LX200 Classic Keypad or Hand Controller

       

 

Visit Doc G's website for more of his research on the keypad (or hand paddle or hand controller) and just about anything on the care and design of the LX200 Classic keypad. Should open a new browser window over this one.

Subject: Analysis of the LX200 Keypad Operation (classic) 

From: Doc G, Date: Oct., 1997

Special thanks to Paul Goelz for his perceptive questions, suggestions and contributions over the course of this analysis.

This is an analysis of the LX200 Classic keypad including a description of its various components and how it communicates with the LX200 main computer. Some commentary on the use of long cables is included.

The pad has 19 buttons. There are 20 connections between it and the converter chip (no surprise there) It looks like all of the connections go to a 40 pin chip. The chip has an oscillator (crystal) to control the frequency (period) of the digital circuits in the keypad. There are also a few 74 series logic chips. These are relatively old style TTL chips but good solid ones. One of them, an SN74LS14N, (Hex Inverting Schmitt Trigger), has one output and one input connected to the cable. These are buffer/pulse shaper chips which are designed to send a well shaped signal down the cable and accept the return signals with a Schmitt threshold. This is a very standard design. The Schmitt input levels are 0.8 and 1.6 volts and the output send levels are 0.5 and 3.5 volts. The other two wires in the cable connect ground and positive power supply voltage. The voltage on the line is full battery voltage. This is regulated down to 5 volts with a 7805 type three terminal regulator. Additionally there are a few chips which drive the display.

The 40 pin chip might be a commercial keyboard chip which generates 8 bit words. It is hard to tell about the 40 pin chip, because it has no manufacturer identification on it. The parallel words are probably converted to serial with the 74HCT166 and sent to the computer in the base. Switch closures initiate appropriate digital signals. The return serial signal is converted to parallel and sent to the display. The keyboard is a relatively dumb digital circuit with the main computing done by the computer in the base.

I have set up the keypad with a precision oscilloscope and measured the signals transmitted to and from the keypad to the computer. One of the lines sends signals to the computer and one receives them from the computer. These signals are of course in serial form. The code is a simple high/low signal with a time period of 100 microseconds. (10kHz pulses)

I did not try to translate very many of the signals, this could be done, but it appears that a single 8 bit word is sent each time a key is pressed. For example West sends 10100000, East sends 1010110, North sends 11010000 and South sends 11010100. This is consistent no matter what speed mode is set. The speed key and every other key sends a different code to the computer in the base.

The main computer responds only some of the time. When the keypad is in a display mode such as time or coordinates, the computer sends a string of code continuously to the keypad. Thus it appears that the computer essentially controls the display. The keypad does no more than send the key strokes and run the details of driving the display.

The pulses are not especially short, 100 micro seconds, and are crystal controlled in the keypad via the keypad to parallel word converter chip. The wave form of the pulses is very good with a cable of 25 feet. There is capacitance in the interconnect cable. For typical 4 wire flat telephone cable the capacitance is 20 picofarads per foot. Thus for a 100 foot cable the capacitive load on the send chip is 2000 picofarads. The chip has an output current rating of 0.8 ma which will give a rise time of about 1.5 microseconds in the output pulse. Since the pulses are 100 microseconds, this risetime is very reasonable. I do not see why a long cable can not be used except for the D.C. voltage drop in the cable. That is, the poor ground it causes.

The voltage drop in the cable is as expected from the resistance of the cable (standard flat 4 wire telephone cable, #26 gauge) which is 4 ohms per 100 feet. With a current of 74 ma 100 feet gives a drop of 0.3 volts. As it turns out several ground points on the telescope are as much as 0.2 volts different. I personally believe that not enough attention has been paid to grounding in the electronics as well as some other details of the design.

