I've got a fault that I can't see how it could occur. A few of the keys on an HP9810 keyboard are intermittant (they work better if pressed hard). Of course this is a contactless keyboard, so it's not just dirty contacts.
I think I understand how the keyboard should work. Each key is really a transformer, with 2 windings made from PCB tracks. Pressing a key brings a 'core' close the tracks, increasing the coupling. The secondaries of all the transformers are wired in series to form the 'sense loop' that's connected to the encoder board.
The primaries of each pair of keys differing only in the LSB of the keycode are also connected in series (but one of the transformers is wired to invert the phase of the output pulse -- that's how they are distinguished). The 37 pairs of primaries are then wired (with a diode in series to prevent sneak paths) in 37 locations of a 10*4 matrix. This matrix is scanned by the encoder board electronics.
OK, the problem is that a few keys don't work. They're not all in the same row or column of the matrix. They're not even in pairs in the same matrix location (which would tell me to check the associated diode and/or the primary tracks). They're random. And it's not the key plungers or the 'cores' -- swapping those round doesn't help -- the same locations on the PCB still fail.
I've even tried cleaning the PCB surface -- no change. I don't see how it can be open-circuit connections as that would take out more than 1 key always. And, yes, it's not a firmware problem causing the machine not to recognise certain keys -- there is no key-pressed signal from the encoder board when a faulty key is pressed. I am running out of ideas, can anyone else suggest anything?
HP9810 keyboard fault
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07-09-2002, 05:07 PM
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07-09-2002, 10:10 PM
Okay, so I don't know one blessed thing about a 9810. But here's my 2 coppers worth: Stupid question: Do you have access to a working 9810? Answer = No: Sorry, out of ideas. Answer = Yes: On the good 9810, get out your trusty oscilloscope. What is the frequency, pulse width and amplitude of the signal on the primary side of the keyboard trace? Compare these values to the unit that is not working. What is different? I'll bet you'll find something here. Probably the amplitude or rise time. I'm sure it will be a time domain problem since we are strictly AC here. As you say, totally random. Since it is really a variable transformer based on your description, the faster the key comes into proximity with the traces, the faster the input to output coupling, hence the change in behavior when you press hard. Sorry if you've already been there, done that. But I would like to know what you find wrong. Curious I am. ▼
07-09-2002, 10:53 PM
As a follow up to my previous post, I really think this is some type of pulse width or amplitude problem. My rationalization: If a key press is by way of coupling a pulse from one matrix line to another, the keys "transformer" must develop magnetic flux in order to pass the current. The rate at which the transformer develops this flux is affected (to a very small extent) by the speed at which the key is pressed. Ideally, when the unit is working properly, speed is not an issue as there is sufficient voltage to induce flux and perform the pulse coupling. But in the case of this unit, if the voltage supply is low, or varying, the rate at which the iron core approaches the pulse can make a difference. It is the rate of rise in the induced current that causes the flux to reach a point where coupling can occur. I think it comes down to a case of (dv/dt). We don't have it on the (v)oltage side, but by decreasing the (t)ime side the key works. I think.
07-10-2002, 12:14 AM
Tony, I don't have any clues at the moment to help you out but I do have a working 9810 and high-quality test equipment. Let me know what measurements I can make on mine that will help you to diagnose the problem further. -Katie
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07-10-2002, 06:36 PM
Thanks for the offer. Let me make a few more tests on mine to attempt to narrow down the area, and then I might well ask you to check a few signals on the board in your machine. Of course anything I discover will be reported here.
07-10-2002, 06:34 PM
No, I don't have a working 9810... But I do intend comparing waveforms between the defective keys on this keyboard and the good ones. I hope to see something a little marginal. ▼
07-11-2002, 09:25 PM
Tony, the fact that the keys work if you "_really_ press on them" makes me wonder if it couldn't be hairline cracks in the PCB traces. Does the PCB deform under pressure? Best, --- Les [http://www.lesbell.com.au] ▼
07-12-2002, 02:08 PM
Yes, I thought of cracked traces. The only problem is, the 'primary' of a defective key is in series with that of a 'good' key, and _all_ the 'secondaries' are in series. So a simple open-circuit trace would not accound for the odd defective keys I have. ▼
07-12-2002, 02:48 PM
Since none of the low-level component testing indicates a problem with the keys or the circuit board, what about looking for something higher-level and subtle. How about noise on one (or more) of the power supply lines. Those old filter caps don't last forever, and that can cause all sorts of weird behavior like you’re having. Alex Knight found that to be the problem in his (and my) HP-46. ▼
07-27-2002, 04:37 PM
A few weeks ago I was asking about a 9810 keyboard that had 3 keys that only worked if pressed hard. Today I was able to spend some more time on it. In the meantime I've read the HP journal articles on the 9820 (same design of keyboard) and discovered that the ▼
07-27-2002, 10:17 PM
True the LM711 is no longer made but it's easy to find the commerical temperature version ("C" suffix) in the 14 pin DIP form. The DIP form of this has the same pin ordering as the metal can, pins 1,7,8 and 14 are not used. It should be easy to extend each lead stright down into the circuit board -- there won't be any crossovers. Look for LM711CN on the internet. Here's one place that has them for $.49 each: http://www.goldmine-elec.com/images/Pages016-019.pdf If you the need the pin-outs on this let me know.
