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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 / C-500 Product Family single-chip calculator circuits.

GI 250

Device-under-Test:

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
	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
KK	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 Vacuum Fluorescent Display (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

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			n.a.

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:

GI 251F

Device-under-Test:

Keyboard: The keyboard assembly of the Litton Royal Digital V uses 18 spring-supported plastic keys pushing fingers on a stamped sheet-metal piece against large, gold-plated contacts on the Main-PCB. The sheet-metal piece itself is connected through two additional contacts with the Main-PCB.

Keyboard Matrix Litton Royal Digital V:

GI 251F
	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
KK	22	K
KA/D48	17	+
KS/D37	16	−
KM/D26	18	×
KD/D15B	19	÷
KEQ	15	=

Display: The Litton Royal Digital V makes use of an 8-Digit low-voltage Vacuum Fluorescent Display (VFD) manufactured by Futaba and known as Type 8-CT-01, soldered with its 19 pins directly to the Main-PCB. Both the 4 Digit Outputs D15, D26, D37, and D48 and the 8 Segment Outputs SA to SG and SDP of the GI 251F are inverted with two Mitsubishi M58212 chips and connected through a bank of 18 discrete transistors to the . Eight inverting transistors are used for the D1 to D8 VFD grids, with two extra, non-inverting transistors selecting the upper or lower digit group of the GI 251F. Eight inverting transistors are used for the A to G and DP VFD anodes, biased to around -44 Volts.

Display Layout:

Futaba 8-CT-01

The Segment drivers A-G and DP (Decimal Point) are connected to the Futaba 8-CT-01 display in the pictured way. Segment H is not used.

Display Fonts:

Type	Calculator	Number Fonts	Decimal Separator	Thousands Separator	Entry Overflow	Calculating Overflow	Minus
GI 251F	Litton Royal Digital V			n.a.

Keyboard Scanning: The GI 251F uses a keyboard scanning matrix with all of its 18 keys connected directly to a corresponding pin, while the common signal is connected to a dedicated Keyboard Enable pin. Seven of the 18 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 251F is blanking the display completely before and in the middle of 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 two 4-digit groups in D15 → D48 direction at a rate of about 420 Hz:

C-500

Device-under-Test:

Keyboard: The keyboard assembly of the Litton Royal RC 80 uses 18 spring-supported plastic keys pushing fingers on a stamped sheet-metal piece against large, gold-plated contacts on the Main-PCB. The sheet-metal piece itself is connected through two additional contacts with the Main-PCB.

Keyboard Matrix Litton Royal RC 80:

C-500
	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
KK	22	K
KA/D48	17	+
KS/D37	16	−
KM/D26	18	×
KD/D15B	19	÷
KEQ	15	=

Display: The Litton Royal RC 80 makes use of an 8-Digit low-voltage Vacuum Fluorescent Display (VFD) manufactured by Futaba and known as Type 8-CT-01A, soldered with its 19 pins directly to the Main-PCB. Both the 4 Digit Outputs D15, D26, D37, and D48 and the 8 Segment Outputs SA to SG and SDP of the C-500 are inverted with two Mitsubishi M58212 chips and connected through a bank of 18 discrete transistors to the . Eight inverting transistors are used for the D1 to D8 VFD grids, with two extra, non-inverting transistors selecting the upper or lower digit group of the C-500. Eight inverting transistors are used for the A to G and DP VFD anodes, biased to around -44 Volts.

Display Layout:

Futaba 8-CT-01A

The Segment drivers A-G and DP (Decimal Point) are connected to the Futaba 8-CT-01A display in the pictured way. Segment H is not used.

Display Fonts:

Type	Calculator	Number Fonts	Decimal Separator	Thousands Separator	Entry Overflow	Calculating Overflow	Minus
C-500	Litton Royal RC 80			n.a.

Keyboard Scanning: The C-500 uses a keyboard scanning matrix with all of its 18 keys connected directly to a corresponding pin, while the common signal is connected to a dedicated Keyboard Enable pin. Seven of the 18 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 C-500 is blanking the display completely before and in the middle of 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 two 4-digit groups in D15 → D48 direction at a rate of about 420 Hz:

C-550

Device-under-Test:

Keyboard: The Litton Interton PC 2008 makes use 19 gold-plated snap action switches mounted directly on a second PCB of the calculator together with a sliding switch for power. The [C] and [CE] keys are hardwired on the Keyboard-PCB together and act as a single [C] key.