The ground drop is of concern since it subtracts from the response voltage window of the Schmitt circuit in the SN74LS14N. A drop of 0.3 volts is significant as related to the 0.8 and 1.6 volt thresholds of the receive chip. I think this could be a real concern. On the other hand, I do not see a problem with extending the cable if the cable can be made to provide a good ground. I think the SN74LS14N can drive 100 feet of cable with a capacitance of 20 pf. per foot easily. The data sheet information clearly supports this belief. In summary, I believe a longer cable will not cause data transfer problems do to pulse shape or speed. The voltage drop in the longer cable is a possible source of problems even though several persons have used cables of 100 to 150 feet without problems. I believe the long cable (100 feet or more) should have a ground that is better than that provided by the typical 4 wire flat telephone cable. Suggestions for solving this possible problem are given at the end of this paper.

The keypad draws 74 ma at 5 volts. Thus it consumes 0.37 watts. The rest of the heat in the keypad comes from the three terminal regulator. In the case of a 12 volt supply this is 0.52 watts and for an 18 volt supply it is 0.96 watts. Thus the heat load on this regulator is significantly larger that for lower operating voltages. The regulator heat and that of the circuitry, 0.37 watts, are the sole sources of heat in the keypad.

I am slightly concerned about the heat dissipation in the 7805 three terminal regulator. With an 18 volt supply the dissipation is about 1 watt. The chip gets quite hot to the touch. This makes it the major source of heating in the keypad. The warmth is directly at the location of the regulator. I am devising a small heat sink for this device since I insist on operating at the 18 volt supply recommended by Meade. (for the sake of the drive motors) (Though I am having second thoughts about this issue as well) Here again I think the entire design of the LX200 electronics package was not looked at from a synergistic viewpoint. The design is good but several separate parts do not quite fit together in some cases. (again my personal opinion)

(If I had a circuit with grounds not at the same voltage, I would find out why, damn fast.) Also, if I designed a circuit to operate at 12 to 18 volts, and had to regulate directly down to 5 volts I would provide a more appropriate power supply or better heat sinking of the regulator devices.

In summary, I see no reason to get too concerned about the keypad design. It works well. The only problem which might arise with wear is the functioning of the buttons. They do seem to have a good snap action however. I have not had trouble with them or with any functions of the keypad. I have noticed sluggish display response when the temperature got to 0 degrees F. (not more sluggish than the actions of the observer however)

Subject: Use of Long Cables (classic) 

I have heard rumors of keypad failure do to using a long cable. I would expect failure to show up as incorrect codes being passed rather than massive failure of the keypad. It is possible that too much voltage drop in the ground line will cause code errors as described above. But I would not expect damage to the keypad. Is it possible that damage with a long cable, as rumored, was do to reversing the power supply? This would definitely destroy the voltage regulator and possibly the circuitry in the keypad. There is no protection whatever within the keypad for an error of this sort. There should be such protection since it can be done with a single diode.

There have been a number of reports of using long cables without trouble. I recommend caution. Coiled cords of 15 to 25 feet, normally used for telephone handsets, should be satisfactory. If a cable of greater length is made, it must be wired correctly. A cable with a good ground should be utilized or a separate ground from the keypad to the control panel with low resistance should be run parallel to the signal cable.

To be sure that a good ground is maintained and that the pulses transmitted do not deteriorate, I recommend using a cable with multiple conductors so that several of them can be combined in parallel to give a ground connection with a resistance no greater than 1 ohm. The cable selected should have a capacitance no greater than 12 to 15 picofarads per foot. A suitable cable would be Belden 8104, 8105 or 8106 or equivalent, depending upon the length of the extension. Other cable choices will be quite satisfactory if they have the necessary electrical characteristics.

The cable must also be competently installed and appropriate cable sockets and plugs installed.

I hope this analysis helps clarify operational considerations for the keypad. Now on to the main computer board and motor drivers.

Subject: Keypad Codes (classic)    

From: Doc G, Date: Nov., 1997

Visit Doc G's website for more of his research. Clicking this link should open a new browser window over this one.

Also visit Telescope Service technical pages.

Keypad codes:

I have looked at the code sent from the keypad to the computer for all of the buttons on the keypad. I have set the keypad into various modes and set it at various selections within the menus. Each key sends the same code no matter the setting of the keypad. The computer interprets the key presses depending on what sequence it sees.

