LX200 Grounding Issues

       

Subject: Grounding the LX200 and Computer  

From: Doc G

<cfryeix.netcom.com> wrote:

> Doc G. said to make sure that the telescope and the computer are separately

> grounded. That's real good advice but how does one do that. Both the Meade

> 120VAC/18VDC power supply and the laptop computer power supply only have

> 2 prong plugs, not three. How do I ground a two prong plug to the copper

> rods I have driven into the ground?

Unfortunately many DC power supplies like those you get at Radio Shack for $10 or the one from Meade for $70 do not have a correct 117 volt power plug. They are cheap, generally imported units, and they are not designed for outdoor use. I doubt if they are even legal for outdoor use. Certainly not approved. You will always find a disclaimer that says something like "do not use outside." The SBIG power supply DOES have a three prong power plug.

This becomes a bit silly since if you normally use the scope outside. And on dark often damp nights. So what to do?

Here is what I do. NOTE: This is not a suggestion and I am not responsible for anything whatever that happens electrically to the scope or operator.

I always use a 117 volt socket that has a Ground Fault Interrupter (GFI). You can get these on power strips and on extension cords. I ground accessories to the telescope and to the tripod with a bit of wire and a ground lug or clip. I also plug all 117 volt things, LX power supply, computer and ST imager supply into the same power strip.

I do not pound a rod into the ground. That would only be necessary if you were viewing during a thunder storm and wanted protection from lightening.

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More from Doc G:  

The flared plug is a substitute for the three prong plug, more or less. The idea is that the plug can only be put into a socket one way. This keeps the hot side and the neutral side of the power line in correct order. But, and it is a big but, This only works if the sockets you plug into have the hot and neutral to the correct pins. Most modern homes and buildings that are competently wired have this correct. Now the neutral wire, the white wire, is neutral and not really ground. The green wire is true ground and is grounded to the metal parts of the socket and to the round pin that is ground on a three prong plug.

The neutral is connected to ground at the fuse box in a house or building. Under normal conditions the neutral is close to ground in potential. But under short circuit conditions the neutral line which is shorted to the hot line jumps up in voltage to half the line voltage. Thus it is under the stress conditions of a short circuit that you get a half-hot neutral line. The green ground on the other hand is supposed to protect the user from things like this. If the socket or plug is wired wrong of course, the neutral is actually hot and vice versa.

So, in summary, the safest power lines, cables, extensions etc. are those with three wires and a well grounded green wire.

Subject: Electrical Grouding Between Scope & Camera Question --part 1 of 2 

From: Roger Hamlett <ttelmahntlworld.com> Date: Nov 2003

> I use to power my LX200 Classic from a 12V car battery (without
> the 12-18V converter, it's an 8") and my laptop computer from another
> 12V car battery (with a converter). The scope and laptop are connected
> via the RS232 line.
>
> I also have a SBIG ST7 camera powered from the battery to power the
> scope. And the camera is connected to the scope for autoguiding and
> to the laptop via the parallel line.
>
> So far nothing has fried. But considering the scary electrical design
> of the LX reported in the Archives, I am having doubt about my
> connections. I would be very grateful for your comment about it.

The power requirement of the LX200, is 12 to 18v. A 12v battery, is OK, provided it is well charged, and the connections are good, but there is little 'margin'. If the battery voltage starts to fall, the motors can lack the torque to actually reach their target, stall and overheat. This gets worse with the larger scopes, where the loads are greater. So, _provided_ your battery is well charged, and the scope is kept well balanced, there should not be a problem.

The second difficulty with the LX200, is that it's 'chassis ground' connection (which also connects to the RS232), is connected to the top of the 'current sense' resistor. Hence if your laptop, and the scope were on the same battery, and your laptop was not using a converter (most of these provide isolation), you could end up with the scope current being drawn through the RS232 ground. You look to be OK here, since you have both a separate battery, and a converter module. The same 'ground' signal, is also present on the 'guide' port, with the same potential problem. It sounds as if what you have is 'perfect'. The ST camera, and the laptop, share the same 'ground', and are powered from one battery, while the scope is on the other. If you put a voltmeter between the -ve terminals of the two batteries, you would find a small voltage between them, which would vary as the scope draws more/less power. Keeping the scope on a separate battery, ensures that this does not cause a problem.

