This Apple 1 Serial card uses the VMA line to replace the on board Keyboard and Display with an RS232 Connection.The VMA line is normally shorted to +5V but can be used by replacing the short with a Resistor and Capacitor. The capacitor is needed due to noise problems on the board. The RC can replace the short to +5 on the Apple 1 board with little visual impact using 0402 or 0201 surface mount R and C (see Photo) or can be connected between pins 12 and 14 of B1 with standard parts. When the VMA line is pulled down all I/O, RAM and ROM are disconnected and can be replaced by other circuits. The Serial Card looks at the Address lines and will pull down the VMA on cycles with address $D010 to $D012 when the KBD or Display functions are enabled to disable the on board I/O and replace it with equivalent information from the Serial Card. If the Serial port is connected by RS232 to a Computer, a Terminal window can replace the Apple 1 screen and the computer keyboard becomes the Apple 1 keyboard. Now the display and keyboard rate can approach the Baud Rate of the Serial connection. A video of 9600 baud operation is at:
https://youtu.be/FIVN6NgGdK4
The FRAM board uses the same VMA line to change DRAM and monitor ROM addresses from the Apple 1 board to the FRAM board on a cycle by cycle basis. A demo is at:
https://youtu.be/mPXJ1EYHrhw
IMG_1032.jpeg
Apple30th9600a.mov