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  #1  
Old Wed 11 July 2007, 15:30
Aubrey
Just call me: Aubrey
 
Jhb
South Africa
Lower Cost Electronics?

Hi Guys,
Congratulations on a lively forum!
Working in a CNC shop, my problem is not the mechanicals but the electronics
Gerald suggests the PMDX-122 breakout board and the Geko G-Rex G100 drivers.)
These cost US$81.00 and US$399.00 (times 3) for a total of US$1278.00 plus postage etc.
Converted to local currency (using ZAR7.00 to the US$) this becomes ZAR8946.00 before postage and taxes.
I have already had bad experiences with "store bought" driver boards that I cannot get working properly, the latest being a Velleman P8000 which I need to get my injection molder to work (18 months and counting but thats another story for another time )
Does anyone have a tried and tested setup they can propose that can be built by an electronics philistine like myself without having to raise a second mortgage.
I am based on the West Rand of Johannesburg, South Africa and am not adverse to a little "bartering" (my mechanical skills for your electronics expertise)
Anyone interested?
Best
Aubrey
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  #2  
Old Wed 11 July 2007, 19:44
tpworks
Just call me: Tom
 
Atlanta, GA
United States of America
Aubrey
G100 DESCRIPTION:
The G100 is a 6-axis motion controller. It uses USB or Ethernet communication with a host PC to execute motion control and I/O commands. The axis outputs use a Step/Direction interface to motor drives. Each axis has an associated quadrature encoder input. The G100 also has 22 general purpose inputs, 16 general purpose outputs, 4 analog to digital inputs and 4 digital to analog outputs. Each digital input and output has an LED indicator

therefore, 1 G-Rex @ $399 4) G201 @ $114 1)PMDX-122 breakout board $81
total = $822

Tom
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  #3  
Old Wed 11 July 2007, 20:30
tpworks
Just call me: Tom
 
Atlanta, GA
United States of America
in the previous post drop the g-rex100 and your cost will be $537.00 because you don't need both. Either G-Rex100 or pmdx122.
Tom
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  #4  
Old Thu 12 July 2007, 00:42
Gerald D
Just call me: Gerald (retired)
 
Cape Town
South Africa
Welcome Aubrey.

The GRex100 is not a driver. Even as a "controller" it is an over-kill for a MechMate. The PMDX122 is doing the work of the "controller". (not strictly true - they are apples and oranges)

You need a PMDX122 and 4 drivers type Gecko201. I suggest these because they are a tried and trusted setup, and there is support & experience in your country.
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  #5  
Old Thu 12 July 2007, 13:39
Aubrey
Just call me: Aubrey
 
Jhb
South Africa
Thanks for feedback

Gentlemen!
Thanks for the quick responses.
If nothing else, my post proves my point about being a below average novice as far as electronics is concerned.
Tom.
It seems the G100 is the limousine and the PMDX122 is the VW Beetle - both will get you there but one has more bells and whistles.
Either gets the job done and both are driven by a program on my computer.
Now the G201 is fed by the G100/PMDX122 using 2 bits (step and direction) and the stepper motor is plugged into this. Unfortunately, 3 axis = 3 x G201 units.
Right so far.
Quite a reduction in cost but still above my "hobby" pocket.
Gerald.
Thanks for the "Welcome".
Why do I need 4 Gecko201 units?

Question - Does the G100/PMDX122 receive actual GCode from the computer and do the conversion to step/direction for each G201/stepper motor or is it sent a small number of bytes for each "step" where the individual motor step/direction is contained in a bit pair. I suspect the latter is true.

If this is the case then the G100/PMDX122 is therefore a simple communications router (allbeit with some necessary extras) that simply takes the info received from the computer port and sends the appropriate 2 bits to each G201.

The G201 then takes over and figures out the next sequence of windings that need to be energised and what polarity to give them to step the motor in the direction indicated by the "direction" bit value received. This is then sent to the stepper using high power switching circuitry ( I believe I've seen figures of 80 volts at 20 amps quoted in one of the threads ) which in turn is translated into movement.

Is this (VERY BASICALLY) what is going on?

