In the previous post I mentioned at the end that the BIOS, running on the ATMega328p, was only capable of communication one way. There are a couple of mailbox memory locations that are used to pass keystrokes to the 8-bit CPUs (Z80 or 6502), yet no way of receiving data back, other than an acknowledgement that the keystroke has been processed. In order to make the BIOS a bit more BASIC-friendly, I needed to add …
In the previous post I’d checked the DRAM and the MOS logic chips, some of which are responsible for memory decoding. The RAM was fine, and one of the logic chips had failed. I was, however, still seeing what looked like an anomalous signal on some of the address lines. I couldn’t think of an easy way to diagnose the 6510 CPU in situ, so decided to desolder it from the motherboard and test on …
The BBC Micro and Master computers were designed by Acorn from the outset to support a second processor via the proprietary Tube interface. Typically Acorn’s external second processors are in enclosures that matches the aesthetics of the BBC Micro and contain a power supply, processor, RAM and ROM. Essentially a computer without a keyboard and video output. The only IO is via a custom ULA that provides four bi-directional FIFO buffers. The second processors connect …