The code has nine bits. It always starts with a high (1) and ends with a high (1) when the key is pressed and a low (0) when the key is released. Every press sends a code and every release sends the same code but for the final bit. Here are the codes. Only the presses are given, for release drop the final 1.

Keypad Codes

North	110001101
South	100110101
East	101011101
West	100010101
Enter	101001111
Mode	101011001
GOTO	100111001
Next	111011101
Prev	110001011
Guide (0)	100010011
Center (1)	101110011
Map (2)	110110011
CNGC (3)	100110011
Find (4)	111010011
Focus (5)	101010011
Star (6)	110010011
Slew (7)	111110011
Ret (8)	111100011
M (9)	101100011

Subject: Keypad Hot Plugging/Unplugging (classic)  

From: Doc G, Date: Dec., 1997

LX200 Keypad -- Pluging it Hot (turned on)

I suggest great caution in doing this. I would in fact go so far as to say DO NOT PLUG THE KEYPAD WHILE THE SYSTEM IS HOT!!

Here are the reasons. Because of the design of the power supply in the keypad and the fact that two digital signals are also transmitted through the connecting cable, there is a distinct probability that hot plugging might destroy the receive chips in both the keypad and the computer main board.

The problem is that the full power supply voltage of 12 to 18 volts is connected through the cable. The plug is not the correct type for hot plugging. A hot plug must have a ground connector (pin) which makes connection before any other connections are made. The telephone type plug is not of this type.

What can happen is that the power line might be connected momentarily before the ground. This momentarily applies, throught the electronics, the full sypply voltage to the input and output terminals of the send/ receive chips in both the keypad and the computer board. These chips are designed to operate and do operate at a 5 volt maximum level. In this instant, the chips could burn out. There is also the possibility of static electricity discharges taking place when the cable is plugged in at either end.

I believe that the system was designed to be turned on and off only with all connections in place. Meade gives this warning. This may or may not be a good design decision, but it is a fact. (I think it not good)

Also, if for some reason the keypad cable looses its ground, the same result may be effected. My personal feeling is that the modular connector is not the best choice for this application.

As an aside, I might note that many pieces of equipment have this same problem. When hot connection is desired, an appropriate plug with a good first on, last off ground must be used. All cables carrying power voltages should have this type of plug. In 40 years of consultation on electronics problems I would estimate that half the problems relate directly to power supplies and improber grounding.

SUMMARY: Do not hot plug your LX200 equipment.

Subject: Hot-Plugging Keypad Modifications (classic)  

From: Bill Ezell

I finished examining the keypad/control panel interface, and now have mods for protecting the main board as well as the keypad. The additional mods can be made right on the control panel; no need to touch the main board. I also discovered some interesting (well, at least to me!) things out about the scope in general, and a procedure you can use to do some very rough diagnostics.

First, a correction. In my original post, I said to look for diodes hanging in space connected to a big IC in the middle of the keypad board. Everything's correct except for the 'big'. The big IC is the microcontroller for the keypad, but the diodes connect to the smaller 14 pin IC, which is the line interface chip. This doesn't really change anything, but it might eliminate some confusion. It's hard to see this without removing the circuit board from both sides of the case; I don't recommend doing that.

Diagnostic procedure:

You can isolate failures in your LX200 a bit. Remember all the reports of the dreaded 'Meade' message on the keypad, and then nothing else?

The keypad itself displays this message on power-up. I ran my keypad without it even being connected to the scope! So, if you see the 'Meade' message, and you have some LED's illuminated on the keypad, it's working. Either your main board in the LX200 is dead, or the interface chips that connect the keypad to the scope are dead. Listen when you power up. If the scope does its index mark seeking, indicated by the motors running for a few seconds, then the main board is probably working, and you've suffered hot-plugging failure or some other interface IC failure, most likely a BIG static discharge to the keypad cable.