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

> As long as you keep your two car batteries connected in
> parallel i.e. positive to positive, negative to negative you have
> nothing to fear. The converter to your laptop will supply the
> laptop needs; and any voltages that occur on the port pins will
> be correct and referenced to the "same "ground as is every other
> device you are using. Even if you isolate the batteries there is
> little chance of a 'cookout' however strapping the two batteries
> together forces all to have the same ground reference. And if you
> were to power your LX200 from the mains converter you could still
> tie your (one) battery ground to the scope/fork ground as this would
> ensure a perfect ground reference. Just remember that the negative
> terminal of your battery is your ground reference.
> --Doug

NO!... Do _not_ do this. You risk damaging the equipment.

The 'key' is to understand that the LX200, does not use battery negative as it's 'ground' reference. It has a sense resistor (used to feed the display showing the current drawn), between the -ve connection, and the 'ground' connection on the RS232 plug, and the CCD plug. If you join the battery -ve connections, and attach an un-isolated device to one of these ports, you risk the power being drawn through this connection, rather than through the power connection. This is how you can blow the RS232 port, or the guide system...

This is the big 'LX200 caveat,' when wiring.

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

> If I understand correctly, I should not connect the Pictor 201
> Autoguider and the 16" LX200 to the same 12V battery because I risk
> blowing the LX200 Guider port or the Pictor, right?
>
> Is there another way to solve this problem apart from carrying around
> two 15kg car batteries?

Yes. You are OK, if the laptop has an isolated converter driving it, and if the camera is using a relay box or opto coupler. It is a really annoying 'feature' of the LX200, which is why there have been so many posts about it in the past.

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

Subject: Electrical Grouding Between Scope & Camera Question --part 2 of 2 

From: Doc G

----- Original Message -----
From: Bostjan <ac0email.si>
> Thank you so much Doc and Roger for your info.
> I am now starting to realize what has probably burned my 16" LX200 main
> board _FOUR_ times...
> Let's suppose I short the 0.1 ohm sensing resistor as you mention.
> Will I then be able to connect the LX200 and the rest of the equipment
> (Pictor 201 etc.) to the _SAME_ 12V car battery without the risk of
> burning something?
>
> The reason you get half the current indication is that you are partly
> shorting out the current sensing resistor. That is the bad part of this
> whole circuit problem. Some users have simply done away with the current
> meter by replacing the 0.1 ohm sensing resistor on the circuit board with a
> short circuit.
> This connects the battery plug ground directly to the true
> circuit ground which is the metal part of the base. The internal ground
> for the electronic circuits board is established through the screw that
> holds the board in place. It is the one going through the voltage
> regulator chip.
> The latter connection method is also very bad circuit design tactic.
> This is indeed a crummy design and has caused many problems over the years.
> I hope they have done better with the GPS version.
> The LX200 "classic" is dead, long live the "classic."

That is certainly a good solution to grounding problems. You will loose the current indicator, but it is generally useless anyway. It will indicate a gross overload due to mechanical jamming of the mount. In the case of such an overload, the fuse should protect the scope. The current indicator is also used during a dynamic balancing operation suggested by Meade. That balancing technique is also a very bad idea, in my opinion. It greatly stresses the drives. Simple two position static balancing is adequate.

It is quite astonishing that the original design was issued. I have designed electronic equipment for over 50 years and such a design has never been considered proper in the electronics industry.

Subject: LX200 Grounding --part 1 of 10 

From: Doc G

Ed Fitzgerald wrote:

> My questions have to do with proper grounding procedures for the LX200 and
> accessories. The MAPUG topical archives treat this briefly, with the
> following suggestions:
> a) Plug everything into the same power strip.

Good advice!

> b) Use ground fault isolation receptacles.

Absolutely good advice!! Called GFIs.

> c) Use clip leads to ground the scope and tripod to true AC ground.

