Open source PLC hardware

In reply to Gilles Allard and Ken E: With a wiki, you don't have a "secretary". That goes against the whole concept of it. You would have an "administrator" for the wiki, but that would just be whoever controls the server. If it was hosted on ControlWiki, then the administrator would be whoever it is that controls that server. The administrator would hand out passwords, but have no further involvement.

One of the direct participants would tell the administrator who to add or who to remove, and the administrator would just ignore everyone else. Since Ken E seems to be the direct participant with the most interest in this, I think he should be the one in charge of that. As for me, I'm just a bystander who is offering a bit of advice that can be taken or ignored as you wish.

Those people who have passwords would then just edit the contents of the wiki however they wanted to. The idea of a wiki is that you have a group of people who trust each other not to vandalize the contents. Then you just go in and write something and say to the rest of the people "what do you think about this"? Someone else could write an alternate version and that could be debated.

I used this process with several other people to develop a communications protocol. If all you do is send disjointed e-mails back and forth, then things can get very confusing. Instead, you just use the wiki to write what you mean, and let the others edit it or write alternative passages. You can then use e-mails to ask things like "do you agree with section 4 now?". It's rather like having a meeting where you are drawing on a blackboard. Things change continuously, but everyone is working from the same version. It sounds chaotic, but it works quite well in practise.

Another way of doing this is Google Docs. That isn't a wiki, but it does allow collaborative editing of documents. If you're really interested in that, just go and sign up (it's free) and start doing it.

The way the Free/Open Source Software world works is that while you do talk about things, it's the people who come up with the actual code (or drawings) that get the final say on things. It's a lot easier to debate the merits of a design once there is something actually down on paper to talk about. A lot of problems only show up once you start to design the details. So, Curt draws something and says "this is what I think", and Ken draws something else and says "this shows why my idea is better". And since I didn't come up with anything, you can pretty much ignore what I said if you don't agree with it. If the peanut gallery doesn't like the way things are being run, then they're welcome to take what is there and go out and come up with their own design. There's no vendor lock-in, so time will tell in the end who was right.

You can think of it as the ultimate free market in ideas versus the sort of top down political control that prevails in monopolised markets. You compete and cooperate simultaneously, but the best ideas are supposed to win on their own merits rather than through market manipulation and legal trickery.

So, what I think you are looking for is:

1) Collaborative document editing - either a wiki or Google Docs. You probably only need this during the period of the most intensive development though.

2) Some place to host files such as drawings, etc. while you are working on them. You want to make sure that you are using the most recent version as opposed to hoping that what you found in your mailbox is up to date.

3) You're going to need someplace that people can eventually download the drawings and documentation *from* when you are done.

4) You are going to need a web site to promote the project. That would be a page that explains what the project is and what it offers people. If someone takes one of your designs and does a Google search for it, they should be able to come up with that web site.

5) You will need some means where people whom you have never heard of can contact you if they have questions or problems.

6) All of the above should be free, as I don't think there is a budget for this.

The above is how a software project is operated. However, I don't see why a hardware design project couldn't operate the same way. In the end, it's all just digital files. There are lots of free code hosting services, but Sourceforge is the only one that I know of that also provides a web site.
 
C
OK, I have a complete, except for capacitors, backplane. And I'm thinking of starting over. Why, you might ask? I took a perfunctory look at standard enclosures and boxes to make an enclosure. The results were not promising. I need an enclosure about 14x4x3. I am hoping to find something OTS. The closest I've found is a Hammond 1441-20 which is 17x4x3. That would be pretty long for a 8 slot chassis. So, I mused on shrinking the backplane to 1" card spacing rather than 1.25". Then I could use a 12x4x3. But, I turned up nothing close for that either. Ideas? The local place that does forming is not real interested. And they hinted that I couldn't afford it, which isn't that surprising. Next to consider would be to have the local HVAC shop make something. But something available to all would be much better. I even thought of using steel channel used for steel framing, but, again that's not OTS. Has anybody seen anything that might work?

Regards
cww
 
W

William Sturm

I can host files for the time being, it is not a problem at all, but a Wiki site or maybe SourceForge would be a better long range plan. The controlWiki has been mentioned, is that appropriate? Can SF be used for hardware designs?