Best
Aubrey
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  #6  
Old Thu 12 July 2007, 14:37
Gerald D
Just call me: Gerald (retired)
 
Cape Town
South Africa
Very basically, you have it right! Repeat: very basically....

The x-axis has two motors, one at either end of the long gantry, that's what holds it square to the table. 4 motors need 4 drivers.

80V, 7 amp max for the G201's - we only use about half, or less, of those amps.

The PMDX is actually quite dumb - the signals on the 25 wires of the printer cable are relayed directly to the G201's, with the PMDX only doing some "signal conditioning". Some people even manage to do completely without a PMDX or GRex, but they spend a lot of time chasing smoking gremlins.
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  #7  
Old Thu 12 July 2007, 15:06
Richards
Just call me: Mike
 
South Jordan, UT
United States of America
Aubrey,
You're in for a wild ride! Building do-it-yourself controllers using off-the-shelf parts is not hard at all. Gerald has documented his projects to the point that even someone like me can build a controller.

The one major part that you left out is Mach 3 software. That software generates all of the control signals, including the step and direction signals to actually run the stepper motors. It reads the G-code file and does all of the necessary computer stuff so that the machine makes the proper movements.

I've been a long-time tester of various Gecko and PMDX products. I've found that Gerald's recommendations are both safe and sound. You won't go wrong by using the parts and pieces that he recommends. As he said, the G100 is overkill and is not needed. (Personally, I really like the G100 and the G101/G102 that I have, but at this point, there is no way to use most of the functionality built into those devices. The PMDX-122 that I have does everything that a normal CNC router would ever require.)
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  #8  
Old Thu 12 July 2007, 15:19
Aubrey
Just call me: Aubrey
 
Jhb
South Africa
Thanks Gerald
I did have a quick look at the plans while they were printing last night and now that you mention it, I remember the dual motor setup --- Sorry!
I intend doing a bit more studying later tonight.
Now things are starting to make sense at last!
I know that a computers parallel port is quite easily "blown" so it makes sense that one of the main "other" functions of the interface would be to protect the port.
A few weeks ago we had a techie in to repair one of our Guilermeister CNC lathes which had a problem with one of its cards. His "breakout board" had a parallel port connection and a whole bunch of IC's which he told me were "optical isolators" and a bunch of pins to which he connected wires onto the suspect card. The whole contraption was built on veroboard.
Anyway, he made a bunch of connections between the suspect card and this thing, powered up the card, ran some app on the laptop and BANG! Smoke etc.
My immediate thought was "Dell sells another laptop" but this guy wasnt phased. He plugged in another board, ran the same app and the thing lit up like a xmas tree. He then plugged the card into the machine and within a minute or two we were in production again.
Seems like the veroboard/optical isolator thing may be able to replace the
PMDX-122 (US$81.00) or G100 drivers (US$399.00) requirement.

Now all we have to do is find a less expensive alternative for the four G201 (US$114.00 each) parts and my budget may just cover things.

Thanks so far for your patience.
Best
Aubrey
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  #9  
Old Thu 12 July 2007, 17:05
Aubrey
Just call me: Aubrey
 
Jhb
South Africa
General Note/appology/explanation

I've just read this whole thread again and if I was someone else, I would think that I was an idiot. I'm griping about prices all the way.
I'm just recovering from a year and a half of unemployment. (The last thing you want to be is white, male and over 50 in the land of affirmative action! Last word on that subject and no, I'm not a raving conservative either!)

I want to build my own CNC machine so that I can make frames for my wife's paintings.

No micron accuracy - I have enough of that at work! Half a millimeter will do me fine.
Speed? So it takes 4 or 5 hours to mill a frame to take an A4 painting - SO WHAT! As long as it does it.
Stepper Motors/Computer - Got them.
Materials to build the machine (and it WILL be a MechMate) - I've got a good boss AND I'm good friends with the management of the scrap company that collects our shavings. You would be surprised how they appreciate you keeping the steel, stainless, aluminium and titanium seperated for them.
Component Machining - I have my own 500mm lathe with a milling head. As mentioned before, I have a good boss and HE has a 3 axis Victor (amongst others) with a 2 METER BED. Please visualise NICELY MACHINED RAILS!
Bearings - Have a nice collection in the garage (which is 81 square meters and has a 3 phase supply)
All that is holding me back are the parts between the stepper motors and the computer. If I had that, I wouldnt be typing this, I'd be making and assembling parts.
Now I am very lucky in that I can probably construct the mechanical side of the mill for the price of taking the family to MacDonalds a time or two.

