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. One of them, an SN74LS14N, (Hex Inverting Schmitt Trigger), has one output and one input connected to the cable. The other two wires in the cable connect ground and positive 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 it, because it has no manufacturer identification. The parallel words from the chip are probably converted to serial with the 74HCT166 and sent to the computer in the base. It looks like the return serial signal is converted to parallel and sent to the display. The keyboard is a sort of dumb controller with the main computing done in the computer in the base ot the telescope. There is a tiny computer-like chip in the keypad that drives the display.
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. (10 kHz 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 100010101, East sends 101011101, North sends 110001101 and South sends 100110101. 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. 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 is relatively dumb, doing no more than sending the key strokes and running the details of setting 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. I do not see why a longer cable could not be used except for the voltage drop in the cable. i.e. The poor ground it causes.
Note that the commands to the computer are made up entirely of the 19 codes generated by the keypad. The great variety of actions are accessed by sequences of key presses. Each key has functions which depend upon previous key presses and the particular mode that the computer is in at the moment. Thus just a few keys can control many functions. This is not very efficient since some times simple and often used functions have to be accessed through 3 or 4 key presses. There are in addition some key sequences that are not documented in the Meade instructions. These are described briefly at the end of this discussion.
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. 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. The designers have not been very careful about grounding in the electronics to the case.
The ground drop may be of concern since it subtracts from the response voltage window of the Schmitt circuit in the SN74LS14N. I think the concern about ground voltage drops is real. On the other hand, I do not see a problem with extending the cable with a wire that provides a good ground. I think the SN74LS14N can drive 100 feet or more of cable with a capacitance of 15 pf. per foot all right. It has a fanout of 10. The specifications for the chip indicate that it should be able to drive several thousand picofarads.
I am a bit 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 since it is not provided with any heat sink at all. This chip is the major source of heat in the keypad. All the warmth is right 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. I think the entire design of the LX200 electronics was not looked at from a synergistic viewpoint. The several separate parts do not quite fit together in some cases. (my personal opinion) (If I had a circuit with grounds not at the same voltage, I would find out why, damn fast.)
In summary, I see no reason to get too concerned about the keypad. It works well. The only problem which might arise with wear is the functioning of the buttons. They do seem to have a reasonably 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 that the observer however)
I have heard rumors of reports of keypad failure do to a long cord. 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. But I would not expect damage to the keypad. Is it possible that damage with a long cable, as rumored, was do to improper design such as 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 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 used. The shield and drain wire in a long cable should be adequate.
Undocumented key presses are accessed from the blank mode. When in blank mode, pressing enter results in "Set Brightness" which refers to the brightness of the keypad itself. When 7 and 2 are presses in order a menu is available as follows: 1 Learn, 2. Update, 3. Erase, 4. User 21600, 5. Motor Test 0, 6. Demo. The purpose of these functions seems to be some sort of testing of the telescope. This much is known. The motor test runs both motors a bit, then runs the altitude motor, bypassing the electronic stops. With this you can slew the scope into the base on purpose. Pressing the N, S, E or W keys cancels the motor test function and stops the motors. The Demo function is for an auto tour. The time between slews can be set and when it times out the scope slews to the next predefined object in the general area and so forth. It is not advised to try to use these functions.
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