Interesting tidbit:

The ground of the 18V plug on the front panel is NOT at ground as far as the rest of the scope is concerned. The outer part of the connector runs throughout a 0.1 ohm resistor BEFORE connecting to electrical ground. This was done to allow measurement of the current being used by the scope... it's used to generate the reading on the bar graph display. 99% of the time, this is irrelevant. However, if you have any external equipment that uses the same ground as the 18V connector as its signal ground and also connects to one of the ports on the scope (CCD guider, for instance) you can have signal noise problems.

Main board hot-plugging protection:

I assume Meade has made this change along with the keypad mod, but if so, they did it on the main board, not the control panel board. My scope's finally back on its new pier and it's not coming off for a while, so I can't confirm this. I would suggest doing this mod ONLY if you have older electronics that don't have the keypad mod. The purpose of this mod is to be sure the voltage on the signal lines doesn't exceed their maximum rating. The voltage will be 'clamped' to no greater than 5.7 volts.

Take off the control panel by removing 4 screws, just like you would do to change the battery. Unplug the panel from the ribbon cable that connects it to the rest of the scope.

Hold the board so you're looking at the back of it and the bottom edge is down. Look near the center bottom where the keypad connector is attached. You'll see 4 solder pads in this configuration:

          O          O
               O  <>  <- square pad

These correspond to:

 +18  SIGNAL
                    SIGNAL  GND

Note that the square pad is GND, unlike the square pad in the keypad. The two SIGNAL pads are the ones you will attach to.

Now look diagonally up and left a bit for 5 pads laid out as:

 O          O  <- these two are +5V
                         O
          <> <>

The three in the right column correspond to the three pins of the VREG1 IC on the front of the circuit board. BE SURE you have identified the correct pads. The only purpose of this IC is to provide +5 volts to the AUX connector, but we need it for the mod. (How convenient!)

You need 2, 50V 1 amp diodes. Just as for the keypad mod, the actual values aren't critical; anything of at least 50V is fine. You can get these at your local Radio Shack.

Connect the BAND end of one diode to either of the +5V pads. Connect the other end to one of the SIGNAL pads on the keypad connector. Do the same for the other SIGNAL pad. You should now have the BAND end of two diodes connected to +5V, and the non-band end of each connected to one of the SIGNAL pads. This is a fairly safe mod; you would have to make 2 mistakes to do serious damage, but if you connect the diodes backwards, your keypad won't work. Prolonged operation with reversed diodes could cause the interface IC's to die, so if your keypad doesn't display the 'LX200' message in a few seconds, turn the scope off and recheck your work. Be aware that the two square pads at the VREG1 location are connected to +18V. Be very sure you don't use these by mistake!

Reconnect the panel to the ribbon cable. Be VERY CAREFUL to be sure the connector is attached correctly, and not shifted over by a position. If you plug it in wrong, you'll probably fry your main board.

Subject: LX200 (classic) Keypad Failure/Suggested Fixes-- part 1 of 4 

From: Tim <timtelescopeservice.com> Date: Aug 2002

Keypad failure is likely to be one of 3 things.

1. Corroded connections at the power panel or keypad. Make sure the modular connector jacks are clean.
2. Failure of the 74LS14 chip marked U11 on the main board.
3. Failure of the 7LS14 chip marked U6 on the controller.

These are the most obvious failures and, of course, it could be some else. You didn't say whether the LCD screen on the controller sequences through its usual display or not, or if the scope does it's usual RA hunting on power up. The answers to these questions may help to trouble shoot the problem. See my telescopeservice.com website for more on the keypad.

-------------------------------------------------

Subject: LX200 Keypad Failure/Suggested Fixes -- part 2

From: Wayne Watson <mtnviewsearthlink.net>

Thanks for the tips. The hand controller cycles through the top level menu. I can set the RA/Dec. All looks right until I hit the slew buttons. How can I pinpoint the failure if it is on one of the chips you mention? From a previous experience with this type of a problem, I think the U11 chip is an LS14 on my board. Are a 74LS14 and LS14 the same? I remember that getting a LS14 is not so easy.