Why have the people who have already constructed/are busy constructing or are looking at constructing a MechMate actually choosen the MechMate?
1) They are well designed and the plans are superbly presented especially considering that they are "open source".
2) They can be constructed without access to very expensive equipment (which I fortunately have).
3) The community have figured out all sorts of ways to do things in a cost effective way. You only have to look at some of the threads about guide rail preparation to see the enthusiasm and ingenuity which this machine is generating. Absolutely Phenominal!!
4) Business owners are building them for thier workshops. I'll tell you a secret! You dont build a successfull production business if you cannot see the advantages of the MechMate design over one made of rough plumbing hardware.

Now lets be fair.
The MechMate does not have the design or operational specifications of the commercially available machinery out there. BUT it does not have the price tag either!
If you follow the instructions and build it correctly, it will give you the same finished product but it will do so more slowly. It cannot take a 25mm cut at 100 meters per minute. You will have to take smaller cuts at a lower feed rate but you will produce the same end product. In addition, you will be able to pin the "I Built This" medal on your chest and save a whole cartload of money into the bargain.

So why can the same savings not be made as far as designing and producing a good design which will be adequate for the job as far as the electronic control circuitry is concerned.

There must be a number of forum members that have enough electronics knowledge to be able to put a design together that does an adequate job safely that does not cost an arm and a leg.

To sum up.
The MechMate can produce the same quality as a commercial machine at a fraction of the cost. What we now need is drive circuitry that can produce the same stepper motor impulses as the commercially available units also at a fraction of the cost.

Thanks for reading, I'm now putting my soap box away and am hitting the sack.
Best
Aubrey
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  #10  
Old Thu 12 July 2007, 17:46
Richards
Just call me: Mike
 
South Jordan, UT
United States of America
Aubrey,
There is inexpensive and then there is junk. The Gecko/PMDX/Mach 3 route is very high quality and inexpensive as far as electronics go. I very much doubt that you could buy anything that is adequate for the job for any less - but you could spend many times what the Gecko/PMDX/Mach 3 parts cost and not get any added value.

It's kind of like the rollers on the MechMate. You COULD go down to the local hardware store and buy some nylon rollers intended for light weight sliding screen doors that would work - for about one minute before being squashed and ruined. The same goes for electronics. Everything that Gerald has listed is really about as basic as it gets. You do want reliability don't you? There is no joy in constantly fixing broken/burned out parts. What Gerald has listed works. That's really all that there is to say about it. Everything that he specifies is necessary if you want a good, working, reliable machine. You can cut as many corners as you want, but you'll end up buying the suggested parts anyway. Why not save yourself some grief and do it right the first time?
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  #11  
Old Fri 13 July 2007, 00:26
Gerald D
Just call me: Gerald (retired)
 
Cape Town
South Africa
Mike, I have a lot of sympathy and understanding for what Aubrey is saying. We have the MechMate broken down into base components that can be sourced in practically any corner of the world, except for the breakout board, drivers and motors. In your case, you can source them in your country - in our case there is the eternal quest of how to do it locally.

To get a small article (like a PMDX or Gecko) out here with reliable delivery, we have to use a courier and not the postal service. Add $30 minimum. If that device is perhaps faulty, add $60 minimum (and 3 weeks) to send it for repair and return.

It becomes very appealing to try and build-your-own with locally stocked $5 chips. For a guy who has old steppers lying around, you couldn't do too much damage with a wonky driver.
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  #12  
Old Fri 13 July 2007, 06:22
Richards
Just call me: Mike
 
South Jordan, UT
United States of America
Gerald,
I'm probably too fixed in my ways sometimes. Building electronic circuits has been a large part of my life. There were many years when I did almost nothing except design stepper drivers and the associated interfaces that went with them. When cost is the main factor, I don't know how anyone could beat the price of the PMDX-122 or the Gecko G202. Over here, just having a run of prototype boards run usually costs around $750 - just for the board fabrication. Most of the time, it takes at least two tries to get everything working properly. Add to that the time and frustration of sourcing all of the parts and you end up wishing for simpler times - before electronics.