Ken, I also thought of optocouplers. There are many applications that have no need for them, but some OVP might be a nice idea. Curt has series resistors, some zeners or Transzorbs might be a good idea for extra reliability. Maybe we will eventually need boards with and without optocouplers. Optical coupling doesn't affect the rest of the design at this time, it just adds cost. For larger installations with multiple power sources, optocouplers may become mandatory.

Bill Sturm
 
W

William Sturm

The enclosure is an issue, I do not know of any ideas that are truly a perfect fit as of yet.

I was thinking of two concepts: One is a Hammond box with card guides, such as a #1590 or #1591, http://www.hammondmfg.com/1591icep.htm. These are not large enough for 8 slots probably, but they would be a reasonable start. We could also mount the backplane in a snaptrack, it could be any length but our widths will be defined for us.

Another idea would be to use Snaptracks or Phoenix card holders to hold the individual boards and a ribbon cable bus, this would of course require a parallel bus and address decoding on the boards.

We should agree on something before we go too far...

Bill Sturm
 
C
Well, that is a constructive suggestion. And the I/O boards that I did years ago and published here did have opto isolation. But many current PLCs do not, so I wouldn't get too uneasy about it, I would say it is the exception rather than the rule. It is also handy to use optoisolators that have back to back LEDs inside because you can connect the inputs as either sink or source. But there are a couple of things that shade against isolators for this first board. One is the space available for connectors, in my view isolated inputs that share a common are not isolated. So when I do that card, it will have to have 32 terminals or less inputs. The second is the speed factor. Photo Darlington couples turn on fairly fast but there is no place for the stored charge to go, so they are pretty slow, and worse, somewhat variable, in their turnoff characteristics. I want to characterize the system the first go around, so I am using the bare inputs with dividers. And no 15mSec filters. This is a "High Speed" card. And there is isolation between the bulk power and the chip power. The local 78L05 regulator provides a great deal of isolation between the two. That's the three terminals above the expander. The common grounds are a fact of life. I don't believe in floating grounds for industrial electronics with mixed wiring. If this expander proves suitable for continued development, I'll do an isolated version. I want to get something going to ground the discussion. In the meantime, if you want to tape up an input card, feel free. Here is what all the pins do on the connector. This is still subject to change.

1 16 +24VDC

2 INT B this is one of the interrupt on change pins, not used yet.

3 A2 Address bit for Microchip MCO23S17 only. Will reflect card slot.

4 A0 ""

5 SI SPI MOSI

6 /CS SPI Chip Select, Active Low.

7 Not committed will probably carry +5 from card for address bits. 8,9 GND, -24VDC
10 NC

11 SCK SPI clock

12 SPI MISO

13 A1 Address bit for Microchip MCO23S17 only. Will reflect card slot.
14 /Reset Active Low.

15 INT A this is one of the interrupt on change pins, not used yet.

We have to be very careful with the numbering as a right angle connector can change left row for right. These are for the RA connector on the _solder_ side of the card as that affords visibility to line up the connector and also makes the bus layout better with no component side jumpers. This is one reason not to cast things in stone until you see how things will fit together a bit. I have a preliminary housing arrangement that will want the 1" card spacing version. Still looking for one that will work with 1.25" spacing as drawn. I'm going to send Bill new pics with the 16 pin connectors on the cards. Had to take time to fire up the lathe and milldrill and make stove parts. A wiki is fine by me, but I'm busy spec'ing parts to finish the layouts.

Regards
cww
 
I was a bit harsh on the Opto isolation, there is probably room for both in a system. Every PLC or remote IO system I've used in the last 15 years has been opto isolated though (unless it was analog). We need to be careful with noise coupling from field devices (conducted (common ground), capacitive, radiated). I don't mind administering a Wiki or Google Docs. I'll look into it tonight.