Over the last six months I've had increasing difficulty with the slew arrow keys. Usually, I get around the problem by just changing the slew rate back and forth. About a month ago, I took the controller out (the scope was powered off) to take to a sky show as a back up to a friend's LX200 who has having trouble with his controller. We never used mine, but it sat unconnected on my table in the observatory. When I decided to plug it in and try out the scope the other night, it failed as described.

---------------------------------

Subject: LX200 Keypad Failure/Suggested Fixes -- part 3  

From: Tim

I think the problem is the membrane keypad. The fact that most of the other keys work and the hand controller cycles thru the menu means there is no problem with the communications between the hand controller and the main circuit board. The membrane keypad works like a switch. When unpressed each key is an open circuit and when pressed offers a low resistance (30 to 60 Ohms or so) to ground. I think that over time and with use the low resistance gets progressively higher until the switch closure can no longer be detected. I don't know exactly how these particular keypads are manufactured, but there is most likely some kind of low resistance material plated onto an area on each side of the membrane and that with each flexure it wears/flakes off the surface.

I don't know of any way to fix it short of replacing the membrane itself. I believe theses are a special part manufactured for Meade and are not generally available from anyone else. The way the detection circuit works is that each switch is pulled up to +5V through a 10K Ohm resistor and the closure is detected by a 74HC166 parallel in shift register. The low resistance switch and the pullup resistor form a voltage divider that switches between +5V and some low voltage level and the 74HC166 detects a logic low when each input is pulled below 2.5V. Sorry to be so technical, but it may be possible to effect a fix by replacing the pullup resistor with some higher value, like 20K Ohms. This will hopefully restore the voltage divider so that the new switched value is now below 2.5V. The part to replace is RN2, a 10Pin, 9 Resistor Network in single inline package (SIP).

To answer your question Yes, the 74LS14 is the same as LS14, and they are not all that hard to get.

--------------------

Subject: LX200 Keypad Failure/Suggested Fixes -- part 4 of 4

I think the diagnosis I performed via e-mail with a few MAPUGers led the correct solution. It was a controller problem. Ultimately the simple act of removing the screws from the back of the device, inspecting it, and putting it back together managed to shuffle some component (probably the keypad membrane) into obedience. It's back working properly. Such is life with the LX200.

Subject: Keypad failure- REPAIRED! (classic) -- part 1 of 2   

From: Paul Markov, Date: Sep 2002

Here's an update on the repair of my hand controller or keypad. (see below of the description of the failure)

The diagnosis and repair was made by a friend (& LX200 owner himself) who is quite experienced in this field. Careful measurements with an oscilloscope showed that all ICs and other components were functioning properly. The problem was traced to the keypad itself. The keypad is connected to the printed circuit board via a "ribbon cable". It's not a real ribbon cable, rather it's clear plastic with strips of metallic paint sandwiched between the plastic. Each metallic paint strip carries a signal back to the board. The problem with my hand controller was a broken signal line, specifically the ground signal (the wider metallic paint strip on the outer side of the ribbon cable).

Expansion and contraction of the plastic and/or metallic paint made the problem intermittent. Generally fixing something like this is very difficult because you cannot solder anything to the ribbon cable (since it's just plastic and a thin strip of metallic paint). Trying to access the signal line where the break is by sanding away the plastic coating does not work because the process also sands away the actual signal line.

A little creativity helped with the fix: peal back the keypad skin from the controller case, underneath you will see the signal lines going to each key, plus the ground signal. This underside is actually three layers of plastic. Top layer - key signal lines, middle layer - spacer (keeps each signal line open, unless a key is pressed), bottom layer - ground signal line. Carefully separate the bottom layer so as to expose a small part of the ground signal line. Use some metallic sticky tape and "tap" into the ground line. Now take a short piece of wire and wrap it around the other side of the metallic tape. Take the other end of the piece of wire and solder it to the connector (the connector that connects the entire ribbon cable to the printed circuit board).

Put the keypad layer back together and carefully reassemble the controller. Basically what was done here was to by-pass the broken metallic paint strip in the "ribbon cable" located between the keypad skin and the connector. The life expectancy of this fix is unknown, but so far it has functioned well for a few hours. I suppose that a similar method could be used to repair keys that no longer respond, depending on the nature of the breakdown.