Someone could build his own breakout board using optocouplers and wirewrap techniques. In that case he should probably use the G203 stepper driver so that he wouldn't have to find optocouplers that could source 15mA per circuit. It would still be a pain, but it would be possible. But I wouldn't try building a stepper driver.
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  #13  
Old Fri 13 July 2007, 08:54
Gerald D
Just call me: Gerald (retired)
 
Cape Town
South Africa
How about building something around the Allegro A3979, or SGS Thomson L6208N as examples of potential low-cost DIY drives?
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  #14  
Old Fri 13 July 2007, 09:59
Richards
Just call me: Mike
 
South Jordan, UT
United States of America
Here's a one-chip circuit that will drive the Gecko G203 stepper drivers. It is not elegant, but it will 'probably' save the computer in the event of a problem.

You would have to map the step and direction signals with Mach 3. The G203 uses a common ground on pin 10. You would also have to furnish +5VDC from the computer to run the 74LS245 chip.

Because the G203 uses opto-isolation for the step and direction inputs, they would be automatically isolated from the computer. The 74LS245 chip buffers and amplifies the signals from the parallel port to the G203.
Attached Images
File Type: jpg bob (600 x 453).jpg (21.5 KB, 1025 views)
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  #15  
Old Fri 13 July 2007, 10:24
Gerald D
Just call me: Gerald (retired)
 
Cape Town
South Africa
Thanks for that Mike. Another good tip I've heard to save the computer, is to use an expansion card with the parallel port, and not the parallel port that is on the motherboard.
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  #16  
Old Fri 13 July 2007, 20:37
tpworks
Just call me: Tom
 
Atlanta, GA
United States of America
If you are handy with a soldering iron and your motors are 3 amps or less you could save a few dollars here.http://www.bright.net/~agarb/STMD/Purchase.html
I assembled 3 for my mini mill.

Also here is an informative site for electronics. http://pminmo.com/
I am using his BOB I purchased the bare board and ordered the components from mouser electronics. draw back is no charge pump.

Tom
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  #17  
Old Sat 14 July 2007, 01:10
Gerald D
Just call me: Gerald (retired)
 
Cape Town
South Africa
We have built a "charge pump" for an earlier BOB - the basic components for a charge pump are under $1. Seriously.

Here is the circuit we got from Les Newell:

charge pump.gif

Okay, maybe more than a dollar, but not much more
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  #18  
Old Sat 14 July 2007, 08:55
Marc Shlaes
Just call me: Marc
 
Cleveland, OH
United States of America
Send a message via Skype™ to Marc Shlaes
Maybe this is obvious somewhere in the forum... I have looked. What is a charge pump? What does it do?
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  #19  
Old Sat 14 July 2007, 10:26
Gerald D
Just call me: Gerald (retired)
 
Cape Town
South Africa
Marc, this very question was asked at the Geckodrive forum 2 weeks ago. Before I copy some of the replies across here, let me have a stab at answering.....

Have you ever left a printer switched on while booting or re-booting a PC? You know those random little noises and moves the printer makes while the PC sorts itself out? Well, your CNC machine, driven by the parallel port, is also going to make some random moves (maybe even switch the router on) if the PC is re-booted. The "charge pump" is a safety device which prevents the CNC from doing anything until Mach3 "pumps" a known signal. The signal charges a (leaky) capacitor. When the capactior is fully charged, it says everything is okay to run. If Mach3 is not in control and not pumping the signal, the capacitor leaks down and the system stops.