Nice if you can do it. A good site should offer this capability.
More comments from Doc G ---First , let me say that I am very sorry that you asked these questions. That is because I am now going to get into trouble with all faithful MAPUGers and especially the devoted inner circle. These are my opinions based on 50 years of electrical and electronic design and industrial consulting. Please do not take them personally.
The Meade electrical and electronic design is at best non-standard, primitive and incorrect. The LX seems to be a collection of outmoded designs and parts and has been modified, sort of fixed and otherwise kluged into working (sort of) over the years. It is due for a major electrical and electronic revision. That said, I will try to answer some specific questions.

> From my perspective, I don't think this subject has been adequately
> addressed. I have the following questions:
> 1) What physical parts of the LX200 is true ground.

Ah yes "true ground" is elusive in the LX. The main body of the telescope might be expected to be "true ground" but it is not unless the DC power cable is plugged in. Otherwise the body of the LX floats. Neat Eh? The grounding of the body of the LX seems to be through the battery connector plug which has three terminals and shorts two of them together when the plug is inserted.
Now a word about this power plug. It is an abomination. It is the most poorly designed cheap and crummy plug ever invented since the ubiquitous RCA jack used on audio equipment. It is a loosely fitting sliding contact which is prone to being intermittent. BAD PLUG!! Never the less, like the RCA audio plug it is everywhere since it is cheap, cheap, cheap!!
I have thought about replacing this plug with a "real plug that had a solid ground for years but it is not easy to do this since the control panel is firmly pressed together and glued in the finest tradition of electronic design.

> 2) Is this ground the same as the 18-volt return line?

So the answer is sometimes. The body of the LX is connected to the 18v DC
return (ground/negative) line when the power plug is in place and happens to make good connection.

> 3) Is this ground the same as Pin #4 on the 6-wire RS232 modular connector?

This ground, which? Actually yes, the ground pin in the 232 connector is the circuit ground within the control panel. The 232 signal probably goes directly to the computer board where there is a 232 send/receive chip. This connection is through the short flat cable going from the control panel to the computer board.

By the way, notice that this short cable has a makeshift shield around it. It is quite amazing that such a shield should be necessary. Computers have very long cables inside without shields. This is indicative of an after thought because the thing did not work for some unknown reason and was "fixed" with the shield.

> 4) Can I access a convenient grounding point on the scope with clip leads?

There simply is no convenient grounding point. You have to add a "body" ground by adding a tapped hole and ground lug or clamp. This can be easily done on the control box chassis. All of the "body parts" of the LX are connected together via the bearings and so forth. Notice that all of the several connector "ground" parts on the control panel are in fact insulated from the panel except through the circuitry. Thus these should not be used as places to connect external grounds.

> 5) Why does the tripod have to be grounded?

I see no reason to ground the tripod if the body of the LX is grounded properly. In any case it will probably be connected to the body of the LX through the big bolts holding the LX onto the tripod.

> 6) Do I need to have a separate ground connection for my laptop? Its AC
> adapter (like the Meade) doesn't have polarized AC prongs (where one prong
> is wider than the other).

When the DC adapter does not have a ground pin and is not polarized, there will not and must not be a connection from the ac side to the DC side. There may be and likely will be some ac leakage current however. Thus it is good practice to ground the DC output side of the adapters together. This is normally done through the connecting cables such as the 232 cable.
However, note that cables like the 232 cable are not designed to be hot plugged. Only special cables which are designed for hot plugging should be hot plugged. The 232 cables and any RJ type connectors and also the crummy battery cables are not to be hot plugged.

Often there is a considerable leakage current or a surge of current when adapters are plugged in. This surge may be strong enough to destroy a 232 chip which is not fully protected. None of this equipment should be hot plugged.
Static electricity is also a serious problem. Even after a person "discharges" himself by touching a ground on the equipment, just shifting position can cause a renewed potential difference that can cause serious static discharge damage.

> 7) For many field trips, true grounded AC receptacles are hard to come by at
> good observing sites. What is a good grounding setup for these cases?