Disclaimer: if you have to try this fix yourself, do it at your own risk. It's very easy to actually cause further irreversible damage while attempting the repair.

-----Original Message-----
From: Paul Markov

I'd like to share with you my keypad failure (LX200 Classic) experiences so far.

During a recent observing run some of the buttons on my recently stopped responding, but otherwise all worked fine. Turning off the scope & then back on would solve the problem for a few minutes, then some keys would stop working again, until eventually all the keys stopped working. During the next observing session, all worked fine for about half-hour, then all keys stopped working again. Just as an experiment I stuck the keypad in the freezer for a few minutes, plugged it back in the control panel, and all the keys worked fine (I was testing for some heat-induced failure mode).

A quick check of the insides shows nothing wrong, so the next step is to check some of the ICs with an oscilloscope. A friend of mine is experienced in this field and is will be doing this work in the next few days (thanks Doc G for the keypad schematics!) His first guess is that either chip in U3 or U4 (74LS166N) is defective. Actually, there was a clue of what may have gone wrong - when we opened the keypad, there was a small loose nut moving around inside the keypad (this is one of the nuts that holds the LCD in place). So perhaps the nut shorted something out.
-----End of Original Message-----

------------------------------------

Subject: Keypad failure- REPAIRED!-- part 2

From: Brian Bond <brian.bondblueyonder.co.uk>

Whilst you have the hand controller open, change C2 to a 30 volt component. This capacitor is stressed at 18V, (along with 2 others in the system), and when breaks down, either blows the main fuse, or....destroys itself, gets extremely hot, then melts the membrane of the keypad before finally burning itself and the PCB. At the same time, add a heat sink to the 5V regulator. For further information go to <http://www.brianbond.eu.com/> then LX200 info.

Subject: Classic Keypad Broken Conductor Repair 

From: S. Jamiem <SJamiem16aol.com> Date: Jan 2004

About a month ago, the East slew button on my LX200 Classic Keypad stopped working. I was able to slew the telescope East with the main computer and all other keys worked properly. After reviewing all the info on this site I could find on the keypad, I decided the problem was with the flexible keypad connection. After unplugging the pad from the connector on the controller board. I ran continuity tests and confirmed normal operation for all of the keys except the East slew.

Examination of the flexible connection revealed a very small break in the conductive paint. It didn't seem that there was anyway to repair this break. Shortly afterward, I was watching TV and glanced at my wife as she was sewing a quilt together. She attached the various layers with a pin to hold them for sewing. Then it hit me.

I asked for the thinnest pin she had and carefully pushed the pin into the conductive foil on one side of the break and back through on the other side of the break. I then carefully gripped the pin in pliers and broke it off about 0.3 inches total length and left this piece in place to bridge the break. The pin bridged the gap on the outside of the bend in the flex circuit so that pressure would be maintained after reassembly. It is also possible to punch the foil in several places on each side of the break to get redundant contact with the foil. After checking continuity and reassembling the key pad, tests on the scope showed it was now fully functional and has worked properly every since, including at tonight's 10 Degree temperatures.

The pin is very thin and appears to be about 1/3 of the foil width but I understand these are not hard to find.

Subject: Replacing Bad Keypad Buttons (classic) 

From: Scott Pinkham

I had a keypad on which I was able to "replace" the GoTo button. Here's what I did:

1. Using an Exacto knife, I carefully cut away the plastic underneath the membrane switch (I cut from under the switch, after disassembling the two halves of the keypad). I cut a square big enough to fit a small tactile push button switch.
2. I then epoxied the new switch in place, making sure that it was positioned so that pressing the on the membrane on top would fully depress the switch.
3. I wired the connections from the new switch to the ribbon cable connector that plugs into the keypad "motherboard". One wire goes to a common (either power or ground, I don't remember which), the other to the signal line for the GoTo button.

