Posted by Bill Lash on August 08, 1999 at 16:34:47:
In Reply to: I think diodes should no longer be recommended. posted by Tom61 on August 08, 1999 at 14:51:11:
: I've never quite figured out how diodes are supposed to help with ghosting, since they just limit which way elctricity can travel. I think people were thinking diodes were more like gates(this is the wrong word for this), where they could redirect electricity away from the wrong parts.
I understand how the diodes are supposed to fix the problem, and it is precisely because they "just limit which way electricity can travel". If you press one button, electricity flows out of a scan line, through the closed button and into a detection line. Ghosting occurs because the electricity flows out of a scan line, through one of the closed switches which shares a detection line with another closed switch, the electricity then flows up this other closed switch into the scan line for that switch, then down through another closed switch into a different detection line. If you can prevent electricity from going through the switch from detection line to scan line, you have prevented the problem.
Now, that being said, we are trying to use a keyboard for a purpose that it wasn't intended for. The people who designed the keyboard weren't trying to make it so you could hold down a lot of keys at once and were trying to eliminate as much cost as possible (diodes are cheap, but not free). They made their design decisions based on this philosophy. The example of the National Semiconductor Application Note shows how the code running in the keyboard controller can convert ghosting to masking, for example.
If someone finds a keyboard that works well, let us all know so we can try to find the same type. It wouldn't surprise me at all to find out that really old keyboards would work better than newer keyboards. Designers tend to get more cost concious over time, and the early keyboards may have had diodes in the matrix to solve the "n-key rollover" problem, but later, to make the keyboard cheaper, they reorted to software means.
I do have a keyboard hack that is part way finished, but I haven't gotten around to finishing it, (I've got the matrix mapped, but don't have an enclosure for it). If I ever finish it, I will use diodes, but it very well may not help. I know it is a necessary condition to prevent ghosting, but if the keyboard controller is doing any tricks, it probably won't work.