I too think the chassis/mechanical is far dwarfing the electrical spec, and that electrical/mechanical is an iterative design process. I do think we need to nail down a few things or else there will be a lot of revisions going around. I started a CAD drawing of a front view of a chassis and I think we can easily do 0.75" pitch boards, something like 3" - 3.5" wide. You can get either 16 or 18 conductors in a pluggable "spring cage" connector, even with retention screws (very nice to know your connectors won't vibrate loose). For sheet metal I can mill/shear out prototypes but we will want to quote price in hundreds to make sure the project will make some sort of reasonable budget.

KEJR
 
C
Well, I was racking my brain to think of a rack that wouldn't double the cost or require fabrication and that anyone has access to. I'm still looking, but I came up with something that will work at least to test and maybe in some applications. I was thinking of long narrow chassis and dismissed steel studding as hard to work with, and many have never seen it. Then I thought of wire duct, which we have all seen. I checked things out and we could use the narrow finger wire duct in a 3x3 inch size as a rack with a .5" pitch so it would work with a 1" board spacing. the backplane would mount in the trough and the fingers would support the boards. This could be mounted in a piece of solid side 4x3 or 4x4 duct and the cover could be slit into 1" pieces to make the board panels.

It could be arranged so that the space between the two was at the bottom and provides a trough for the wiring. It would look kind of bush league, but it's relatively cheap, would require little fab and no machinery and would be available anyplace. And it's fire rated, etc., Some one might even make end plates.

Regards
cww
 
C
Hi Ken

I think .75" would be a bit tight for a card with a double stack connector. Those would be needed for either isolated inputs or multiple commons. And it would be nice to have room for some sort of LEDs to show input status. Tell me what you think of the wire duct idea I came up with. It doesn't even need painting. And if anyone has a scrap chunk of 3x3 narrow finger or 4x4 solid wire duct that they can spare.... It comes in 2 meter chunks and they are kinda spendy that way, although you could get 5 enclosures out of each piece. The spec I was using was for the stuff that Automation Direct sells. I think they get it from Hubbel. I would think Panduit and the others are compatible.

Regards
cww
 
Curt:

I haven't looked at double stack connectors because the pluggable versions seem to be kind of expensive (Pluggable TBs in general aren’t' cheap...). Most high density Isolated Input cards I have seen are isolated from other types of devices, not from themselves. 24V inputs usually aren't noisy as they are mainly hooked to hall effect switches these days. In this way having one common per Input card is OK for everything I have done. If someone chooses to have multiple commons this will eat up more pins.

I can't argue with the expense of the wire duct idea. I am concerned about the mechanical strength mainly, and the perceived quality as well. We use some European duct at work that is somewhat delicate without the cover on it, but we might have some bigger "Panduit" style laying around that is more rigid.

I designed a 3 slot backplane chassis about 7-8 years ago that had no press brake bends to it and the cost to have it fabricated by a local sheet metal shop in QTY 25 was under $15. The cost for this system will be more, obviously. I'm used to seeing backplanes and chassis costing anywhere from $100-$300, so this initially doesn't seem out of the ballpark. The old design had a single cover plate that went over all of the boards (This part did have some bends on it), and I think for this system we want to have each IO card be removable without taking a cover off of all the boards. It might not be bad if we could come up with a standard connector cutout and always enforced pluggable connectors. Otherwise a "mass cover" is going to be difficult to use (who wants to remove wiring on board "A" because board "D" failed?).

I can't help but wonder if we could source some VME style extrusions. These have all of the hardware, tapped holes, rigidity, etc. The only thing we would have to do is fabricate some custom end plates, which at a prototype and production standpoint is very very simple. I know VME is prohibitively expensive as a whole system, but the actual extrusions in long lengths should not cost hardly anything. I'd be surprised if 8' of the stuff was more than $15. What I don't know about this idea is as follows:

- Can you get the extrusion?
- Can you get VME/CPCI card guides that are short? (We don't want 6"-7" deep cards!!) Card guides are built into the Euro rack system, so you can't just use any old card guide for short boards.

KEJR
 
For any project that I've been involved in that required custom electronics, the most expensive things were usually the engineering, assembly labour, and the enclosure. The actual "electronics" was usually the cheap part.

I don't think the wire duct idea would be too popular with electricians because the boards would flop around too much. There is nothing to guide the assembly and hold everything securely.