Other than about an hour of my time, I spent maybe $2 on fixing the keypad, and it's still working after a year. I'm sure this same procedure could be used to repair any of the buttons on the keypad, but of course, it's only worth trying if it's not under warranty.

Subject: Classic Hand Controller Button Membrane Repair 

From: John Mahony, Date: Jan 2005

> My three year-old LX200 classic hand paddle is showing its wear. The
> keypad cover is getting brittle and flaking off of the N,S,E and W
> buttons. The membrane underneath seems okay. Is this a cosmetic
> problem only, or is there a risk with humidity/dew.

If it bothers you, you can put some clear packing tape over it. I managed to rip part of mine, removing some clear self-adhesive rubber "buttons" I had put on so I could feel the direction buttons in the dark (a very nice mod, if you want to try it, you can get them in hardware stores, normally used on the bottoms of appliances to keep them from sliding around or marking surfaces). The first were a little too thick and didn't feel right when I pressed them, but then I found some thinner ones (~1/8") that looked just right, so I replaced them. But the adhesive on the old ones was a little _too_ good. The rip was nearly a complete hole where one of the old buttons had been, so I used packing tape over it and added the new button. That was over a year ago and it's worked fine since.

Subject: Panic Stop? 

From: Rob Getz, Date: Mar 2005

Question: What is the best way to stop the Classic mount in a GOTO if I realize it is going to hit the fork?

Answer: Just hit the GOTO again and it will stop.

Subject: Elimination of Annoying Keypad Beep (classic)

From Brent Boshart, Date: July, 2000

The beep from the LX200 keypad when satellite tracking (esp. continuous tracking) could just about drive one crazy. So, I installed a small switch to enable/disable it. See:

<http://sattracker.hypermart.net/> Should open a new browser window over this one.

Subject: Multiple Keypads for the LX200 Classic   

From: Doc G, Date: Dec., 1997

I have reviewed my rather extensive note on the LX200 keypad and have come up with a simple circuit that allows for two Keypads to be connected to one LX200.

I have become somewhat less sanguine about the computer/keypad connection with the LX200. Never-the-less this circuit works fine, but I do not take any responsibility for its application. Do any tampering with the keypad(s) at your own risk.

The basic communication between the keypad and the telescope take place through one of the four wires that are in the interconnecting cable. The cable has a ground line, a power line and two signal lines. One of these lines carries information from the computer to the keypad. The other carries commands from the keypad to the computer.

To find these lines, observe the keypad from the end with the keys to the top. The connector plug is a modular plug with four connector wires inside. They will be at the top with the keyway at the bottom. The socket wires are from left to right: power for the keypad, output signal from the keypad to the computer, input information to the keypad from the computer and on the right the ground connection.

The input to the keypad can be connected in parallel to another keypad without difficulty. Both keypads will display the information the computer is sending.

The output from the key pad comes from a Hex Schmitt chip (SN74LS14N) which has a positive 5 volt output which will deliver at least 0.8 ma in the high (1) state. It sinks to ground with a high current capability. The command signals generated by the keypad are 9 bit strings of high pulses with high being +5 volts and low being ground. Thus it is not possible to simply connect the two keypad signals in parallel. One output will short out the other.

A simple diode adder circuit will solve this problem. The two keypad output signals should each have a diode in series with the anode connected to the keypad output line. The cathodes of the diodes are then connected together and go to the input line to the main computer. A 10K resistor should be connected from the cathode side of the diodes to ground. Small general purpose signal diodes can be used. The length of cable between the adder circuit and the LX200 should be short.

This circuit is a simple diode adder. Whichever keypad sends a signal, the adder will pass the positive pulses to the computer. I have tried this circuit and found it to work. I assume no responsibility for it working nor any liability for malfunction of the circuit.

Visit Doc G's website for more of his research. Should open a new browser window over this one.

Subject: LX200 Classic Hand Controller or Keypad Replacement Source

From: Mark <astronomyqteaser.com> Date: Sep 2004

Try Scopeheads Telescope Warehouse as he usually has a few for sale there:
   <http://stores.ebay.com/Telescope-Warehouse/> He has blems for parts and full working controllers