Open source PLC hardware

In reply to Bill 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 mostly agree, but I go back and forth. You might want to limit the current so you don't pop your backplane or connectors. The power required for output cards is rather huge (even at 100mA per channel) though and you would need some kind of reasonable power budget. Nowadays with 1Watt valves being very common you don't need 250ma per output.

At a minimum, Power supplied through the backplane should have Logic and IO Power separate!!!

> 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. <

Agreed.

> What about a bracket that holds the cards solidly to the backplane, who needs an enclosure? We could ultimately use a <

Some folks make a card guide that bolts to your backplane. These would have to be relatively cheap to compete with a sheet metal enclosure IMHO. For the 8 slot chassis you would have to be probably under $2 a piece to keep it competitive with cheap card guides and simple sheet metal. I had only really considered this for OEM (and hobby) types of applications where cost was king. For professional life I'd spend the extra $20-$30 and get an enclosed sheet metal box to help prevent stray wire strands and screwdrivers from landing on my boards. This is a personal/professional choice and really there is room for both approaches in the spec as long as the boards stay the same for both.

How would you accomplish board retention with simple card guides bolting to the backplane? This was what stumped me on this idea.

KEJR
 
K

Ken Emmons Jr.

In reply to Curt:

> 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 <

I'd like to get the package to under 4" if possible. I also would like opto isolation as an option (different card design). Perhaps if we bring the IO power in separately on the backplane we can use 16 conductor 0.2" pitch connectors on the front and still have room for all the hardware for the option of a fully enclosed unit. Maybe I'll have some time this weekend to do a CAD layout to see what kind of room we need once card guides, front plates, and card cage retention is all factored in.

KEJR
 
C
Hi Bill,

> 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. <

It's a field expedient.

> 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??? <

If we give up the concept of plug in cards, I might as well make it on one monster board. I did find card guides for a 2.5" board for about 15 cents each in qty. 1k, but they want a drilled panel to mount to. Back to sheet metal. I also found a killer deal on wire duct, but it's .4"(9.2mm) pitch and thats a really odd number. Looks like about $200 for 3 protos of the 3 boards. Plus parts. Less than $10 each in qty., plus parts.

Regards,
cww
 
C
The commercial offerings seem to be all aimed at deep cards and not to our application. I have found 2.5" card guides for $.15 each in qty. they snap in drilled or punched holes. It should be cheap and easy to bend a simple channel that takes the backplane and guides and cards and mounts in the enclosure of your choice. Any hvac shop could do this. But the drilling would be a problem for those without a shop. I can buy a simple brake to bend this for $30 or so. I can get 12" wide rolls of galvanized steel economically as "flashing tin" at a big box store. So there are things that could be done, but it would be nice not to be in the sheet metal business. I would like for anyone to be able to put this together with local sourcing or internet sources. I may buy the brake anyway as this gives me an excuse :^)

Regards
cww
 
W

William Sturm

Curt,

is that an RC network on your inputs. I see some spaces labeled R1-R16?, but I'm not sure what 17-32 is. I think series resistors are an obvious choice for current limiting, then maybe some zeners or Transzorbs in parallel to ground for OVP. Also, should we use pins 7 and 10 for an earth ground to the boards, primarily to be used to sink voltage spikes, so they don't have to go to the supply common?

Bill Sturm
 
C
While on the subject of power, I could use some opinions. I have opted for linear 3 terminal regulators for the cards. This is reasonable, I think, because the 5V requirement is very small, around 10ma., less for an input card. I would like the whole unit to run on a single 24V supply for convenience. But, when you add a CPU card, things get a little more complicated. The CPU cards I have been looking at are wonders of low power, 2 or 3 watts. But at 5V that means we should allow as much as 1 A or even more because we don't know that the CPU chosen will be that low. The problem comes in because we have 19 V dropped across the regulator. If we use the simple approach of another three terminal linear regulator, it would dissipate just over 19 watts. Not a huge amount of power, but the regulator would require a fairly large heat sink to operate over our temperature range. And its kind of a waste to have a 20 watt heater in a low power PLC. I could design in a switcher with reasonable efficiency, but that would involve magnetics and the potential for noise, emissions, etc.

The chickens way out would be to leave it to the user to power their CPU card. Another would be to use two supplies. I don't like these much either, but it would get the heat outside the case. So, I thought I would see what the potential users might think:^). It's a very novel approach in automation.