Here are some other explanations:



By Jim Connerton:
It is a signal that only presents itself after Mach is fully booted up and in control of the printer port. It's main reason for implementation, was to overcome extraneous signals on the printer port from causing the motors from moving or twitching. As windows boots up it goes and talks to the printer port. So a break out board that has the charge pump safety function, will disable those undesirable printer port pulses at start up, and will only activate the motor drives and IO after it receives the charge pump signal (I think it is a 12K pulse stream) from Mach, in essence Mach is saying I am in control, it is OK to allow motor and IO functions. Something like that. It is a funny name, it does sort of charge up a sensing circuit on the BOB, and Mach is pumping it out? That's the best I can do.
Jim Wellman:
A "Charge Pump" is the term used with respect to Mach software to describe a mechanism for implementing a safety lockout of undesired motion when the Mach software is not in control of the parallel printer port.
Sometimes this function is also known as a watchdog, although the exact implementation may differ slightly.
The Charge Pump mechanism is desirable because the system connected to the parallel printer port may respond to signals generated by the normal power-up sequence of the computer, or because some application other than Mach may try to run the printer port. This can have undesirable or even dangerous effects if the CNC machine moves or the spindle powers on unexpectedly.
The Charge Pump mechanism works by requiring that a constant pulse stream be present on one of the output pins in order to enable functions attached to the other pins. Typically the power up sequence or an application trying to print will only toggle an output once or twice rather than presenting a constant stream of pulses. By waiting until a constant stream of pulses on the Charge Pump output is seen by the interface, the interface will prevent or lock out undesired actions. The Charge Pump mechanism is available as one of the functions on many Breakout Boards.
Examples of charge pump circuits that drive a relay were provided for use by Mach users by Mariss Freimanis of Geckodrive Inc. and a PDF document describing them can be downloaded here:
http://www.artofcnc.ca/ChargePumpSafety.pdf George you can find alot of information www.artsoft.com goto the forum pulldown and wiki it has a vast amount of information covering many topics. Hope this helps.


Bob Campbell:
When Mach has control it sends out a stream of pulses (about 10 kHz). In
the case of our breakout board we use what I call a one shot IC (flip flop)
that keeps getting reset as long as the pulses keep coming in. You can
think of this as a charge on a capacitor.

Once the stream of pulses stops the flip flop gets reset and turns off the
charge pump signal on our breakout board.

On our breakout board we use the charge pump signal for several purposes.
First of all we inhibit all outputs until the charge pump signal is preset.
We also inhibit and pulses going to the stepper motor drivers. I call this
an anti chatter feature.

In my enclosure I use the effect of the charge pump to turn on my power
supply only when Mach has control.

The charge pump is a great safety feature.


Steve Wille Padnos:
OK, you've had the Mach-specific answers (which also work for other
software, such as EMC2), now for the Electrical Engineer explanation

It's called a charge pump because you're pumping a small amount of
electrical charge into the circuit with every pulse. This goes along
with the analogy of a capacitor as a bucket of charge. A charge pump
circuit in this application is a leaky bucket. Each pulse from the PC
puts in a small amount of charge, but there's an intentional constant
leak. The detector looks for the voltage to be above some level, which
it will only be if you stick in enough charge pulses to keep up with the
discharge rate of the capacitor (the leak is the resistor that forms the
RC time constant with our "bucket" - the capacitor).

I wonder if that helps or hurts your understanding?



Enough?

The referenced document by Mariss does not show the 1kohm resistor (R1) that less shows in his circuit - that resistor is seriously needed if one doesn't want interference from a VFD to become a valid charge signal. The first PMDX122's did not have that resistor, and a bad experience here in Cape Town led to the PMDX now being modified. See this thread in the archive. One of the reasons I promote PMDX - Steve listens and reacts.
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  #20  
Old Sat 14 July 2007, 14:52
Marc Shlaes
Just call me: Marc
 
Cleveland, OH
United States of America
Send a message via Skype™ to Marc Shlaes
Makes perfect sense. Thanks tons. So, simply put, it is a safety interlock that prevents any signal transmission to the machine until Mach is ready to be in control.