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07-28-2002, 01:40 PM
Thanks for the info. Of course it'll probably cost me a _lot_ more than $0.49 to transfer the money, and to have the part shipped, etc. Still, it won't be too expensive in the end. But I'll explore my own junk box a bit more first, I've got all sorts of things ▼
07-28-2002, 05:14 PM
Here's another (better?) option. Use an LM319H as a replacement. It's what National Semi recommends and it's still available in a TO-5-10 (like a TO-100) case. It has two separate open-drain outputs instead of a common output like the LM711, but because they're open-drain they can just be tied together and it should work identically to the LM711. What are missing on this are the strobe inputs, it has independent grounds instead, but I don't know if the 9810 is even using those. The only other issue is that you’d have to cross over two pins to math the ordering of the LM711. Digitkey has them for $3.00 is single quantities and I’m sure that there’s a vendor in the UK that has them too. ▼
07-28-2002, 09:23 PM
I had considered using the LM319/393/339 comparators. Unfortunatelty the 9810 _does_ use the strobe inputs, which makes life a little harder. I mentioned that there were 2 keys at each matrix location, one which causes an in-phase pulse to be generated, the other which causes an antiphase pulse to be generated.
07-28-2002, 12:44 AM
Regarding the transformer coupled keyboard, in a slightly related way, I've got an HP3480A nixie tube digital voltmeter which has BCD outputs and remote control inputs which are isolated with transformer coupling. The transformers are implemented with ordinary axial lead inductors. The hot side "windings" are all on one PCB and the other side on another PCB. The inductors are laid out so when the PCBs are in place, they are close enough together to couple. To achieve the required spacing, or for other mechanical reasons, the PCBs are so far apart that the inductors on one side have long leads holding them up off the PCB. To reduce the number of transformers required, some logic serializes several bits to one transformer, then de-serializes them on the other side. I have another much more recent meter, HP3456A, which also uses transformer coupling between a guarded (mostly) analog chassis and a digital display and I/O chassis. HP uses the terms "inguard" and "outguard". In this case, each end has two toroidal inductors mounted on the PCB. A four wire harness goes from one PCB to the other. The two pairs of wires go onto the PCB at each end but each pair only connects to the ends of a short length of wire that passes through a toroid as a one turn winding. The signaling is like HPIL in that each channel transmits in one direction. It is like the IBM PC keyboard in that one start bit plus eight data bits are transmitted and the receiver is a 9 bit shift register, when the start bit reaches the ninth flip-flop, the output interrupts a microprocessor to read the data bits. But where the PC keyboard has a separate clock signal, this circuit recovers a receive clock from the serial stream. There is an edge for every data bit because a pulse of one polarity is transmitted for a "1", and the opposite polarity for a "0". This is similar to the "Manchester encoding" used in Ethernet where the goal is to achieve a zero DC level, which is also part of the reason for "eight-to-fourteen" encoding used in compact disk. By using 14 code bits for 8 data bits, they accomplish run length limiting and have leftover bits (or redundant codes) which are selected to bring the DC average to zero, which is important for the IR receiver. Just thought you'd like to know! I'm going to try making a switch from your description of the 9810 keyboard. In high school I built a project that worked on the principle of a shorted transformer secondary. It was a length of 1/4" steel rod bent into a large "U". At each end was a coil of about 100 turns of #20 wire. The "U" was attached to the top of a box with the ends pointing down. A short length of aluminum tube (I used the can of a small electrolytic capacitor) circled the steel rod, free to slide from one side to the other. A motor driven switch energized the coils alternately with about 24 VAC. When the coil under the aluminum tube was energized, the tube would fly to the other side. Another electromagnet project I have plans for, but haven't built, although I have seen one in operation, has an solenoid-type electromagnet mounted on a horizontal support, the coil vertically oriented so the pole is looking down on an open space. A photocell is mounted on a vertical support which is holding the horizontal support. The photocell controls the current to the electromagnet so that the current is in direct proportion to the light hitting the photocell. There are two control pots: one controls the overall gain, and the other controls the response time, from the photocell to the coil. You place a light source so it illuminates the photocell with light passing inder the electromagnet. Now that it's all built, you hold a light steel object, supporting it from the bottom, under the coil so that it is partially blocking the light to the photocell. The perfect object is the tiny globe that comes with a pencil sharpener. Then you adjust the gain till the electromagnet lifts the object, which reduces the light on the photocell, which lowers the drive to the electromagnet. By adjusting the controls you can make the object float. Then you can set it spinning and it will spin for quite a while. I imagine eddy current are what finally slows it down, since it is moving in a magnetic field. With the gain control you can move it up and down a little. With the delay control you can make it bounce up and down a little. ▼
07-28-2002, 01:47 PM
Many older digital voltmeters had transformer-isolated input stages. My old Solartron has a VCO (as a frequency-to-voltage converter) in the input module, the output of which is transformer coupled to what is effetively a frequency counter (!). But my somewhat more recent Fluke (but still old enough to have a pair of 4040's to control it) had optoisolators on the 'guard crossing PCB'.