For low volume, plastic is usually more expensive because you can't do much without making a mould. Metal is usually cheaper if you can do everything with a bandsaw and a brake.

The best thing that I can think of is to use an open rack card cage. If you use a eurocard size PC board, you can buy these already assembled off the shelf. You can also buy just the parts. Here's an example:
http://www.boss-enclosures.co.uk/Enclosures/19_Sub_Racks/200_Series_Racks

If you look at that link, you will see some bare racks. The racks consist of sheet metal end plates joined by aluminum extrusion rails. The screws go through the sheet metal and into the ends of the rails. That means you can cut the rails to any length and still add your own sheet metal end plates. The end plates are simple and custom end plates could be bent up on a brake. If you download the catalogue you will see the individual components. No doubt there are plenty of other sources of equivalent parts.

If you use a 25mm board spacing, then 8 boards will fit in 200mm. A standard length of the rail is 432mm. You could cut those in half to get 2 pieces out of each length. That would make each one a half width size for a 19 inch rack (which might be useful to some people). Card guides are also available (you should be able to get these from Vero as well).

If you make your own end plates you can of course make the card height anything you want. However, you would want to have a good look at the card guides if you want to make the depth less than a standard eurocard dimension. These sorts of little mechanical odds and ends are things that people don't want to have to spend a lot of time modifying.

If you know of some good surplus electronics stores, you might be able to get some old racks that you can scavenge parts from to build a prototype.

As an alternative to this, OKW make plastic enclosures that look like shoe-box PLCs. The boards would mount flat with the terminal blocks coming out the top and bottom. This looks very nice, but you are stuck with their enclosure design and probably can't source alternatives easily.
 
W

William Sturm

Curt said: "in my view isolated inputs that share a common are not isolated. So when I do that card, it will have to have 32 terminals or less inputs."

I agree that few PLC's have isolation from channel to channel, like an Opto 22 rack.  But I think most have isolation from the I/O circuitry to the logic circuitry.  I think this is what Ken was referring to.  That style of isolation only requires 2 extra terminals.  Otherwise, how would we use sourcing outputs > 5 volts?  And I am not saying that we absolutely need any isolation, but that it might be a good idea.
 
Bill Sturm
 
Yes, Isolation from the backplane P.S. to the IO power supply is usually what you are after (or at least for me and some colleagues).

KEJR
 
M. Griffin:

This is exactly what I was saying in my reply this morning. As you said, finding shorter card guides is the key. If anyone can find this out it would be very useful for the project. Also, pricing the extrusions in bulk would be helpful, but I can't imagine they are bad.

KEJR
 
S
"In my view isolated inputs that share a common are not isolated."

While not trying to get into a discussion on the definition of "isolated" I don't see value in individually isolated inputs. I definitely see the value of isolation from the logic, but input from input?

Most inputs have a common, uh, common anyway, so having two terminals per channel just makes more wiring. For me, individual channel isolation would create a lot more extra work (in wiring) than the commomed ones would create when I occasionally (think once a year) have to install a relay or optocoupler because they aren't isolated. You could have a specialty input card for people who need a lot of isolated inputs.
 
C
Yes, the finding is the big problem. A Google search for card racks turns up thousands of hits and sorting through the mess would take a lot of time. I'm not too worried about looks at this time, PLCs tend to be ugly and get more ugly once you have the real world attached. If there is any substantial volume, more solutions become possible. It will substantially complicate things if we have to have things made for the project. Volumes are extremely unpredictable, who knows, I might end up having the only one, or someone might want dozens for a special project. The only part that needs to be unique for the hardware is the front plate. I still have some places to look for an OTS box at least, and that would leave only the front plates. It would be easy enough to just saw slots in a slab of nylon or HDPE for the card guides, but those slabs come pretty dear these days. If a person can come by a source of the snap in card guides used in PCs, they would probably be cost effective. One good thing on the connector front is that a standard .200" layout will work for both pluggable and non pluggable connectors of several types and the choice can be made on need or cost. Where did you see spring clamps reasonably? I'm going to call around and see if I can get a sample of the two types of wire duct to at least judge how that works. The connectors will also impact the front panels somewhat. I have a milldrill and can make presentable front panels, but that isn't a universal solution.

