Thursday, March 30, 2023

Forty Years of Personal Computing - Das Blinkenlights

SWTPc 6800 front panel, sporting sixteen LEDs.
Each LED indicates a 4 KB address block access.
You can also see a yellow power LED was added.
The SWTPc 6800 Computer System sports a rather spartan black and silver front panel. Unlike contemporaries of its day, there were no lights.

I'd already modified the front panel of my computer. I mounted the CIS-30+ cassette interface inside the case, which accounts for the assortment of holes in the upper right. There's a yellow power LED next to the reset button. Before I went to the ISEF 1979, I replaced the power switch with a key switch. I added the yellow LED so I could tell when the machine was on. 

While building the MC6809E V1 board, I decided to add some lights. Hooking up LEDs to the address bus didn't seem useful, since addresses change quickly. Plus, the DAT complicated the issue -- do you show logical or physical addresses?

I ended up with a simple solution. Running the top four CPU address lines into a 4:16 decoder, each LED indicator represents a logical 4 KB address block. This connects to the CPU board directly, before the address signals enter the DAT. 

Board holding 74LS154 and LEDs.
I drilled the front panel for sixteen LEDs. On the backside of the front panel, I mounted a 74LS154 4:16 decoder and wired up the LED cathodes. The board is held in place by the LED wires. A 16-pin ribbon cable goes back to the CPU board and attaches to a socket.

This socket has address pins A12-A15 from the CPU, the VMA* signal, the DAT write decoder signal, +5 volts, and ground. The VMA* and DAT write signals go to the two Enable* pins on the 'LS154. This means the address must be valid, and not writing to the DAT to enable.

The LEDs anodes are connected together to a resistor to +5 volts. Originally, this was a 470 ohm resistor. I found that the LEDs lit very dimly. Each LED is not asserted all the time. VMA* is not asserted at all times, and pulses every clock cycle, which limits how bright the LED can illuminate. I decreased the resistor value and experimentally arrived at a value of 22 ohms.

The resulting front panel is informative. I can see when the CPU is busy-waiting on I/O ports, if it is crunching away at some calculation, or if it has crashed entirely. And if the CPU executes a CWAI instruction to wait for interrupts, all the LEDs go dark.