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LCD goes black when Pluggy McPlugface is inserted

Posted: 27 Mar 2019, 12:51
by manumenzella04
I finished building my Gigatron Kit a few days ago, and I could not be happier with it :) thank you so much.

However, I am having trouble using a keyboard; after inserting the Pluggy McPlugface adapter, the screen goes immediately black. A few things are strange:
  • The serial port works fine with the included controller.
  • This only happens if the ATTiny is in the socket; just plugging in the empty board does not cause any issues. This rules out simple issues like shorts on the board, etc.
  • The computer doesn't stop execution when the screen goes black; the LEDs continue to flash with the normal pattern, and unplugging the board makes the image come back right where it left off.
  • Putting a scope on the H and V sync signals, I cannot see any obvious difference when the board connected. I was not able to trigger on the signal effectively though, so I might be missing something here.
Initially, I thought that my LCD screen might be incompatible; however, I think that the H & V sync signals are only outputs from the Gigatron to the ATTiny, right? So no signal should be flowing back to them.

I'll do some more experimentation today, including trying other LCD screens, but I was wondering if anyone had encountered something similar before.

LCD Screen: https://www.amazon.com/gp/product/B01L6 ... UTF8&psc=1

Re: LCD goes black when Pluggy McPlugface is inserted

Posted: 27 Mar 2019, 15:07
by marcelk
This is something we haven't heard of before. Only a brief black out is expected when plugging into a life system. If another VGA screen shows the same issue, the ATTiny85 must be faulty in some way. Well, assuming it isn't accidentally reversed: the pin 1 indicator should be closest to the PS/2 jack. They tend to get damaged when reversed BTW. Feel free to drop an email if you need a new one.

Re: LCD goes black when Pluggy McPlugface is inserted

Posted: 28 Mar 2019, 02:01
by manumenzella04
Thanks for the quick response, Marcel!

Interestingly, another LCD screen did not show the same issue; the keyboard still didn’t work, but that might be a different problem since I am using a USB-to-PS/2 converter. I’ve ordered a new PS/2 keyboard — I’ll update the thread when it gets here on Friday.

I’m really curious as to why this might be happening, though. Do you know where I can find some more information about how the H and V sync signals are reused for serial communication? I was under the impression that these were strictly outputs, but maybe the ATTiny is outputting something into them.

Re: LCD goes black when Pluggy McPlugface is inserted

Posted: 28 Mar 2019, 10:58
by marcelk
These signals are strictly inputs to the ATTiny. The code is in the repo: Utils/BabelFish/BabelFish.ino

Code: Select all

//                       +------+
//              ~RESET --|1.   8|-- Vcc
// PS/2 data       PB3 --|2    7|-- PB2  Serial data out
// PS/2 clock      PB4 --|3    6|-- PB1  Serial latch in
//                 GND --|4    5|-- PB0  Serial pulse in
//                       +------+
//                       ATtiny85

Re: LCD goes black when Pluggy McPlugface is inserted

Posted: 29 Mar 2019, 03:32
by manumenzella04
This actually turned out to be a cold solder joint. Apparently, the GND pin on the DB-9 connector for the Pluggy McPlugface tested positive for continuity, but must have had enough resistance to cause a significant voltage drop and prevent the ATTiny from booting.

They keyboard is still not working, but I suspect that’s just because of the USB adaptor. I’m hopeful that the ATTiny was not damaged; I can see a signal (~60 Hz) on the PS/2 port, so that’s probably a good sign.

I’ll update the thread when I’m able to test a native PS/2 keyboard, hopefully tomorrow.

Re: LCD goes black when Pluggy McPlugface is inserted

Posted: 29 Mar 2019, 07:04
by marcelk
manumenzella04 wrote:
29 Mar 2019, 03:32
I can see a signal (~60 Hz) on the PS/2 port, so that’s probably a good sign.
It is a good sign. In order to avoid having to deal with two clocks potentially arriving at the same time (Gigatron and keyboard), BabelFish disables the PS/2 clock each time it expects the vertical blank.

Re: LCD goes black when Pluggy McPlugface is inserted

Posted: 30 Mar 2019, 04:08
by manumenzella04
The new keyboard works!

Thanks so much for helping me debug this.

I can’t wait to tinker with the software now! :)

Re: LCD goes black when Pluggy McPlugface is inserted

Posted: 01 Apr 2019, 08:44
by Tor
manumenzella04 wrote:
29 Mar 2019, 03:32
They keyboard is still not working, but I suspect that’s just because of the USB adaptor. [..]
I’ll update the thread when I’m able to test a native PS/2 keyboard, hopefully tomorrow.
USB adapters are completely passive, there's just wiring inside. The actual PS/2 protocol is handled by the USB keyboard, as a fall-back mechanism. So for a USB-PS/2 adapter to work, the keyboard must include PS/2 support, and most USB keyboards did in the past. But nowadays some vendors probably skip the old PS/2 support to save a little on manufacturing costs.
Your next post confirmed that your new keyboard solved the problem, good to hear! I have been hoarding PS/2 keyboards for my various retro computing projects, after those non-PS/2-compatible USB keyboards started to take over the market.