Well, I would say that grounded and ground fault protected plugs ARE available at a GOOD observing site. The Madison Astronomical Society has a dark site with many outlets at pads and ALL are grounded and ground fault protected. A site that does not have such outlets is not a good observing site even if it has good skies. ;-o
Now if you really want to find more problems, just start to add CCD imagers, and all kinds of other electrical toys to the LX and you have a REAL grounding nightmare. Most of this equipment is of marginal electrical design in my opinion. But some, like the SBIG imagers actually have a true and continuous ground. Amazing that this stuff works at all!

--------------------------------
Subject: LX200 Grounding --part 2  

From: David W. Bonnell <Bonnellnist.gov>

I will throw in my two cents worth here, noting the Doc has really covered the ground well (as always), but a couple of concepts may still be unclear to some.
(1) All DC grounds should meet in one place. A good choice, when using multiple devices that are interconnected is to do as Doc suggests, tap a hole in grounded "body metal" somewhere, put in a screw, and make sure a wire from each device's "ground" potential is attached here (or nearby "electrically"). Note that you will need to ascertain just where to get that ground from some devices. An inexpensive multimeter from Radio Shack is a good way to explore for possible problems. Do remember that many devices are deliberately designed not to have grounded cases (for static isolation/protection), and you need to be careful just how you arrange to "ground" them. If a "continuity" test (with power supplies off) shows a low resistance connection to your central DC ground, you may be OK, at least as regards power ground loops. Note that for almost all DC equipment, it is NOT necessary for that ground to be "earthed." The DC voltages involved are all too low to be dangerous, and "earthing" a DC ground can be a marvelous way to pour current into the ground. Using the AC "ground" is a really bad idea --

(2) AC "ground" should NOT be mixed with DC ground. It is very common to have several volts of "hum" on the neutral wire, and sometimes even the green ground, if there is one, whether from a generator (e.g., battery-powered inverter) or even good mains wiring. This hum is not often good for DC devices using 5 volt signals. If you need a good AC ground (and don't have one), a couple of feet of rebar, sharpened at one end, and driven into the ground can be a good ground. Put a hose clamp or something similar at the top (out of the ground) end, be sure to wire brush or sand rust... off the rebar metal (and the hose clamp!), and use the clamp to attach a substantial (at least 16-18 gauge, better is 10 or 12 gauge) wire (can be bare copper, or whatever - in a pinch, heavy fence wire or something will be better than nothing), and run it to a ground pin on your AC supply (could be the case, or look at any receptacle - the center screw hole is supposed to be ground!). Then, your ground fault receptacles will not only protect against current carrying grounds, but will insure those grounds are earthed. It is possible for ungrounded GFI gear to suggest no fault, until YOU make the ground. If you don't have GFIs, and let the power supplies float, you are asking for it. Any AC-using device should either have a ground wire, or be totally isolated (greater than 20 MOhms) from any of the AC Pins (except, perhaps the green ground pin). Floating AC supplies are a pain, but do generally have a polarized plug. Be sure your AC source has the wide pin as neutral! and things should be fine. Doc's comments that ungrounded AC supplies are OK ungrounded is generally good advice for commercial hardware.

If you are using multiple AC sources (ugh!, bad idea), be extra careful, and be sure that they are either completely isolated, with the grounds joined. It's also a good idea to at least be sure that the neutrals are both near ground potential! It is not uncommon for different AC receptacles to be miswired interchanging hot and neutral. A real opportunity for serving smoked equipment.

Sidebar: It is generally good practice NOT to run DC, AC, and signal wires near each other. Often the biggest mistake made in setting up an installation (whether observatory, or a big night at a favorite site) is to conveniently bundle all wiring together in parallel runs. In general, having cables fanned out (the usual haphazard way they fall is good), and AC as separated from DC and signal wires as possible will avoid many "glitches." Trying to compensate for nearby AC is possibly one of the reasons for extra shielding on external computer wiring.

