our tear-down. No reason to remove the power supply from the chassis, everything is cleaned and waiting to be put back together into a working calculator. In the meantime we could discover the different parts of the Compucorp 325 Scientist.

The large drum printer of the Compucorp 325 Scientist was manufactured by Shinshu Seiki Co., Ltd. since 1982 better known under the EPSON brand. The printer mechanism uses a spinning drum behind the paper. On the drum are all of he numbers and letters required for each column. In front of the paper is the ribbon and then a row of hammers. The hammers fire towards the paper just when the desired character is spinning by the back of the paper. The hammer strikes the ribbon pushing it into the paper and making an imprint of the character on the drum on the paper! When the printer is deselected it powers down and stops spinning the drum.

The Compucorp 325 Scientist uses a high quality keyboard with a tactile and audible feedback. Please notice that a lot of possible switches are not accessible through the keyboard plate. They seem to be used on other models from the Compucorp 300 line.

The keyboard is composed from a plastic mold and a printed circuit board to hold the switches. Not shown are the small springs to support the contacts.

The display of the Compucorp 325 is a 16-digit Panaplex II style manufactured by Burroughs in the United States. We know this display from early Texas Instruments calculators like the SR-20.

Under the keyboard assembly one printed circuit board (PCB) is located. It scans the keyboard and the display with the TMC1869 and TMC1884 circuits. Some discrete transistors and IC's drive the high voltage of the Panaplex II display.

The brain of the Compucorp 325 uses a stack of 4 PCBs. The top PCB drives the printer with the TMC1868 circuit. Some discrete transistors drive printer inputs.

The second PCB of the Compucorp 325 assembles the processing unit of the Compucorp 300-line calculators. The four IC's numbered TMC1866, TMC1867, TMC1870 and TMC1872 feature a ROM programmable unit. The fifth IC named TMC1871 is known as interface circuit to the memory.

The third PCB of the stack gives the Compucorp 325 its identity as a programmable calculator for scientists: Five ROM (Read Only Memory) are used for program storage of the calculator operating system (8KR04D, 8KR12C, 8KR089, 8KR258 and 16KR104, all from General Instruments).

The forth PCB accommodates eight RAM (Random Access Memory) to store both data and user programs. Each INTEL P2102 RAM chip hold 1024 bits.

An additional PCB holds most components of the power supply. Please notice some transistors attached to the metal chassis for cooling.

The label of a wonderful programmable desktop calculator:

If you have additions to the above article please email: joerg@datamath.org.

© Joerg Woerner, December 21, 2002. No reprints without written permission.