07-28-2002, 02:44 PM
Ellis: Look for Popular Electronics magazines around 1974 or the "Electronics Experimenters Handbook" from the same PE editors, and you may find an example of your magnetically suspended earth globe... I am not sure about the exact issue it appeared in. Thank you for a nice reading about electronics, coils, etc.
07-10-2002, 07:53 AM
Tony: I don't know the 9810, but the keyboards of similar equipment of the date were capacitive-coupled instead of magnetic (transformers) as your posting implies. I am just guessing, once again, but I suggest you may check whick kind of coupling is used in this case. ▼
07-10-2002, 06:27 PM
I've worked on plenty of capacitive keyboards (mostly Keytronics) to know how they work and what the PCB traces look like. Typically you 'scan' one plate of the 'capacitors' and look for voltages on the other plate. I've seen this done with TTL and 4000-series CMOS chips, I've seen it done with a variety of ASICs. ▼
07-11-2002, 08:20 AM
Tony: I knew you are very well qualified about hardware and many times I enjoyed your detailed descriptions. I was puzzled about the "inductive" coupling keyboard, but please don't take my previous suggestion as disrespectful. ▼
07-11-2002, 02:04 PM
Tone-of-voice doesn't come across very well in this forum :-). I was not being disrespectful at all. But I am sure it's an inductive keyboard and I wanted to save other people from wasting their time by making suggestions appropriate to the capacitive type. ▼
07-11-2002, 04:46 PM
Tony: Certainly tone of voice doesn't come across very well, and perhaps my case is worse because of idiomatic issues. In my previous message, I was trying to apologize, I tried to mean I may had been inadvertedly disrespectful to you without such intention. I do respect your skills, and also know from your previous postings you have been extremely kind to others. My guess about "capacitive", while well-intended (I worked with such keyboards too, and also found some with Hall-effect switches!), was clearly wrong. ▼
07-12-2002, 08:05 PM
Hey you, guys! I sometimes amaze myself with posts in here. Reading Andrés' and Tony's posts brought me back to a time when men respected each other in such a manner they would feel embarrassed if misunderstood. That's the spirit that maintain this forum a place to stay all over. Good readings. Tony, Andrés (I read some news about Argentina; are you alright?); is there anything about inductive, capacitive coupled keyboards that can be found at Internet? I am a lot curious, and all info in your posts teased me. I have been dealing with rubber, membrane kbd solutions, and I do not know about capacitive and inductive ones. Thanks for any info. ▼
07-13-2002, 10:13 AM
Luiz: Thank you for your comments. I agree one of the pleasures about this site is that most of the people here are true gentlemen and ladies (should we say gentlepersons?). I have not detailed information about the inductive or capacitive keyboards, the latter consisted in a printed circuit board where there were islands of copper (usually covered with an antisolder mask or such). The space beneath each key formed two semicircular islands, which will become "capacitively" copuled when the key travelled down. The bottom of the key included an aluminum foil (usually attached to the plastic by means of a plece of plastic foam). When this foil approaches the PCB islands (there was no electric contact), these become coupled. I suppose specific ICs apply signals to scan the keyboard and to detect the signals from the "sense" side of each key. About the situation here (and certainly I will not discuss anything political or controversial in this Forum), I may say usual life goes on; we look awful in international TV news, but not everything goes that bad. We have deep economic problems, and our international purchase power decreased to 25% from usual, just in a few months. There are usually peaceful protest marches and a very large underemployment. Many qualified professionals are going to work overseas. I hope things will improve in a couple of years, but only after a societal agreement which is still insufficient. There is a wrong tendency here to blame external factors (which may exist indeed); I think we should stop such blaming and start working to solve our own problems.
In the meantime, tourism is almost incredibly cheap from anybody coming from other countries, so all you are welcome! |