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I would like help designing a Color Computer shield for the Arduino Mega.

For those who don't know the CoCo is a M6809 based computer from the 1980s.

I imagin a shield with a 40 pin card edge connector (for the CoCo cartridge port) on one end, and header pins for the Arduino on the other end.

I am hoping to design a very passive shield with little or no configurability. But I don't want to limit the kinds of projects avaiable.

Here is the CoCo cartridge port:

A0-A16 - The 6809's address bus.

D0-D7 - The 6809's data bus.

E Clock - Normally a 0.89 MHz clock. The clock generator in the CoCo is software selectable to double this.

Q Clock - Clock which leads E by 90 degrees.

CART* - a signal to the CoCo to immediaetly jump to ROM mapped in CTS* space.

+5 VDC - +5 volts DC, 300mA.

R/W* - Read/write signal from the CoCo.

CTS* - Cartridge (ROM) select. This goes low when the CPU is reading from $C000-$EFFF.

SND - Analog input routed to the TV speaker.

SCS* - Spare cartridge select. This goes low when the CPU is reading or writing to/from $FF40-$FF5F.

SLENB* - When put low by the external device it will have full control of the data bus. This allows overriding of internally (in the CoCo) mapped devices.

What would be a sane mapping to the Arduino's pins?

Some projects I have in mind:

• ROM emulator. Tied to CTS*.
• Data aquisition from the Arduino's analog input. Prolly tied to SCS*.
• Emulate a UART.

Bristol, UK
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Nice idea!  You may need to add an 8-bit latch to go between the bus of the 6809 and the I/O ports of the Arduino.  That way, the strict timing of the bus cycles won't matter to the Arduino.  I've been thinking about a way to do this on a 6502 machine (Compukit UK101) as a way to emulate the 6850 serial UART.

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