DATAMATH CALCULATOR MUSEUM
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DATAMATH CALCULATOR MUSEUM |
Characterization of General Instrument 250 / C-500 Product Family Single-chip Calculator Circuits
The Datamath Calculator Museum DCM-50A (PLAYGROUND) supports the Characterization of the GI 250 / C-500 Product Family single-chip calculator circuits using the DCM-50A Playground C-500 Family Adapter mounted on top of the DCM-50A PG Frame Carrier and connected to the DCM50A Playground KBD102 Keyboard. The optional DCM-50A Playground Digilent I/O Extender supports Characterization and Reverse-engineering of GI 250 and GI 251 single-chip calculator circuits.
Device-under-Test:
| • Package Markings Top:
250 GI 7211 • Package Markings Bottom: • Donor Calculator: Litton Royal Digital III, March 1972 |
Keyboard: The Litton Royal Digital III makes use 19 gold-plates squares/rectangles etched on its Main-PCB and a wired stylus with a metal tip to operate the calculator. The 19 key contacts are connected with series resistors to 19 pins of the GI 250 single-chip calculator circuit, while the stylus is driven with a pulsed output signal of the GI 250 and a transistor working as buffer (voltage follower).
Keyboard Matrix Litton Royal Digital III:
GI 250 |
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| Pin# | KB_EN | |
| K0/SB | 12 | 0 |
| K1/SF | 9 | 1 |
| K2/SG | 10 | 2 |
| K3/SA | 11 | 3 |
| K4 | 5 | 4 |
| K5/SC | 8 | 5 |
| K6 | 4 | 6 |
| K7/SD | 7 | 7 |
| K8/SE | 6 | 8 |
| K9 | 3 | 9 |
| KP/SDP | 14 | . |
| KC | 20 | C |
| KC | 22 | K |
| KT | 21 | ↔ |
| KA/D48 | 17 | + |
| KS/D37 | 16 | − |
| KM/D26 | 18 | × |
| KD/D15B | 19 | ÷ |
| KEQ | 15 | = |
Display:
The Litton Royal Digital III makes use of four low-voltage VFD tubes
manufactured by an unknown company and the grids connected with four additional
transistors to the
respective 4 Digit Outputs D15, D26, D37, and D48 of the GI 250. The anodes of
the segments are connected directly to the 8 Segment Outputs SA to SG and SDP of
the GI 250 and biased to approximately -25 Volts.
Display Layout:
| Unknown VFD |
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| The Segment drivers A-G and DP (Decimal Point) are connected to the unknown display in the pictured way. |  |
Display Fonts:
| Type | Calculator | Number Fonts | Decimal Separator |
Thousands Separator |
Entry Overflow |
Calculating Overflow |
Minus |
| GI 250 | Litton Royal Digital III |
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n.a. |  |
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Keyboard Scanning:
The GI 250 uses a keyboard scanning matrix with all of its 19 keys connected
directly to a corresponding pin, while the common signal is connected to a
dedicated Keyboard Enable pin. Seven of the 19 keys are using dedicated Key
Input pins, four are shared with the Digit Outputs and eight are shared with the
Segment Outputs. To allow for the use of the Digit and Segment Output pins as
Key Input pins, the GI 250 is blanking the display completely before each
Display Scanning Cycle while reading the keyboard. Scanning the keyboard is
indicated with a short pulse on the Keyboard-Enable output pin while briefly
disabling all of its segments and digit output drivers before and after the
pulse.
Display
scanning: Display scanning is performed in is performed in D15 → D48
direction at a rate of about 830 Hz:
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• State Time = 2 Clocks = 0.020 ms @ CK=100 kHz • Keyboard Time = 8 States = 0.160 ms @ CK=100 kHz • Digit Time = 13 States = 0.260 ms @ CK=100 kHz • Scan Time = Keyboard Time + 4 Digit Times (D15 to D48) = 1.2 ms @ CK=100 kHz |
Work-in Progress
If you have additions to the above article please email: joerg@datamath.org.
© Joerg Woerner, July 8, 2025. No reprints without written permission.