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strange 1200XL problem. random keyboard sounds when no key pressed


ACML
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I have an Atari 1200XL motherboard that has a unique problem I've not run across before. I can just power up the mobo, no keyboard hooked up and after about a minute I hear what sound like random keyboard "clicks". They can be slow to rapid fire (very random). Only occasionally it shows on the screen a carriage return or random letter, but 95% of the time you hear the click but not characters are on the screen. This again, is without the keyboard connected (remember the 4051s are on the 1200's keyboard). It last for about 5-10 seconds then settles down. I've swapped POKEY, PIA, SALLY, GTIA and ANTIC and still has same issue. Any ideas?

Edited by ACML
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The keyboard has the scanning ICs mounted on it. If you don't plug in the keyboard, you leave the inputs floating. So, it is normal to get 'ghost' keys. I've seen it many, many times.

 

Bob

Thanks Bob! I'm surprised with the number of 1200XLs I've worked on, this is the first time I found this "feature". So, with the keyboard plugged in, the issue should resolve itself? Would this also hold true for a 400/800 with the keyboard unplugged? This is great news. I was thinking the motherboard had a problem.

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How does the XEGS overcome this? Pullups or something?

It would have the same problem, except on the XEGS Atari added a keyboard enable pin (goes to TRIG3) which needs to have +5 connected to it before the keyboard will be acknowledged. When you plug in the keyboard cable there is a connection from that pin to +5 being made at the same time.

 

- Michael

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Thanks Bob! I'm surprised with the number of 1200XLs I've worked on, this is the first time I found this "feature". So, with the keyboard plugged in, the issue should resolve itself? Would this also hold true for a 400/800 with the keyboard unplugged? This is great news. I was thinking the motherboard had a problem.

 

Only the 1200XL has this problem. The XEGS would as well, if you were to jumper the enable pin without the keyboard plugged in. All of the other series have pull-up resistors on POKEY's key response lines, so short answer is they don't share this problem.

 

- Michael

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Only the 1200XL has this problem. The XEGS would as well, if you were to jumper the enable pin without the keyboard plugged in. All of the other series have pull-up resistors on POKEY's key response lines, so short answer is they don't share this problem.

 

- Michael

Is that because the 4051s are on the keyboard and not present if not connected?

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I don't think the enable pin would matter. XEGS still does a keyscan, the TRIG2 input is only for the benefit of the OS startup when it decides whether to give you Basic or the game.

The schematics show inline resistors and some caps near the kb connections to Pokey. XEGS looks to have linear inductors (?) Maybe that gives the steady input?

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Is that because the 4051s are on the keyboard and not present if not connected?

The pull-ups are always required on the KR1 AND KR2 key response lines due the otherwise floating nature of not pressing any key on the scanned matrix, but on both the XEGS and the 1200XL these exist on the keyboard side and not on the motherboard like all the other A8's.

 

 

I don't think the enable pin would matter. XEGS still does a keyscan, the TRIG2 input is only for the benefit of the OS startup when it decides whether to give you Basic or the game.

The schematics show inline resistors and some caps near the kb connections to Pokey. XEGS looks to have linear inductors (?) Maybe that gives the steady input?

I know through my own experience with the TK-II-XEGS project that once you enable the keyboard, those pull-ups (or a simulation there of) needs to be present, otherwise you'll get the ghost key presses as Bob mentioned. So the key scan may well happen whether or not the enable pin is brought high, but when not the OS ignores them. I also know this to be true because when I plugged in my first prototype without the enable pin going to +5, key presses were completely ignored.

 

Edit: Or at least I thought so, since this could be the nature of the game. I'll have to try loading the game slot in my U1MB with Basic instead and see what happens.

 

Edit2: Actually the more I thought about this my memory fully kicked in, and I seem to recall that I could get the external keyboard to work without the enable pin being present by pressing SELECT on power-up which would put me in Basic where the keyboard would work. But as I think about this more, Missile Command will normally respond to a CTRL+T to toggle into trackball mode, it would be interesting to try that with a keyboard plugged in, but not enabled. Long story short rybags could be correct, but the investigator in me wants to do some experiments to find out.

 

 

Is it possible for this floating condition (keyboard unconnected, computer left on) to cause damage to Pokey?

No damage will result if left floating. Case in point, the XEGS's KR lines are always floating when no keyboard is present.

 

- Michael

Edited by mytekcontrols
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"Enable" - what does it do, is it just the +5V supply for the multiplexors or something?

There is an independent power pin to supply the 4051 MUX's with power. Pin 13 of the keyboard connector (enable line) is only used as a signal line back to the XEGS motherboard to tell it the keyboard is plugged in and present, with no other connection to the scan circuit.

 

- Michael

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My thought is still that line couldn't inhibit the floating inputs problem.

 

But curious now. I don't think the Pokey on my XEGS is socketed but I do have over 10 spares Pokeys and can grab one and bend some pins up to try and simulate the situation on my 600XL.

 

It might not inhibit the floating inputs, but it could instruct the OS to simply ignore the keyboard responses when not enabled (which is back to my original line of thought). Obviously the XEGS OS is not the same as the XL or XE. There has to be a little bit of extra code in there to deal with the built-in game and the external keyboard enable line, as well as what to do if SELECT is held on power-up with or without the keyboard plugged in. This last part was all new to the XEGS. Other than that it should act just like a 65XE operating system.

 

- Michael

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