Keyboard Matrix Interton PC 2008 (Version 2):

C-550
	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, CE
KK	22	K
KA/D48	17	+
KS/D37	16	−
KM/D26	18	×
KD/D15B	19	÷
KEQ	15	=

Display: The Interton PC 2008 (Version 2) makes use of Bowmar Nine-Digit Calculator Numeric Seven-Segment LED Optostik Display module, very similar to the type R7H-112-9. It uses seven individual GaAsP (Gallium Arsenide Phosphide) Segment LED chips and one GaAsP Decimal Point LED chip per character bonded directly to a double-sided FR4 printed circuit board (PCB) and placing a one-piece red acrylic protecting lens on top of the assembly with four heat stakes. The leftmost display position is missing like the R7H-112-9 the upper right segment, but sports the decimal point. The display module is connected with 17 pins to the Main-PCB of the calculator, but the left most pin (Digit D9 Common Cathode) is not used. Two ITT 491 (SN75491-style) "Segment Drivers" with four channels, each are connected to the D15, D26, D37, and D48 digit outputs of the C-550 chip with two additional transistors connected to the UD_EN and LD_EN outputs of the C-550, selecting the upper or lower digit groups of the ITT 491 drivers. The outputs of the two ITT 491 drivers are connected to the Common Cathodes D1 to D8 of the LED display. The A to G segments (Anodes) of the LED display are driven with a CA3082 transistor array while the Decimal Point (Anode) is driven by a discrete transisistor.

Display Layout:

Nine-Digit LED Module Bowmar Optostik

The Segment drivers A-G and DP (Decimal Point) are connected to the Bowmar Optostik display in the pictured way.

Display Fonts:

Type	Calculator	Number Fonts	Decimal Separator	Thousands Separator	Entry Overflow	Calculating Overflow	Minus
C-550	Interton PC 2008			n.a.

Keyboard Scanning: The C-550 uses a keyboard scanning matrix with all of its 18 keys connected directly to a corresponding pin, while the common signal is connected to a dedicated Keyboard Enable pin. Seven of the 18 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 C-550 is blanking the display completely before and in the middle of 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 two 4-digit groups in D15 → D48 direction at a rate of about 420 Hz:

TMR 012

Device-under-Test:

Keyboard: The Litton Interton PC 2008 makes use 19 gold-plated snap action switches mounted directly on a second PCB of the calculator together with a sliding switch for power. The [C] and [CE] keys are hardwired on the Keyboard-PCB together and act as a single [C] key.

Keyboard Matrix Interton PC 2008 (Version 1):

TMR 012
	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, CE
KK	22	K
KA/D48	17	+
KS/D37	16	−
KM/D26	18	×
KD/D15B	19	÷
KEQ	15	=

Display: The Interton PC 2008 (Version 1) makes use of Bowmar Nine-Digit Calculator Numeric Seven-Segment LED Optostik Display module, very similar to the type R7H-112-9. It uses seven individual GaAsP (Gallium Arsenide Phosphide) Segment LED chips and one GaAsP Decimal Point LED chip per character bonded directly to a double-sided FR4 printed circuit board (PCB) and placing a one-piece red acrylic protecting lens on top of the assembly with four heat stakes. The leftmost display position is missing like the R7H-112-9 the upper right segment, but sports the decimal point. The display module is connected with 17 pins to the Main-PCB of the calculator, but the left most pin (Digit D9 Common Cathode) is not used. Two SN75491 "Segment Drivers" with four channels, each are connected to the D15, D26, D37, and D48 digit outputs of the TMR 012 chip with two additional transistors connected to the UD_EN and LD_EN outputs of the TMR 012, selecting the upper or lower digit groups of the SN75491 drivers. The outputs of the two SN75491 drivers are connected to the Common Cathodes D1 to D8 of the LED display. The A to G segments (Anodes) of the LED display are driven with a CA3082 transistor array while the Decimal Point (Anode) is driven by a discrete transisistor.

Display Layout:

Nine-Digit LED Module Bowmar Optostik

The Segment drivers A-G and DP (Decimal Point) are connected to the Bowmar Optostik display in the pictured way.

Display Fonts:

Type	Calculator	Number Fonts	Decimal Separator	Thousands Separator	Entry Overflow	Calculating Overflow	Minus
TMR 012	Interton PC 2008			n.a.

Keyboard Scanning: The TMR 012 uses a keyboard scanning matrix with all of its 18 keys connected directly to a corresponding pin, while the common signal is connected to a dedicated Keyboard Enable pin. Seven of the 18 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 TMR 012 is blanking the display completely before and in the middle of 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 two 4-digit groups in D15 → D48 direction at a rate of about 420 Hz:

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

© Joerg Woerner, July 8, 2025. No reprints without written permission.