2022-03-27

Someone made replicas of some Acorn System boards, including the VDU. He also made an enhanced version. I bought one of the latter from eBay.

It is 80 × 25 text, holding a single screen in 2k bytes of RAM. It uses four 2114 RAM chips. I would have preferred a single 6116 or 6284 chip, as these are more common.

The characters are 6 pixels wide, which helps pixels stay clearer in the limited bandwidth monitors. The character rate is 2 MHz and the pixel rate is 12 MHz. There is a circuit to multiply the 2 MHz processor clock by six, though it is not as good as dividing 12 MHz down to drive the CPU clock. 

The characters are 10 lines high, giving 250 lines total.

I bought this board as an experimental platform. It saves me a lot of wiring.

I have a strong preference for square pixels. The correct pixel rate is 14.75 MHz for 80 × 8 = 640 pixels in 43.39 microseconds PAL display time. For 80 × 6 = 480 pixels in the same time, pixels are no longer square but the pixel rate is 14.75 × 3/4 = 11.0625 MHz. The nearest commonly available frequency is the 11.0592 MHz baud-rate frequency.

High-bandwidth video CRT monitors have become very rare, but old televisions with baseband video input are still available on eBay. I shall be modifying this board to work with the old Toshiba colour TV that I have. I intend to go with 40 columns, 8x10 character cells, and a 7.375 MHz pixel rate.

The 6845 can be programmed for a wide range of video timings, so that should not be difficult.

There is even an online calculator to do the work for you: https://mrboot.de/mc6845.php

2022-04-15

Wanting to get it up and running quickly to check it all works before making modifications, I shopped around for 2114 chips. The best deal seemed to be ten for £14.33. Searching for four-bit SRAM, I found some that are larger (16k × 4) and much larger (64k × 4). I bought some of the latter at £3.94 for four. They are 24 pins instead of 18, they have most of their pins in common. This will make them easier to shoehorn in than a RAM chip with and 8-bit data bus. As well as being cheaper, I get 64 times more memory, enough to support pixel graphics.

Much of the minor parts and 74 series discrete logic seems to be for multiplying the 1 or 2 MHz CPU clock signal into a 6 MHz dot clock. I intend to avoid this ugly kludge by dividing a dot clock to produce the character clock.

Shopping around for discrete logic chips is a chore. Suppliers like Mouser and Digikey have minimum order values, and chips from eBay tend to be much more expensive in ones and twos. Often several pounds instead of a several tens of pennies. 

I'm not keen on keeping loads of 74 series logic chips around, so I shall use GAL chips for my experiments.