#rs232

Comparing various 3rd party RS232 adapters for Commodore computers. From left to right:

Commodore S232 Adapter by Jameco for VIC-20/C64/C128

Link232 v1.2 by Brain Innovations for C64/C128

RS232 and Reading from Compact Flash

I decided that instead of adding a MAX232 level shifter for RS232 serial directly to the board, I would be better off building a separate board. This would allow me to use the existing header with either the MAX232 or a TTL-USB adapter.

I got to use an interesting unique part on the MAX232 board — a quad capacitor network in DIP8 package. I couldn't find any similar parts still in production, and couldn't find any data sheets either. I got the part from a local electronics surplus shop, and it's perfect for use with a MAX232 because it's four 0.1μF capacitors in a convenient package.

I don't actually have a computer running right now with a proper RS232 serial port, so I used a USB-RS232 adapter to test with. I did have the Receive lines wired backwards at first, but other than that the breakout works. There is a weird quirk though that I have to connect it and turn on my 68000 computer before plugging it in to the other computer's USB port or else Receive doesn't work. Strange, but not really a show stopper.

So then I moved on to trying to read from the Compact Flash card. It's a no-name 64MB card that I've formatted FAT16. I've been looking at it by running:

$ sudo xxd /dev/disk4 | less

on my iMac to read the actual raw data from the card (especially the boot block and FAT). There is something immensely satisfying to see the exact same data come through my 68000 computer when reading the card as I got from the Mac with a proper card reader.

SAPI-1 (1983)

On the left side, you can see SAPI-1 – a professional computer that was made in the Czechoslovak Socialist Republic and was used mainly for industrial applications. Its architecture was modular and not very far from S100-bus computers. One of the cards contained CPU (Zilog Z80 in this case) while other cards contained RAM, ROM and communication interfaces. It was able to run CP/M which was loaded from 8-inch floppy drives.

This photo is from the last Bytefest show a few weeks ago. We were copying files over a null-modem cable from Amstrad CPC6128. File transfers between different CP/M computers were not an easy task back in the 80s. Each computer had different floppy drives or at least a different logical format of the disks. This was very similar to terminal incompatibilities where every full-screen program had to support tens of terminals in order to be compatible with most CP/M computers. If your computer was not on the list, you ended up with incorrect layout of the text on the screen.

Now that most of the firmware is all set an done, it's time to start working on the final part of the Activity Board: the Raspberry Pi display interface.

@tumblokami​ ordered up some PCBs and gathered the parts to make a few of Geoff Graham’s ASCII Video Terminal board kits. 

Solderless Breadboard Compilation

A useful tool for all kinds of prototyping!  Seen here are examples of (in no particular order):

Executing NOPs on an MOS6502, and building a data entry circuit

Crudely adjusting brightness/contrast of a composite video signal

Improving upon the data entry/modification circuit for my 6502 homebrew computer, and testing with an MCM6810

Approximating the function of a Dekatron tube in count-up mode

Dividing a 32 MHz oscillation down to 8MHz

Controlling a device over RS232 via rotary encoder

Reading the contents of an EPROM, one byte at a time

Combining sync and pixel signals to create a proper composite video source

Headless SGI workstation?

No problem, if you have a null-modem cable and terminal emulator software. SGI O2 (and other MIPS-based SGI workstations) uses the first serial port as the main output for all system messages and OS console if no keyboard is attached (or if firmware is set to prioritize the serial connection). You can access firmware settings, install IRIX to an empty hard drive or boot the system. This is a full replacement for keyboard and mouse unless you need graphics.