(3) Where Doc talks about "hot-plugging" he is referring to making or breaking a connection with power applied. Bad idea for anything you are not SURE is designed to allow it. Plug in everything, then apply power. If a power source fails, better to turn everything off, make the power source change, and then repower everything. A pain, but cheaper and quicker in the long run than sending fried electronics back to Meade or whoever!. If you are not SURE your laptop or whatever will last on internal batteries, hook it to the main power at the beginning. Interestingly, much general electronics today is designed to survive hot plugging (for instance, the old saw about unplugging the laser printer I/O port cable without turning off power is almost never a problem where everything is plugged into good house or business wiring). Hobby (which includes a lot of Astro-stuff) devices are often of a lower standard, and will die easily. Even good commercial stuff can be hurt by the vagaries of outdoor wiring. Thus, Doc's advice to avoid "hot-plugging" is really good advice, regardless of how you are set up. Those who don't need this advice will know what I mean.

--------------------------------
Subject: LX200 Grounding --part 3  

From: Ed Fitzgerald <fitzgeraldmsn.com>

Doc - Thanks a lot for the detailed and pertinent input. I couldn't test ground points on the scope (since it is being repaired) but I tested my laptop. I found that there is a screw hole for some aux connector in the back which is at the same ground as Pin 5 on the DB-9 serial COM port connector. The hole is one of two on either connector side that you'd normally run screws into to hold the connector in place. I could run a grounding strap from this screw lug to an AC ground point on the power strip. If I also ground the LX200 and then tie a serial cable between it and the laptop, do I end up with a ground loop? Also, I found in the MAPUG topical archive that the 18-volt DC power return is tied to ground through a 0,1-ohm resistor (so they can measure current draw). Hence, this point is not a true scope ground. My gut feel tells me that it's best to ground everything I can.

Mark - The AC adapter for the laptop is essentially the same as the one for the LX200, except that it puts out 15 VDC instead of 18 VDC. In neither case are the AC prongs polarized. I thought that the transformers would provide isolation, but this evidently isn't strictly true - especially for in-rush current when you plug something in. If you think about the transformer-coupled DC output of the adapter, the question is "What sets the absolute voltage of the two DC lines, +18 V and DC return?" I think this has something to do with capacitance between the transformer primary and secondary for turn-on transients and then leakage resistance from the primary to secondary - at least in lieu of any other constraint on these voltages.

I think my solution at this point is as follows:

1. Plug everything into one power strip with GFI receptacles.
2. Take the battery out of the laptop and always use AC power.
3. Connect all serial cabling before applying power.
4. Connect a special grounding strap from the computer to AC ground on the power strip.
5. Connect a special grounding strap from the LX200 to AC ground on the power strip.

The remaining question is getting a good ground point on the LX200

--------------------------------
Subject: LX200 Grounding --part 4  

From: Doc G

I have just checked, by taking the thing apart and measuring it, the above comment about a 0.1 ohm resistor in the ground return lead from the internal circuit ground and the return lead to the external power supply. It is also the case that the reticule ground is returned to the internal circuit ground and the 232 plug ground is also returned to the internal ground.
Thus, the return lead to the power supply which is the negative side of the supply is slightly below in potential than the internal circuit board voltage. This potential depends on the current drawn at the moment. For a current of 1/2 amp it would be 0.05 volts.
This means that the return lead to the power supply is not at the same potential as the ground lead pin at the 232 port. This means that the ground on the computer should not be connected to the ground on the LX external power supply. I have normally run my LX and my computer from the same battery with a common ground and not had trouble. But I should be shorting out the 0.1 ohm resistor by doing this. Probably the external wiring resistance is larger than the 0.1 ohm resistor. There is really no reason to measure the current this way since it could be measured in other better ways that do not screw up the ground system.
This is strange and unconventional design in my opinion. I would say it is odd and stupid design. I am really baffled by such design. I no longer feel confident in making predictions about the LX electrical design. I think at this point, I will resign from saying anything further at all about the LX electrical system.