Regards
cww
 
C
Hi Bill

The output card as designed has sourcing outputs at 24 volts. That's to work with the sinking 24V input card. The chip +5V is generated from the rack +24 which only supplies the actual output devices on chip. One dilemma was between internal power or external power. External power has the virtue of simplifying power distribution. but has the issues of ground loops and I've seen many cases of latchup and weird problems when it's done that way. Internal power means high currents on the bus, but ensures that the internal clamping diodes and protections are effective. Since many people don't bother with diodes on inductive loads these days I have empted for internal power for reliability. I could make it a jumper option I suppose, but folks would need to be careful when using external +24V. For sinking outputs it's not such a problem but it's still good to have things returned to the same point. There are a lot of things that must be carefully considered, especially for use by the general automation public.

Regards
cww
 
W

William Sturm

I do like the idea of internal power for the I/O, I guess. It is certainly seems simpler to use for many applications.

I do not like the idea of a wire duct for a rack enclose, they are way too flimsy. We need a better idea here, IMHO.

What about a bracket that holds the cards solidly to the backplane, who needs an enclosure? We could ultimately use a beefier circuit board (~.125") for the backplane and just bolt everything to it. Maybe???

Bill Sturm
 
C
I agree that they aren't needed vary often. But the more common type with common terminals aren't often needed in a properly wired system either. With even reasonable ground bonding it's not a problem. But, what I have seen a need for, particularly when interfacing several boxes, is a mix of sourcing and sinking inputs in the proportion I need and the shared common units aren't very good that way. I seem to always have the type I don't need and need the type I don't have. One solution would be to have say three commons and an internal jumper select, but that would be pretty confusing after the fact. But for now, a simple card will suffice. The burden to add cards is not very high once the other issues are sorted out. One thing I probably won't do are line power inputs. Not difficult, but that opens a regulatory can of worms.

Regards
cww
 
Here's another source of rack hardware. http://www.vectorelect.com/

They have two styles DIN and EIA. Card guides come in snap-in and screw mount versions. The snap-in ones are much cheaper. The following is an <b>example</b> only for snap-in guides.

Card guide: CG2-45S 80.52mm pkg 12 - $18.76.
Strut: TS169-6/90 16.85" ea - $19.61

If you use screw-in guides, then the strut is about half the price, but the guides cost twice as much (including screws and nuts). Lets round the above numbers off to $3.00 per pair of card guides, and $20 per strut.

Hole to hole centres on the strut are 0.75 inches (or 19mm). That allows for 22 cards on the minimum centres. That means that for 8 cards we can cut each strut into 2 full sections with one short section left over. So, for a full rack (with minimum card spacing), we need 2 struts ($40) and 8 pairs of card guides ($24). So, that amounts to $64 plus the cost of end plates, and any top, bottom, back, and side covers used. This can be considered the floor price for "rack" style packaging.

The DIN (Eurocard) packaging is somewhat more expensive and the minimum size is larger (these two points are closely related). I didn't shop around for these prices - I just used the first distributor that Vector listed. It's possible that someone may find better prices than these.

The packaging considerations will obviously have a strong influence on the board design because it will dictate certain board dimensions as well as the pitch between the backplane connectors.
 
Hello,

(In reply to M. Griffin:)

Vector is tailored to the prototype market and in doing so is going to have readily available but more expensive hardware (I'm guessing). I would be willing to gamble that you can get a 20' length of strut inexpensively if someone will source it "onesy-twosy". I'm not saying anyone here should run out and buy 20' lengths of struts, but it would be good to know if there was a bulk source of this stuff for budgetary purposes. I'm sure its the type of thing where you could find out in a few phone calls where people get this stuff. My phone time allotted to this project during work hours is very limited though. One issue is that the guys making these things are used to "big money"... Telecom, Military, Test.... Will they sell us parts... Who knows.

If, in fact, the price can't come down much more than your $64 I think that custom sheet metal is more attractive. I have not done a full analysis including card guides and fastening hardware however.

KEJR
 
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