mystical keyboard keys
in [Home Built PC]
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From: transkawa on 5 Jul 2010 08:47 can someone help me solve this non-mystical so i want to believe problem? The keyboard on my laptop fails sometimes. It's a DeLL Latitude C600 series. Though the laptop is old but the keyboard is only two years or so old. The keyboard fails in the sense that a particular set of keys fail. The keys are T and Y keys. Then the Tab and Caps Lock keys. I make htem work again but discharging them with a screwdriver and the tips of my lips. I do this several times once in a while but this mystical way of making these work is way towards the alchemical. Can someone proffer a solution to the problem? xnt -- happy are those who have endured for they shall reap bountifully --the gospel according to an avatar
From: TVeblen on 5 Jul 2010 09:31 On 7/5/2010 8:47 AM, transkawa wrote: > > > can someone help me solve this non-mystical so i want to believe > problem? > The keyboard on my laptop fails sometimes. It's a DeLL Latitude C600 > series. Though the laptop is old but the keyboard is only two years or > so old. The keyboard fails in the sense that a particular set of keys > fail. The keys are T and Y keys. Then the Tab and Caps Lock keys. I make > htem work again but discharging them with a screwdriver and the tips of > my lips. > I do this several times once in a while but this mystical way of making > these work is way towards the alchemical. Can someone proffer a solution > to the problem? > xnt Paul recently posted a diagram of the wiring of a keyboard, and unlike the ordered, logical, and neat wiring schematic you would expect, the diagram looks like a plate of spaghetti. So while it seems random to you and me, the keys are on the same circuit. And the circuit is failing. Time for a new keyboard/laptop.
From: Paul on 5 Jul 2010 12:39 transkawa wrote: > > can someone help me solve this non-mystical so i want to believe > problem? > The keyboard on my laptop fails sometimes. It's a DeLL Latitude C600 > series. Though the laptop is old but the keyboard is only two years or > so old. The keyboard fails in the sense that a particular set of keys > fail. The keys are T and Y keys. Then the Tab and Caps Lock keys. I make > htem work again but discharging them with a screwdriver and the tips of > my lips. > I do this several times once in a while but this mystical way of making > these work is way towards the alchemical. Can someone proffer a solution > to the problem? > xnt If I was working on it, I'd want to start by reading the part number off the keyboard encoder IC. This is an example of an encoder IC. This datasheet is not as complete as I would like, but it should give some idea how it works. http://web.archive.org/web/20010613141313/www.semtech.com/pdf/sysmgmt/doc5-418-ds-109.pdf The keys are arranged as a matrix of wires (row and column wires). At any one time, most of the column lines are open circuit, except for one column line driving to ground. That means the column lines have no "drive to logic 1" capability. They either "drive to logic 0" or are open circuit. The row lines are inputs. There are some external resistors tied to the lines. If no keys on a column are closed, then all the row signals will be pulled to logic 1 by the resistors. If a single key is pressed, it joins a column line to a row input. If that column line is currently being scanned (driven to logic 0), then the row input will see a logic zero as well. So seeing a logic zero, means the key is active or closed. So that describes a bit about how the keyboard matrix is scanned. The keyboard encoder chip "debounces" the key, and only declares a key closure, if the key remains closed for at least two scans of the keyboards (roughly). Now, this article describes some additional components that can be added. See "Getting Rid Of Ghosting and Masking" on page 8, where a switching diode has been added for each and every keyswitch on the keyboard. The diodes are added, when you want a "deluxe" keyboard supporting N-key rollover. N-key rollover is preferred for gaming applications, where the user can be pressing a number of keys at the same time. The diodes solve the "ghosting" problem, although at the cost of adding $3-$5 worth of diodes per keyboard. The thresholding on the CMOS circuit must be such, that it can handle the slightly higher input voltage of a logic 0, caused by the diodes in the circuit. http://www.dribin.org/dave/keyboard/keyboard.pdf If you can find the datasheet for the main chip, you can get the table of key mappings, which may help you explain why those particular keys are affected. They could all be members of a group experiencing "ghosting", meaning the keys are failing in the closed position, and the keyboard controller has "locked them out" to prevent them from interfering. That would not explain though, why you need to "discharge" the circuit. Discharging a circuit to restore operation, is something you might do if encountering CMOS latchup. But in this particular circuit and application, I don't see a reason for that to happen. And in any case, I don't see how you could add any components to the circuit to actually fix it. To test the PCB of the keyboard, I'd use a multimeter. I'd set it to a low power ohms range (where the circuit will not apply enough voltage to forward bias a diode). Using the meter, I'd test for shorting between rows or columns or the like. The test is potentially limited in sensitivity, by the pullup resistors. I notice in the Dell C600 documentation, they mention the presence of "diagnostic software". Perhaps you could run that software, and check for any error codes related to the keyboard. The keyboard has internal status information, and if the Dell diagnostic software can extract that status information, perhaps it can detect a "stuck key" or "ghost key" problem. And that might give you more clues as to the source of the problem. Paul