Again, thanks.
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  #21  
Old Sat 14 July 2007, 15:21
Gerald D
Just call me: Gerald (retired)
 
Cape Town
South Africa
Ask the French about that L6208 chip for CNC:

http://www.otocoup.com/CarteL6208_e.htm, superceded by:
http://cncloisirs.com/Construction/Cartes%c0R%e9aliser

Whether it does everything the MechMate needs, I don't know for sure, but 2.4 Amp, 50V, microstepping, sound promising.
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  #22  
Old Sat 14 July 2007, 15:45
Gerald D
Just call me: Gerald (retired)
 
Cape Town
South Africa
Another approach:

http://www.cnczone.com/forums/showthread.php?t=24699

And there are sure to be lots more.

What is interesting for me is the dates on the data sheets - we seem to be talking of chips that have become available relatively recently. After the Geckos hit the market.
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  #23  
Old Sat 14 July 2007, 16:56
Aubrey
Just call me: Aubrey
 
Jhb
South Africa
Megapost

Hi guys

NOTE ABOUT THIS POST:
Due to some bottleneck in the internet, the lines out of South Africa are VERY slow. So slow that the MechMate site times out when I try to post to the forum but I am able to load the forum (every 15 to 20 reload tries.) This is incredibly frustrating! Anyway, I am able to read the thread and instead of doing individual posts (that fail) I am preparing all my posts into one MEGAPOST which I will submit once I am able to.

Mike (post 10), I thing that Gerald (post 11) has eloquently echoed my point.
Gerald, we are from the same neck of the woods so you know exactly what I have been experiencing.
This is the THIRD time I am going to start building a CNC mill.
The first 2 got abandoned mainly due to the high cost of the electronic section.
This time I intend getting the electronics FIRST. That way I wont be getting myself all worked up only to hit a very expensive brick wall.

And if that means that I have to start becoming proficient (in a very amateurish way ) in electronics, then so be it!

Mike (post 12) - AT LAST! - Someone with knowledge!
Speaking for myself, if the first interface/driver unit works properly (even if it has speed limitations), I'll be a very happy camper.
As to the "prototype costs", why bother with a printed circuit board for the first unit? If you get it right first time then you have made a mistake! What’s wrong with "Veroboard"? Once all the bugs have been sorted out, then worry about a smart looking board.
Hell, maybe I can resurrect my second attempt at a CNC mill, which was a pcb router.

Mike (post 14) - Now we are stepping in the right direction..........
I had a look and the 74LS245 goes for about US$0.39 (ZAR2.75)
Add ZAR5.00 of veroboard, a parallel connector (rob an old printer), 6 pcb screw connectors (maybe ZARR4.00 each), an old cell phone charger as a power supply and an old plastic lunch box as a casing and you have yourself a half decent start to a breakout board. Cost? ZAR31.75 (US$4.53)

I dare say that most people who are knowledgeable in electronics are either laughing hysterically or are counting the number of good practice rules that we have broken so far (and at this stage it only has one actual electronic component) but for me this is great news.

Gerald (post 15) – The auxiliary parallel cards are getting harder to get.

Tom (post 16) – I’ve looked at both sites and have actually downloaded the entire pminmo site to my hard drive. The number of circuits at pminmo that all do the same thing (I think) is astounding. Then there are “pin compatibility issues” and “jumper settings” and all sorts of other strange things that are as clear as mud to me.
Also, the US$10.00 tag turns to 40 or 50 when it hits your doorstep (IF it hits your doorstep). I’ve been waiting a year and a half for a robotics book. Got summonsed by the local revenue authorities and had to pay the taxes but the book has disappeared.

Gerald & Marc (posts 17 to 20) – Always wondered why a charge pump had to be there somewhere. Often asked myself “Why don’t they just use the same voltage as everything else?”
Short Answer: “Its used as a safety device to prevent the Break Out Board passing signals to the rest of the circuit before it is actually supposed to.”

Next I see that it is used to control the Estop relay. (Emergency Stop I believe)
The Estop relay is further upstream than where I am right now so I will have to come back to it at the appropriate time if that is ok with everyone.

Progress so far:
Post 14 gave us a one chip connection to handle the signal from the pc.
If we use the other 2 bits to handle the spindle and extractor relay functions, we have (VERY BASICALLY) got the minimum control requirements.
Post 17 gives us a charge pump which is vital as a safety device.

