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bfollowell

Atari 8-bit Keyboard Matrix Primer

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I've been troubleshooting a keyboard issue with an Atari 400 I was recently gifted. I quickly realized that, as much as I've learned about our beloved 8-bit Ataris over the past few years, I know nothing whatsoever about keyboard matrices or how to test/troubleshoot a keyboard electrically. It's a matrix, not like each switch/contact has a pair of wires coming down where I can check for continuity upon a keypress. How, exactly, do you checkout a keyboard for a troublesome key, or a set of troublesome keys?

 

I have a schematic for the 400 keyboard, but I really don't have any idea what I'm looking at or how to interpret it. Both sides of the contact seem to be common with other, but different keys. I think I have an idea in my head how it may work, but I'd much rather hear that from someone more knowledgeable.

 

I thought about posting this in the 600/800XL Keyboard variants thread, but that's for a different machine's keyboards, and it's more about the different styles, not about how they work, and I hate to muddy that thread. I figure the XL and 400/800 style keyboards operate in a similar manner, even if they are different under the hood. I also figure that I can't be the only person that could find this information useful, so I'm hoping this thread will help someone in the future. So, while some specific 400 keyboard information would certainly be appreciated, I guess I'm looking for more of a general keyboard matrix primer.

 

Thanks to anyone that can help or point me in the direction of useful links.

atari800xl_kbd_matrix.png.afa7aabd13b1086b7aacaad100428459.pngStandard_400_Keyboard1.thumb.jpg.0f95844cb4929633fddf916a1cd02bb0.jpg

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The image in the left side of the pic is your key..... 

When you press a key, it connects one row pin to one column pin.  The row pins are 1,2,3,4,5,6,7,8; and the column pins are 17,16,15,14,13,12,11,10,9.  The 4 function buttons (reset, option, select, start) are sort of a 'column of their own....  connecting pin 18 with 23, 22, 21, or 20.

 

To test the keyboard you can use a voltmeter, either modern digital, or classic analog....

 

If you want to test the T key, put one probe on pin 4 and the other on pin 12, you should have infinite resistance.... till you press they key, then it should drop to zero.

 

When troubleshooting a keyboard you can find a single key out, a row out, or a column out.  Now in a 400 keyboard, I don't recall that there is much you can do unless possibly if the problem is the ribbon cable between the keyboard assembly and the mainboard.  With XL and XE keyboards, you can find cracked/damaged traces, etc, etc.  I recently fixed 2 XL keyboards.  One was a 'basket case' and I undid a number of bodge wires, cleaned things up, re-isolated the problems, and used solder, and some stripped kynar and larger wire to rebuild flaky traces.  The other keyboard had two keys that didn't work, and with the help of forum members I figured out how to resurrect vs find replacements for the switches.

 

Also, in the Atari, there are two CD4051 chips that decode the matrix.  If part or all of one of these chips is bad, you can have a number of keys out (usually one or more rows, or one or more columns).  In the two XLs I was working on, each had a bad CD4051.  So, if all the keys test good with a meter and you still have keys out (rows or colums), then you have one or more bad 4051 chisp on the Atari mainboard.

 

Hope this helps explain how the keyboard matrix and keyboard troubleshooting works.

 

 

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19 minutes ago, cwilbar said:

The image in the left side of the pic is your key..... 

When you press a key, it connects one row pin to one column pin.  The row pins are 1,2,3,4,5,6,7,8; and the column pins are 17,16,15,14,13,12,11,10,9.  The 4 function buttons (reset, option, select, start) are sort of a 'column of their own....  connecting pin 18 with 23, 22, 21, or 20.

 

To test the keyboard you can use a voltmeter, either modern digital, or classic analog....

 

If you want to test the T key, put one probe on pin 4 and the other on pin 12, you should have infinite resistance.... till you press they key, then it should drop to zero.

 

When troubleshooting a keyboard you can find a single key out, a row out, or a column out.  Now in a 400 keyboard, I don't recall that there is much you can do unless possibly if the problem is the ribbon cable between the keyboard assembly and the mainboard.  With XL and XE keyboards, you can find cracked/damaged traces, etc, etc.  I recently fixed 2 XL keyboards.  One was a 'basket case' and I undid a number of bodge wires, cleaned things up, re-isolated the problems, and used solder, and some stripped kynar and larger wire to rebuild flaky traces.  The other keyboard had two keys that didn't work, and with the help of forum members I figured out how to resurrect vs find replacements for the switches.

 

Also, in the Atari, there are two CD4051 chips that decode the matrix.  If part or all of one of these chips is bad, you can have a number of keys out (usually one or more rows, or one or more columns).  In the two XLs I was working on, each had a bad CD4051.  So, if all the keys test good with a meter and you still have keys out (rows or colums), then you have one or more bad 4051 chisp on the Atari mainboard.

 

Hope this helps explain how the keyboard matrix and keyboard troubleshooting works.

 

 

 

It definitely does, and it confirms the way that I'd kind of gotten into my head that it must work. Thank you for your detailed explanation and for confirming my suspicions.

 

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Well, I got my trusty Fluke out and was able to confirm that my keyboard is actually bad. I suspect a broken trace, or several, somewhere on the board. When I get time, I may go through it with my microscope and see if I can find something that I can actually fix, but for now, it looks like I need to get a call in to Brad at Best for a NOS 400 keyboard.

 

Thanks for the help with confirming how a keyboard matrix works.

 

EDIT

Actually, that didn't take all that long to track down. I decided to run down through the connections and confirm the cable, and I no conductivity on quite a few conductors going from where they solder into the keyboard PCB to the end of the ribbon cable, so I either have some bad, dry solder joints, or more than likely, breaks in the ribbon cable. I'm still going to order a NOS 400 keyboard from Best while I can, but I'll probably look to change out the ribbon cable and connector eventually, and keep this old one as a backup.

 

Edited by bfollowell

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I went through and checked continuity to each pin on the keyboard PCB side and all of the keys work perfectly, so it's definitely just a ribbon cable issue. I'll come up with a different ribbon cable and connector setup and replace the original cable, and then clean up this keyboard and hang on to the NOS keyboard I ordered from Best and keep it as as spare.

 

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