Alternative circuit for SHARP EL-210/EL-215 calculator
This is a circuit/board design for the alternate PCB for the SHARP EL-210/215 calculators. SHARP EL-210 series are manufactured around early 1980th. By using this alternate PCB, you can program any program as firmware of this classic VFD calculator.
There is an dedicated RPN firmware is located in the rpn2026 project.
To make your own RPN calculator, user must follow the procedure :
- Take the main board of the EL-210 out.
- Move the VFD from the old board to this board.
- Set this board into the EL-210 calculator.
- Program the board with the appropriate RPN firmware.
You can found additional design resources :
The VFD is 9-ST-20, manufactured by Futaba. The specification is not disclosed. Pin assignment has been investigated by LI2026ODP project.
- Anode voltage : 25V
- Heater voltage : 2.5v (Differential square wave drive), 21mA.
- Grid cutoff : 4V
The heater must be driven by differential alternate current. This is required to make the flat brightness between right-end and left-end of the VFD. If you drive the heater by DC, the voltage slope of the heater will make the slop of the brightness.
The analog switch provide the low voltage drop out current switching. The timing is made by MCU timer.
The grid bias is required 4. Because the balanced drive of the heater makes 1.65V grid bias, we need only 2.5V more.
The key matrix has been investigated by also LI2026ODP project.
The EL-21x series key matrix doesn't have any protection diode. Thus, to prevent the signal shortage, the GPIO pins for the key matrix have to be configured :
- Row : Open Drain output.
- Col : Input with pull-up register.
To check the button pushed, output the "L" from the Row. If the Column get "L", the button is pushed.
| Row | Column 0 | Column 1 | Column 2 |
|---|---|---|---|
| 0 | CE | X->M | 0 |
| 1 | 7 | 4 | |
| 2 | CM | 8 | 1 |
| 3 | . | 5 | % |
| 4 | + | RM | 2 |
| 5 | C | 9 | = |
| 6 | / | M- | |
| 7 | * | 6 | 3 |
| 8 | - | M+ | sqrt |
To supply the 24V to VFD, set EN24 signal to "H".
In the case you order the PCB to a board manufacturer, you must specify the PCB thinness as 1.0mm. The standard 1.3mm is too thick.
The Grid bias is not deep as expected. It is -0.9V while expected -2.5v. It is left untouched because it is not problematic.