Question: Who can connect the 2 together so that they work as a team?
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  #24  
Old Sat 14 July 2007, 21:18
Richards
Just call me: Mike
 
South Jordan, UT
United States of America
Aubrey,
You could add the charge pump to the 74LS245 by disconnecting pin 19 from GROUND and then connecting the output of the charge pump (signal line on the darlington transistor's collector) to pin #19 on the 74LS245. That is the /output enable line, which must be pulled LOW in order for the 74LS245 to work. When the /output enable line is HIGH, the chip is in the high zener or off condition.

Pins 2 and 3 of the 74LS245 are available to control two other devices. In the schematic, I have them tied to Vcc so that they are always in a stable state. You could connect them to two of the five control lines on the parallel port to control the router/spindle and vacuum. If you used Solid State Relays to control the router/spindle and vacuum, you could connect the + side of the SSR's control circuit to +5Vdc and the - side of the SSR's control circuit to the 74LS245's pin 18 (output of the #2 input pin) or pin 17 (output of the #3 pin). Then, when the input pin 2 or 3 went LOW, the SSR would turn on. When the input pin 2 or 3 went HIGH, the SSR would turn off. The SSR is already opto-coupled, so no addition isolation would be required.

You could also use the two unused pins of the Parallel Port's data bus to control pins 2 and 3, but that would require the software to mask individual bits - which Mach 3 surely does already, but it's just an extra complication that is not necessary if the control signals are used instead of the data bus signals.

Keep in mind, however, that just because something is possible doesn't mean that it should be done. I'm sure if you search threads that discuss SSRs you'll notice that they can cause problems. If you use standard relays instead of SSRs, then you'll have to build an amplifier circuit and an isolation circuit to control the higher current requirements of the standard relay. You'll also have to deal with the electrical noise generated by the standard relay when its contacts open and close.

With this type of simple circuit, wiring mistakes can be very costly. It is not unusual to blow out the parallel port. If the parallel port is directly connected to the mother board, that means that the mother board could also be destroyed.

That's the main reason that I buy instead of build whenever possible. I understand that getting off-the-shelf parts might be hard or even impossible, but building your own can be an exercise in futility. Believe me, I've built thousands of dollars (US) of prototype circuits that didn't work. Some of them were spectacular failures, others were just fizzles - but they all took substantial time to design and significant cost to prototype.

Good luck.

-Mike

Edited:
A simplier method of connecting the charge pump to the 74LS245 would be to connect the collector of the Darlington Transistor directly to pin 19 of the 74LS245 . If you do that, you wouldn't need the reverse biased diode shown in the charge pump circuit. (DO NOT connect the collector to Vcc and to pin 19 at the same time, just connect the collector to pin 19 and to nothing else - in this configuration the transistor would be SINKING current supplied by the 74LS245.) Also note that I'm assuming that the charge pump circuit turns the transistor ON when the charge pump is working (I'm at a customer's site where I'm working without my glasses and I can't magnify the charge pump circuit enough to see the components clearly). If my assumption is false, and the charge pump works by turning the transistor OFF, then you would have to invert the output signal by using another transistor or by using a TTL device like the 74LS04 to invert the signal. In any case, inverting the signal would cost an addition $0.50 at most.

Last edited by Richards; Sat 14 July 2007 at 21:36..
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  #25  
Old Sun 15 July 2007, 01:41
Gerald D
Just call me: Gerald (retired)
 
Cape Town
South Africa
Aubrey, re slow internet connections out of SA to this forum.... I use both MWeb and Telkom on ADSL connections. They both slow down your international browsing when you have reached your bandwidth limit for the month. I havn't had problems with either in the last 24 hours (well below my bandwidth limit). Speak to your ISP, or check if your neighbour/colleague has the same problem.

