The initial system
For Colin MKII, I decided to use a Hitachi 64180 processor. It could address 1MB of RAM via an internal memory management unit, had on board DMA and a couple of serial ports. The Zilog equivalent wasn't yet available, and I wanted to prove to myself that I could design a computer. After many hours of design work, I finally had some design drawings. The design was broken down into four sheets covering:
- The main processor, bus buffering, reset circuitry and the serial ports
- System memory, ROM and real time clock
- System peripherals including the keyboard interface
- Floppy disk interface for up to four drives
The pictures below show the initial circuit diagrams.
I hand wire wrapped the main board, and put it into my card frame, with a hand wire wrapped backplane, of course. I had also written the BIOS for the system. Everything displayed correctly at first power on. I pressed "A" on the keyboard and hey presto, "A" appeared on the screen. BRILLIANT !!
The down side was that when I pressed the "B" key, nothing happened. Power off & on, the same thing happened. I powered off and pulled all the chips. (I was confident that my design was correct). I spent a few hours belling out the multitude of connections and found one wire in the wrong place. This was on the Mode 2 interrupt circuit connected to the data bus buffer between the keyboard and the CPU. Once the one connection was put in the right place, evrything worked OK, and the system booted from a floppy.
I used the original VDU card from my Colin MKI system until I could build one for Colin MKII.
Also shown below is the VDU interface finally adopted for Colin MKII, which was a design by John Adams which featured in Electronics and Wireless World. The disk interface is based on a design from the internet by Hans Summers, which once I modified the design and built the interface, worked amazingly well. CPM can now be loaded and run from Compact Flash cards.
|CPU, bus buffering & RS232|
|System memory, ROM & RTC|
|Floppy disk interface|
|Hard disk interface|
|Colin MK II in the chassis|
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