From: jerry on 6 Jul 2010 19:25 On Mon, 05 Jul 2010 13:47:57 +0100, transkawa wrote: > can someone help me solve this non-mystical so i want to believe > problem? > The keyboard on my laptop fails sometimes. It's a DeLL Latitude C600 > series. Though the laptop is old but the keyboard is only two years or > so old. The keyboard fails in the sense that a particular set of keys > fail. The keys are T and Y keys. Then the Tab and Caps Lock keys. I make > htem work again but discharging them with a screwdriver and the tips of > my lips. > I do this several times once in a while but this mystical way of making > these work is way towards the alchemical. Can someone proffer a solution > to the problem? > xnt I worked in an office once and I had some problems with the key board on my desk. My desk was made of metal. The center drawer of the desk rode on nylon guides, but there was a piece of metal spring at the back of the drawer to cushion it it case someone slammed the drawer in. I sat in a chair with hard rubber roller feet. And my chair was rolling on a plastic chair mat on the floor. When I rolled the chair across the mat I would pick up a static electric charge. And if I then pushed the center drawer of the desk closed so that it contacted the metal spring at the back of the drawer bay, there would be a tiny spark emitted. And that spark produced EMI which the keyboard was sensitive to, and it would stop working. I finally figured it out and got a static discharge mat to go under the keyboard which fixed the problem. good luck! Jerry
From: emekadavid on 16 Jul 2010 12:34
On Jul 5, 5:39 pm, Paul <nos...(a)needed.com> wrote: > transkawa wrote: > > > can someone help me solve this non-mysticalso i want to believe > > problem? > > Thekeyboardon my laptop fails sometimes. It's a DeLL Latitude C600 > > series. Though the laptop is old but thekeyboardis only two years or > > so old. Thekeyboardfails in the sense that a particular set ofkeys > > fail. Thekeysare T and Ykeys. Then the Tab and Caps Lockkeys. I make > > htem work again but discharging them with a screwdriver and the tips of > > my lips. > > I do this several times once in a while but thismysticalway of making > > these work is way towards the alchemical. Can someone proffer a solution > > to the problem? > > xnt > > If I was working on it, I'd want to start by reading the part number > off thekeyboardencoder IC. This is an example of an encoder IC. > This datasheet is not as complete as I would like, but it should give some > idea how it works. > > http://web.archive.org/web/20010613141313/www.semtech.com/pdf/sysmgmt... > > Thekeysare arranged as a matrix of wires (row and column wires). At any > one time, most of the column lines are open circuit, except for one column > line driving to ground. That means the column lines have no "drive to logic 1" > capability. They either "drive to logic 0" or are open circuit. > > The row lines are inputs. > > There are some external resistors tied to the lines. If nokeyson a column > are closed, then all the row signals will be pulled to logic 1 by the resistors. > If a single key is pressed, it joins a column line to a row input. If that column line > is currently being scanned (driven to logic 0), then the row input will see > a logic zero as well. So seeing a logic zero, means the key is active or closed. > > So that describes a bit about how thekeyboardmatrix is scanned. Thekeyboard > encoder chip "debounces" the key, and only declares a key closure, if the > key remains closed for at least two scans of the keyboards (roughly). > > Now, this article describes some additional components that can be added. > See "Getting Rid Of Ghosting and Masking" on page 8, where a switching diode > has been added for each and every keyswitch on thekeyboard. The diodes are > added, when you want a "deluxe"keyboardsupporting N-key rollover. N-key > rollover is preferred for gaming applications, where the user can be pressing > a number ofkeysat the same time. The diodes solve the "ghosting" problem, > although at the cost of adding $3-$5 worth of diodes perkeyboard. The thresholding > on the CMOS circuit must be such, that it can handle the slightly higher input > voltage of a logic 0, caused by the diodes in the circuit. > > http://www.dribin.org/dave/keyboard/keyboard.pdf > > If you can find the datasheet for the main chip, you can get the table > of key mappings, which may help you explain why those particularkeysare affected. They could all be members of a group experiencing > "ghosting", meaning thekeysare failing in the closed position, > and thekeyboardcontroller has "locked them out" to prevent them > from interfering. That would not explain though, why you need to > "discharge" the circuit. Discharging a circuit to restore operation, > is something you might do if encountering CMOS latchup. But in this > particular circuit and application, I don't see a reason for that > to happen. And in any case, I don't see how you could add any components > to the circuit to actually fix it. > > To test the PCB of thekeyboard, I'd use a multimeter. I'd set it to > a low power ohms range (where the circuit will not apply enough > voltage to forward bias a diode). Using the meter, I'd test for > shorting between rows or columns or the like. The test is potentially > limited in sensitivity, by the pullup resistors. > > I notice in the Dell C600 documentation, they mention the presence > of "diagnostic software". Perhaps you could run that software, > and check for any error codes related to thekeyboard. Thekeyboard > has internal status information, and if the Dell diagnostic software > can extract that status information, perhaps it can detect a > "stuck key" or "ghost key" problem. And that might give you more > clues as to the source of the problem. > > Paul diagnostic replies okay. it gives only one problem but that is non keyboard related. not using the laptop right now and not at home but i think the only failing on the diagnostic side is a dst error message; haven't solved that problem yet also. |