--------------------------------
Subject: LX200 Grounding --part 5

From: Bill Ezell <wjesii-nh.com>

This isn't quite true; check out the front panel power schematic on my astro page. The negative (or sleeve) of the power connector is NOT ground. There is a (small value) resistor between it and chassis ground. This is used to provide the current-sense for the little LED current meter on the front panel. Consequently, if you short the DC connector 'ground' to chassis ground (which is also the circuitry electrical ground), you'll lose the current meter output. This isn't any big deal, and it won't hurt anything.
   However, if you start plugging in accessories, aren't using a scope supply that doesn't have an isolated DC - (which the brick does), then you can potentially set up a situation where the ground voltage levels can differ by several hundred millivolts between things. Again, this isn't a disaster but it can certainly introduce electrical noise.
   In my setup, which is permanent, I use a DC supply for the scope whose outputs are isolated from any other ground, and use the scope frame as system ground. All of the ground pins in the various external connectors are (more or less) at this level, since the main board in the scope does eventually tie to the scope base casting.

--------------------------------
Subject: LX200 Grounding --part 6  

From: Doc G

Ron Wagner wrote:
> I presume this is more about ground loops then grounding? I don't suppose
> that an AC isolation transformer on the equipment would help?

You make a distinction which should be considered. The discussion has been mainly about the local ground, ground loops and the like among the various power bricks and equipments. These grounds are for the sake of reducing noise in the system and insuring that the various equipments can communicate among themselves at digital the level.

The secondary but very important issues has been grounding equipment to a true earth ground. -- nominally, the neutral of the ac power line is grounded at the main fuse box. A third wire has been added to improve the ground to the true earth ground. Grounding to a true earth ground is mainly a matter of safety. An isolation transformer can be used to interrupt the ac ground on the output side of the transformer from the house ground. I such a case, the output side of the ac system will have a neutral and a hot side which is nominally determined by the equipment connected to it. Such transformers are usually used to eliminate noise and power system spikes from getting to the equipment.

So, one should pay attention to both sorts of grounding and isolation. The various suggestions made in the bast day have been good ones. Do both. Good grounds among components of the system and a single good ground to the true earth ground if possible.

--------------------------------
Subject: Further Thoughts on Grounding --part 7  

From: Bill Ezell <wjesii-nh.com>

> My gut feel is that I should ground the laptop and let the serial cable
> carry the laptop's ground over to the LX200 Obviously, one would have to
> be very careful to connect the serial cable first before powering up
> anything. I would appreciate comments on this approach. I fried my serial
> chip once, and I'd like to avoid doing this again.

If you're using an isolated supply for the scope (the brick that came with it), then you can use the scope frame for ground. This is electrical ground as far as the scope electronics are concerned. If you're NOT using an isolated supply, then don't. You can see a simple schematic of the LX200 front panel power supply layout in the astronomy part of my web page--
   <http://www.duckpond.mv.com/> Note: should open a new browser window over this one.

Normally, the RS232 common pin is tied to electronics ground, so you really don't need to separately ground the LX200 anyway. The brick is isolated, so unless you get it wet there's no path from the AC supply to the scope. (it's isolated by a transformer that's part of the supply)

--------------------------------
Subject: Further Thoughts on Grounding --part 8

From: Ric L Ecker <rleckerjuno.com>

Ed Fitzgerald" writes:
>I have fabricated a GFI receptacle box and separate grounding wires so >that
>I can ground my laptop and the LX200 to AC ground at the receptacle >box.

You don't need to ground your Laptop because it's already has an isolation xformer built into the computer. It is basically still seeing battery power and/or power supply power which is isolated. When ever hooking up the LX200, Laptop or anything that's a computer, always have the power off and better off unplugged. If you forget to install or want to install a serial connector or dec. motor cord always power off to do so. GFI will only work on plugs that have a ground connector. If you plan on making your own ground you must be very careful and get everything polarized to prevent problems. On the GFI outlet the round plug will be the ground while the small slot is the Hot side of the 115 vac and the larger slot is the neutral. Most plugs (two prong) we have today are polarized so they go in one way. This plug will not work with the GFI unless you modify the plug which is defeating the isolation already built into the electrical device, as the transformer for the LX200. Remember that the LX200, Laptops, CCDs all run on battery power or (basically) a battery eliminator (transformers). Your home PC has a good ground and a GFI is designed to work with this type of product.