Has anyone else had an access speed problem with this forum? (I often battle with the CNC zone forum, but I put it down to all the advertising they carry, or their very large number of users and graphics)
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  #26  
Old Sun 15 July 2007, 08:08
Aubrey
Just call me: Aubrey
 
Jhb
South Africa
Quote:
Originally Posted by Gerald D View Post
Aubrey, re slow internet connections out of SA to this forum.... I use both MWeb and Telkom on ADSL connections.
Are they both set up on the same ADSL router?
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  #27  
Old Sun 15 July 2007, 09:36
Gerald D
Just call me: Gerald (retired)
 
Cape Town
South Africa
Yes, on a Telkom ADSL router (at home). The MWeb helpline told me how to do it. The router defaults to Telkom, but I can manually click on Network Connections > MWeb, to connect the MWeb account instead.

My old home account was always MWeb, my work account Telkom, but both of them allow you multiple logins from up to 3 different routers. Maybe the speed suffers when somebody else is using the same acount?
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  #28  
Old Sat 21 July 2007, 07:02
Aubrey
Just call me: Aubrey
 
Jhb
South Africa
Belated Post

Hi People
Net Connection problem took a week to resolve - just "came right"
Mike:
Thanks for your patience.... appreciated!
I've been building up an image of the circuit and although the aim is to save costs as far as possible, there are places where safety overrides the cost reduction. This applies to the "safety" of the computer as well as to the human.
Here is the "image" so far:

Question: What do I connect the "from computer" (as indicated by the red ??) to?
Best
Aubrey
Good Grief!!!
I previewed this post - the image is awfull!!!
I'll have to ddo a bit of work on it.
A.
Attached Images
File Type: jpg cnc_controler_circuit.JPG (14.8 KB, 979 views)
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  #29  
Old Sat 21 July 2007, 12:26
Richards
Just call me: Mike
 
South Jordan, UT
United States of America
Aubrey,

Here are a few suggestions of things that I would change if I were building that particular circuit:

The 'From Computer' signal can be connected to Parallel Port pins 14, 16 OR 17 (assuming that you're using Mach 3 as the controller software).

Diode D3 on the 'charge pump' circuit needs to be removed. It is used only in other type circuits.

Pin 1 on the MCT2E opto-isolator chips should be connected to 5V. Pin 2 on the MCT2E opto-isolator should be connected to the 74LS245 chip through the 470 ohm resistors. The 74 series of TTL chips can sink about 5mA of current, but only source about 0.5mA of current, which is not enough to turn on the L.E.D. inside the MCT2E. Changing the pins as recommended will allow the 74LS245 to pull enough current through the MCT2E to make things work.

The 4.7K resistors on the emitters of the 2N2222 transistors will limit the current to 1mA, probably not enough to turn on the coil of the relays. Normally, the coil has enough resistance to be used without a resistor. The emitter of the MCT2E can normally be connected directly to the base of the 2N2222 to form a Darlington circuit. You might have to 'play' with those components a little. The trick is to turn the 2N2222 fully on, but not have so much current flowing through the base that the 2N2222 latches on. Sometimes it's necessary to use a transistor with less 'beta' or gain when it is used in a Darlington circuit.
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  #30  
Old Sat 21 July 2007, 15:53
Aubrey
Just call me: Aubrey
 
Jhb
South Africa
Stage 2 Done

Hi Guys
Thanks to Mike the charge pump circuit he donated has been inserted into the controler (and corrections made to the opto-isolator connections on the auxiliary switches)
The image of the circuit that I uploaded to the forum was a total mess - sorry about that. I think I put too much compression on the jpg file.

The updated circuit and my next plea for help can be found at http://ocallaghan.co.za/cnc_images/. There are 2 identical files, controlerdev.jpg (330k) and controlerdev.bmp (1.7M). I put the bmp there in case the jpg is lousy.

I was thinking about maybe using a PIC to do the logic. Reason: It would probably be easier to alter the output characteristics (ramp, decay etc) programatically than by changing a whole bunch of components on the board. Comments please....

When I was getting the specs for my motors I noticed thay they are rated at a lower voltage than some discussed on the forum. I seem to remember a figure of 80 volts from somewhere. Are my steppers to weak for the MechMate? My alternatives are Siemens units off a Guildermeister lathe (physical size about 200 x 200 x 450 mm or so). I think that these MAY just be an overkill

Anyway, have a good weekend.

Best
Aubrey

Last edited by Aubrey; Sat 21 July 2007 at 15:57..
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