I've been spinning my wheels on this for too long and thought I'd ask for help here. I am probably missing something simple. First, I'm a Mac user. I have a MacBook with a USB-TTL adapter based on FT232R. I have it in 5V mode.

Adapter is this: DSD TECGH SH-U09C5 USB to TTL UART Converter
ACIA is a Rockwell 6551 (R6551AP)

USB adapter connections:
GND --> breadboard ground (same ground as ACIA)
TX --> RxD on ACIA
RX --> TxD on ACIA
RTS --> CTSB on ACIA
CTS --> RTSB on ACIA

The ACIA is connected using Garth's decode logic, so it's at $4400 DCDB and DSRB are tied to ground.

This is what I'm a little fuzzy on. I have the IRQB lines from two VIAs plus the R6551 going to a CD74HC21E. My thought here was that this would go low if any of the irq lines went low. The unused input is tied to high, and the line from the R661 is tied high with around 3.2k ohm (3 x 1k, and a 220 ohm, this is what I had on hand).

From my Mac, I'm using pico com. Here is my command line:


So here's what works and what doesn't work.

When I send output from the 6502, I see it on my PC. When I type a letter on my PC (sent to the 6502), I see the 6551's irq line go low, and on my scope I can decode the serial line and see the key pressed. The scope shows the data on RxD, CTSB stays low and IRQB goes low. But IRQB stays low, so I don't think I'm handling the receive interrupt properly. The ACIA repeatedly sends a character back ("D") in a loop. I don't know where it's getting that.

Here is what I have for setup:


Here's what I have for interrupt handling:


I've found a few examples and can't get this working yet. I hope it's something small/silly I'm missing here. Any help is appreciated. I don't care about processing the input yet, I just want to get it out of the ACIA so the IRQ resets. I think I can handle the rest if I can overcome this hurdle.

Here's the scope.
1: CTS
2: TxD
3: RxD
4: IRQB

Quite a lot is working, so that's good.

I'd wonder if the IRQ handler is getting called. Have you an LED to light or any other IO? Instead of your
jsr print_char
as a debug, I'd suggest just doing a STA ACIA_OUT. It should send back the character it received.

It might be good to share your assembler listing: often, the mistake will be using the wrong addressing mode or the wrong address for something. Seeing the hex will help.

It does feel like I'm at the 90% mark on this milestone.

Here's a hex dump of the binary (I'm always amused after spending so much time on something only to see a few kilobytes of data)


I've attached a txt file containing my code at the moment.

I did change the handler to immediately do a "sta ACIA_DATA" and it looks like it does do this from the scope, it's just not being decoded correctly? Looks like its slightly delayed.

That's good - your IRQ is called, you can send back what you saw.

Next mystery perhaps - and I haven't look at your hex yet, but thanks for that - is why the machine starts sending. And it only sends when it has received. And it keeps on sending. That's actually three puzzles.

That TxD line may be a red herring. I tried my test again, hitting '1' instead of 'Q'. The scope shows I sent '1' but received the same shape of pulses. So I was maybe just seeing things there.

As a debugging step, I setup port B on the VIA as a debugging output and connected LEDs. I also setup port a pin 0 as an output. In my interrupt handler, I first pulse port a pin 0, and then load ACIA_DATA and output it on port b.

When I power up, and hit '1' on my terminal window, what I'm seeing is the interrupt running repeatedly (constant pulses on port a pin 0), and 'D' on port b repeatedly.

I'm at a loss here. I had another R6551 (labelled R6551P / R6551-27) and it exhibits the same behavior. I've pared my test program down to nearly nothing. Maybe this is a circuit issue? I may tear it down and built it back up again.



scope:
1: via 1 port a pin 0
2: r6551 txd
3: r6551 rxd
4: irq at 6502

ok, well perhaps I should have tried this earlier. I just ripped the circuit out, and built it again. and of course, IT WORKS. I thought I was going nuts, trying to get a dead simple program working, when it was the wiring that was at fault. In any case, thanks for taking a look.

_________________
http://WilsonMinesCo.com/ lots of 6502 resources
The "second front page" is http://wilsonminesco.com/links.html .
What's an additional VIA among friends, anyhow?

I just ripped it out. I was scratching my head about increasingly rudimentary code, off and on for weeks. That should have been my first clue. I had it rebuilt from scratch in about 20 minutes and the test worked right away. Lesson learned!

It's a good result! (It's more difficult if you've designed and built a PCB, of course...)