--------------------------------
Subject: Further Thoughts on Grounding --part 9  

From: Ed Fitzgerald <fitzgeraldmsn.com>

Ric --Your point about only changing or setting up cables while un-powered is a good one. Even if you do this, there may still be a problem (like the one that fried my RS232 chip) depending on the sequence in which you power things up. Both my LX200 and Toshiba laptop are powered from "bricks" with un-polarized AC plugs. Where this puts DC ground in (say) Pin 5 of the DB-9 serial connector depends on leakage resistance between the transformer primary and secondary in the brick. During power turn-on, this ground reference may be initially determined by stray capacitance in the transformer.

As a test, I plugged my laptop (two-prong un-polarized plug) into my power strip and then measured the AC voltage between the true ground in the power strip and the Pin 5 ground on the DB-9 connector at the back of the laptop. I got a reading of over 50 VAC. A similar test on the LX200 (using the ground pin of the RJ-25 RS232 connector) showed a much smaller voltage. This voltage difference can cause a momentary current surge through the serial cable during power-up until the voltages equalize.

My present approach is to ground the laptop to power strip ground using a separate ground cable and then let this ground be carried over the serial cable to the LX200. If I use my Pictor 201XT autoguider, it is powered by a separate brick and can get the ground reference from the CCD front panel connector on the LX200, which in turn is set via the serial cable and laptop ground. In this way, I have one ground reference point.

--------------------------------
Subject: Further Thoughts on Grounding --part 10 of 10  

From: Ric L Ecker <rleckerjuno.com>

Ed Fitzgerald writes:
>Actually, I measured a resistance of about 1 ohm between the LX200
>front panel screws (i.e., case) and ground as seen coming out of the RS232
>connector, so case is close to ground, but there isn't a dead short
>between them.

I think folks here are confusing earth ground to neg. dc voltage or common which are different and somehow some are making it the same, they are not in this case. Earth ground is in reference to what??? Earth ground is a very archaic reference point considering the makeup of soil and the aggregate of the soil, moisture, salts and the like. Grounding can be a very serious problem depending on what part of the country you come from and what type of ground system you are talking about. A rod driven 6 feet into Terra Firma is not necessary a "good" ground. A proper ground screed cover several thousand sq. yds. is sometime considered a poor ground depending on the soil and age of the ground. What type of instrument would one use to consider a ground to have a resistance of 1 ohm in reference to the panel screw of the LX200. If grounding is a problem like the instance of the ruin serial port of the LX200 I would consider the use of better voltage transformer (brick) that had no leakage current. Most are so well isolated that leakage current seems strange to me and must be an isolated case. A scope that sits on rubber feet does not need to be grounded and if you have leakage current coming from your (brick) just replace it, I'll bet the problem goes away.
P.S. That's why it has an Isolation transformer in the (brick) power supply.

Subject: RS232 Grounding Problem?   

From: Roger Hamlett <ttelmahntlworld.com> Date: Oct 2002

----- Original Message -----
From: Paul Goelz wrote:
>I use TheSky with CCDSoft V5.00.070 to control my camera and LX200. For
>some time, I have been having intermittent problems getting the telescope to
>move in one or more directions using the direction buttons on the focus
>panel of CCDSoft. Usually the problem will go away when I disconnect the
>RS232 wire running from the computer to the scope, massage the ends, and
>plug them back in.

>This sounds like a hardware problem to me. Do I need to spend a ridiculous
>amount of money buying a new telephone-type cord from Meade or one of its
>suppliers, or is this cable available at Radio Shack?

> Before you go too far, make sure you do not have a ground loop or potential
> difference between the two ends of the RS232 connection. I have seen the
> LX200 do some strange things in the face of less than perfect grounds or
> leakage currents.

Good point. In the case of the LX200, it is not 'less than perfect grounds,' that are the problem, but the fact that the internal ground of the scope, is offset from the incoming supply ground, through the current sense resistor (silly design...). Hence if you have a 'perfect' ground on the RS232, and no isolation, the system will attempt to ground the supply, through the RS232 connection. Ouch...

Provided you use something like a laptop, on it's own _isolated_ power supply, then the small offset voltage can be dealt with by the isolation provided by this, but if instead you switch (say) to using battery supplies, with no isolation provided, this becomes a problem.