Olivetti Summa Prima 20


Collection Item

Home | Contact

Brand:

Olivetti.

Model:

Summa Prima 20.

Origin:

Ivrea, Italy. This example was built in Argentina. Olivetti was stablished in Argentina in 1932 and during decades made a complete line of mechanical, electro-mechanical and electronic typewriters and calculators for the internal market and for export.

Introduction:

1960?

Type:

Basic desktop "muscle-powered" mechanical calculator with printer, no display.

Functions:

Add and subtract. Multiply by continuous adding. Subtotal and total.

Logic comments:

Memory organization: The Olivetti Summa Prima 20 uses 2 mechanical registers:

- Input Register (IR): Stores the keyboard input. Any digit you enter is appended to the right of the IR, the remaining digits are shifted one position to the left (or 2 or 3 positions if you press double or triple zero key). The number of actual digits in IR is shown on the display. This register have a precision 10 digits in integer format with no decimals. The IR is automatically cleared after a normal add or subtract operation (except if "X" switch is up), also you can clear it by pulling the spherical clearing lever up and down.

- Accumulator Register (AR): Stores the result of a chain of adding or subtracting operations. Each time you pull the main handle the IR is added or subtracted from AR. It have a precision 11 digits also in integer format with no decimals. This register is cleared by pulling the main handle having the joystick in "Total" position. There is no "All Clear" key, also you can't power off the machine to clear... !

Principle of operation:
When you pull the main handle the machine operates depending on the joystick position, as follows:
- Joystick in Normal (Center) position: IR is added to AR. Also the IR is printed in black color and then IR is cleared for a new input. If "X" switch is up then IR is not cleared to allow repetitive adding or subtracting (see below).
- Joystick in "-" Subtract (right) position: IR is subtracted from AR. Also IR is printed in black color and then is cleared for a new input. If "X" switch is up then IR is not cleared to allow repetitive adding or subtracting. You can push the lever on the right to lock the joystick in subtract position in order to subtract several numbers.
- Joystick in "◄" No add (left) position: IR is printed in black color and then is cleared, unless "X" is up. AR is not altered. This feature is used to print an "item number", "order number" or something else not used in operations but used to clarify the reading of the paper tape.
- Joystick in * Total (up) position: AR is printed in red and then is cleared for a new chain of operations. Also IR is cleared. If "X" switch is up, is automatically returned to the down position.
- Joystick in ◊ Subtotal (down) position: AR is printed in red but is not cleared to add or subtract more numbers to this chain. Also IR is cleared, unless "X" switch is up.

Five position joystick (here is in No Add position)

Multiplication:
This calculator can not multiply but you can do it by succesive adding.
For example, to compute 725 x 5 you can follow this "basic" method:
- Clear the machine: Set joystick in Total position and then pull main handle.
- Set joystick in Center position for normal add.
- Enter 725.
- Set "X" switch up.
- Pull the main handle 5 times. IR will be added 5 times on AR.
- To see the result set joystick in Total position and pull the main handle. Is printed the result 3625.

This method works fine if you multiply by a small number, 5 in this example, but what if you want to multiply 725 x 213? You can complete this operation by pulling the main handle only 6 times using the "abbreviate" method!
Do the following:
- Clear the machine.
- Set joystick in Center position.
- Enter 725.
- Set "X" switch up.
- Pull the main handle 3 times for the rightmost digit of 213.
- Now type 0 (zero) to shift IR one position to the left and convert IR into 7250.
- Pull the main handle 1 time for the digit "1" of 213.
- Now type 0 again to convert IR into 72500.
- Pull the main handle 2 additional times for the digit "2" of 213.
- Now you can see the final result 154425.
The operation you made was this:

725 +
725 +
725 +
7250 +
72500 +
72500 +

This method is quite similar to the operation of the Curta mechanical calculator. Setting a 0 (zero) into IR is similar to shift one position the carriage of the Curta.

Also you can use the "expert" mode used to multiply with a Curta.
For example, if you want to multiply 12866 x 897, you can compute 897 as 1000-100-3, so the sequence is:
- Clear machine.
- Set joystick in "-" (subtract) position.
- Lock the joystick in "-" position by pulling the lever next to the joystick.
- Enter 12866.
- Set "X" switch up.
- Pull the main handle 3 times to accumulate IR * -3.
- Now type 0 (zero) twice (or double-zero key) to shift IR two positions to the left and convert IR into 1286600.
- Pull the main handle 1 time to accumulate IR * -100.
- Enter 0 again to convert IR into 12866000.
- Now set the joystick to center (normal) position, first release the lock lever next to joystick.
- Pull the main handle 1 additional time to accumulate IR * 1000.
- Now you can see the final result 11540802.
The operation you made was this:

12866 -
12866 -
12866 -
1286600 -
12866000 +

Please compare the 3 methods described for multiplication:
If you multiply 12866 x 897 you need to pull the main handle...
- Using the "basic" mode: 897 times.
- Using the "abbreviate" mode: 24 times (8+9+7).
- Using the "expert" mode: Only 5 times! (1+1+3).

Keyboard:

Keyboard and parts:

Digits 1-9: This calculator accepts a maximum of 10 digits including the zeroes.
Zero, double and triple zero: one, two and three zeroes, the calc does not accept input leading zeroes.
Multiply mode: Slide to "X" position to avoid clearing the IR and allow to multiply by repetitive addition.
Clear digit/entry: Slide down to clear the last entered digit in IR. Slide up to clear the full content in IR.
Display: See description bellow.
Joystick: Changes the operation of the main handle:
Center (normal) position: Adds the IR register to AR register.
Subtract: Subtracts the IR register from the AR register. After operation returns to the center position.
Total: The AR is printed and the content is cleared to begin a new series of operations.
Subtotal: The AR is printed and the content is not cleared to allow further operations.
Non add: The input register IR is printed but not added or subtracted to the AR register.
Subtract lock latch: Slide the latch down at the same time you put the joystick in Subtract mode. This will lock the subtract mode to subtract several numbers. Slide the latch up and the joystick will return to the center position.

Display:

This simple display shows the number of digits you entered from the keyboard into the IR.
After a digit 0-9 is pressed the red needle will be desplaced one position to the left, or two positions if you press the double zero key. After pressing +, -, C keys or Subtotal or Total joystick position the red needle will move to the right.
The small rectangle on the left indicates when the result in AR is negative.

In this case there are no digits into the IR.

Printer:

Arguments and results are printed on a printer station with 11 digits plus 2 special positions using a 2-color ink ribbon: black for normal input and red for total and subtotal. The first special position indicates * for Total and ◊ for Subtotal. The second position is for negative indication. Here you can see a typical print-out:

Please note this calc do not operates with decimals, only integer numbers!

Remarks:

(8/10) Very good, tested and fully working.

Size:

11.02 x 7.28 x 4.72 in, 280 x 185 x 120 mm. Weight: 9 lbs, 4.1 kg.

Accesories:

Dust cover, user manual in Italian and quick reference guide in Spanish.

Condition:

(8/10) Very good, tested and fully working.

Download:

User manual "Istruzioni per l'uso della machina addizionatrice Olivetti suma Prima 20" (in Italian), click here.
Quick Reference Guide (in Spanish), click here: Page 1 and Page 2.

Some advertisements of the Summa Prima 20 I found on the web, click on the thumbnail.