The Eurocom1802
What is it?
The design is based on Eltec's Eurocom-I or better, my remake of it.This version is equipped with the RCA's CDP1802. It is best known for the Cosmac Eld and the Comx-35. The reason I designed this board is the simple fact that I found out by accident that I have a CDP1802 laying around.
Next thing is to understand how the I/O works. I know the N, R and X register are involved. But how exactly, I still have no idea. What wasn't explained was what the address range of the supported I/O is. If it is the full 64 KB range, the things are fine. If it is only 8 bits i.e. 256 bytes, then I have to add some lower address lines as well.
Regarding the E signal for the 6821s. The CDP1802 has no equivalent for it. But As I understood the RW and WR signal tell the system that the address and data lines are valid at that moment. By NAND-ing these two signals I created the signal that could replace E.
Regarding interrupts: as I understood, the CDP1802 has one real interrupt input and and four flag inputs. I decided to combine the external NMI and IRQ signals to one signal using two NAND gates as an AND one. The result is fed to the INT input. Then NMI and IRQ are fed to EF1 and EF4.
The CDP1802 has a multiplexed address bus. But in contrary to various well known Intel ICs where the data bus provides a part of the address, the lines MA0..7 provide both the low and high byte of the address. IC2, a 573, and output TPA of the CDO1802 take care of demultiplexing these lines.
The schematic:
The board:
You can email me here.