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Technical Note on Plugs and Cables for the LX200

General comments are first and specific cable connection schemes at the end of this note.

Special note about grounds in the LX200. There is a 0.1 ohm resistor between the return line on the power connector and the internal circuit ground. The 232 ground pin is connected to the internal ground. Thus it is not at the same potential as the return line (negative) on the power plug. This can cause a problem if the LX200 and a computer are connected to the same battery. The common power ground connection between the two will form a ground loop with the 232 ground connection. The potential difference is probably only about 0.05 volts but its existence should be recognized.

Many of the control signals on the LX200 are carried by telephone type connectors known as modular plugs/sockets. These are usually 4, 6 or 8 pin plugs/sockets. These signal cables can be extended to at least 100 feet and often much longer. I have tested cable lengths up to 150 feet for the following cables. All 232 Serial cables including the computer to LX200 cable, the serial cables to the various imagers. The keypad cable requires special attention given below.

Some of these cables are available in complete form from Radio Shack or Newark Electronic supply houses. Cables over 25 feet in length will have to be custom made since none are generally available. A standard handset coiled cord will work for the keypad. This is a four wire cable with 4 pin Modular plugs at each end. The cable is connected as a normal, straight through cable.

So called modular jack extension wires have a 6 pin modular connector at each end but are generally only a 4 wire cable. These will work for any of the purely serial communications ports but will not carry ground and power signals since these signals are on pins 1 and 6 which are not connected or do not exist in these four wire cables.

Custom cables can be made to any length by purchasing bulk 4 or 6 wire flat modular telephone cable and attaching plugs to each end. The cable and plugs are available from Newark electronic supply or possibly other suppliers that stock telephone supplies. In order to attach the connectors, a special crimping tool is needed. There is a Telco tool that will crimp both 4 and 6 pin connectors which should be considered. It is plastic and thus for light duty use. But it is only $18. With the proper crimping tool the connectors are very easy to apply. Without, it is all but impossible.

It is possible to put the connectors onto the wire in two orientations. A right way and a wrong way. You must get this correct. When the cable is a standard cable, the color coded wires will go from left to right at one connector and in the opposite direction on the other when the connectors are looked at side by side pointing in the same direction. Viewed another way, the connectors and wires go straight through with no twists when laid out straight.

NOTE: the cable that goes from the imager to the CCD socket on the LX200 IS NOT a standard cable. DO NOT substitute for the coiled cable supplied. A special cable of greater length can be made but do not attempt this unless you are sure of what you are doing. If a new cable is made, the connections must be EXACTLY like the coiled cord cable supplied for the purpose. This requires a cable with a twist and one pin removed at each end.

The modular plugs/sockets are made to carry low currents and like all plugs should be worked from time to time to keep down oxidation on the contacts. Regular use, plugging and unplugging, should not cause problems and will generally be good for the connectors. Signal circuits are often called "dry" circuits because they carry little current. Even slightly oxidized contacts can cause problems in "dry" circuits. Thus, if these plugs are seldom used they should be worked from time to time to keep them clean. Standard contact or switch cleaner can be used but excess should be wiped away.

Recently there have been complaints about declination drive intermittent electrical problems. Many do not realize that there is a second plug on the inside of the fork where the declination cable plugs in. This plug should also be worked from time to time as well and cleaned. The declination cable carries motor power, ground and DC supply power as well as the two signals from the encoder of the motor shaft. Interruption of any of these signals will cause a dead or runaway declination drive depending upon the type of fault.

The keypad cable requires special consideration. Several persons have reported replacing the cable with up to 150 feet of standard flat telephone cable with good success. Others have reported failure and burnout of the keypad electronics with cables of such great length. Meade advises to not extend the cable great lengths. I have been successful with a cable of 100 feet length. However, there is some concern that the ground wire connecting the keypad to the main control panel will have too much drop in it due to the current drawn by the keypad. I have measured the voltage drop carefully and found that there might be a problem with cables much longer than 50 feet. Thus, if you need to make a keypad cable longer than 50 feet, I recommend you use an additional ground wire, which might be the shield and drain wire in the case of a shielded cable. The small gauge wire in typical telephone cable might not provide an adequate ground connection.

I hope this primer on cables will help solve some cable/connector problems for LX200 owners.

Specific cable connection schemes for the 232 cable

There are a number of confusing diagrams and very confusing designations for cable connections given in the LX200 operation manuals. Specifically, on page 29 of the manual (page 28 in the older manuals) there is a diagram of the connector socket for the CCD connector on the LX200 panel. This connector is shown with the key way to the bottom and the connection wires to the top. This is in agreement with its orientation on the LX200 control panel. However, notice that the connector wires (pins) are numbered in a unique manner. They are numbered from the left to the right as 1, 4, 2, 5, 3 and 6. This is contrary to what would make common good sense. i.e. 1, 2, 3, 4, 5 and 6. The table is correct for the numbering used by Meade on their socket diagram.

On page 85 or 86 of the manual they have another drawing of what presumes to be the socket on the LX200. This diagram is inconsistent with the diagram on the earlier page. The upper drawing shows the key way at the top and the pin numbers at the bottom. This diagram is wrong! The lower drawing shows the pins at the top, which is correct if the key way (not shown) were on the bottom (as it is mounted on the LX 200 control panel). In the lower drawing, the pins are numbered 1, 4, 2, 5, 3 and 6 which is consistent with the earlier numbering, even though peculiar) The table given is correct for the pins as they are numbered in the lower drawing. Note that the Figure is called Fig. 61 in the text. It is actually Fig. 29.

To summarize the correct 232 connections: Looking at the socket with the key way to the bottom and the pins (contact wires) at the top, the second wire from the left is the ground wire and goes to pin 5 on a DB-9 connector and to pin 7 on a DB-25 connector. The fourth pin from the left is the data transmit pin from the LX200 and the data receive pin to the PC. It is connected to pin 2 on the DB-9 connector and to pin 3 on the DB-25 connector. The fifth pin from the left is the data receive pin on the LX200 and the data transmit pin from the PC. It is connected to pin 3 on the DB-9 connector and to pin 2 on the DB-25 connector.

No other pins should be or need to be connected. It is especially important not to connect the first pin from the left to anything since this is the positive hot supply bus of the LX200.

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