Regards
cww
 
Advanced circuits has much better pricing, check out their "bare board" special. 2 layers, no solder mask, no silkscreen.... Very cheap.
 
Curt:

You can't compete with a sheet metal shop that has a CNC punch machine and press brakes. They load up a machine and load the CAD/CAM file and it punches all of your holes and profiles unattended. I'm pretty sure the metal will be under $20. I'd rather target a market where someone buys the backplane and cage from multiple sources who have invested in the quantities to make it a reality (since its open). IF someone has the machines to do it themselves, that is fine too. I just don't think it does the project justice to think that people will be out there hacking together units with saws and drill presses and wondering why cards don't line up. If we can get the smaller backplanes/cages to under $50 this would be a good price point for companies and hobbyists needing a lot of IO in an "industrial" package. I have access to a full shop and could donate a prototype cage to you once the design settles out.

I forgot to mention this, but the pluggable 3.5mm connectors I like are made by Weidmuller and are available from Digikey. I think the Right angle board header is about $5 and the removable spring cage plug is a bit more. I would not mind finding something that is cheaper in a very similar package (yeah, I think I could live with 0.2" (5.08mm) pitch for 16 contacts).

KEJR
 
Bill:

I think this would be a good option for an "embedded only" kind of approach. The fact that you have to screw into the card guides from the side make it difficult for single board field replacement. What is the price for that kind of card support? Another approach would be to buy the card guides that mount to the backplane, and use long standoffs on each end of the backplane and run a piece of sheet metal strip, or bar stock, that attaches to the standoffs and runs along the top of the boards to keep them in. This would accomplish retention and card support as well as field replacements by simple removing the retention bar with two screws. Again, how cheap are the card guides for this?

KEJR
 
W

William Sturm

I'll second Advanced Circuits, they are very good.  They have great deals, such as Barebones, 33 each...

Bill Sturm
 
W

William Sturm

Curt,

I think it is very reasonable to use an off the shelf switching PS for the computer.  Having said that, it would be nice if the rack had a 5 volt supply (or 3.3 Volt) specifically for a host processor.

This may be a good solution:  http://www.dimensionengineering.com/DE-SW050.htm  Have you seen these?  With this idea, we do not have to develop a switcher of our own.  It is not super low cost, but probably cheaper and smaller than any external supply.

Bill Sturm
 
Bill, Curt:

I feel that 24V should be supplied to the backplane, either through the CPU or a connector on the backplane that someone can get at in the field. From this Backplane supply, the CPU card would feed 5V logic supply to the daughter IO cards through separate traces.

- Supplying 24V to the front panel of the CPU card makes it easy to wire, but limits the amount of connectors you can have on the front of the CPU.
- Supplying 24V to the backplane makes more requirements (potentially) on the mechanical layout since you have to leave room for a screwdriver to get into the end of the backplane. Depending on the path chosen this might not be a bad thing.

Something like that TO-220 switcher would be great to get started, and then something cheaper could be developed out of discretes. At this point we should probably just pick Backplane supply vs CPU "feed through" design. I guess I'm leaning toward the backplane to provide connections for 24VDC. If we lay it out and it is ridiculous this might change! :eek:) I guess it could be specified both ways, its just extra pins on the CPU connector.

KEJR
 
C
Hi Bill

Those are voltage dividers to scale 24V inputs to the chip level. All 32 are resistors. The inputs have clamping diodes and the high resistance of the dividers will limit the current. You can socket the chip and replace it for a buck or so. 16 Transorbs or zeners would be expensive overkill. And the chip can be destroyed before they begin conducting. In actual testing, having "stiff" supply rails for the clamp diodes to conduct to has provided better protection. In this design, the backplane and case will be grounded to earth ground and two pins are tied to the backplane ground. The inductance of a separate wired earth ground would make it less effective. I did ESD testing for Control Data for a few years. The chips have gotten a lot better, even hand held devices have very few ESD failures and for extreme abuse, you would probably want to replace the chip anyway.

Regards
cww
 
C
I was using their "Bare Bones" quote. They seem to want to get about $60 an order, In other words 1,2 or 3 boards all cost about 50 or 60 bucks. I could panelize and try that way, but I was just looking for ballpark numbers. 3, 3.5" X 2.5" are $21.04 each. 1 is $54.38. A 10" X 1.5" backplane is $57.50 for one or $24.17 each for 3. So I figured roughly $200 for 3 input, 3 output, and 3 BP. The price does get very reasonable for 10 or so, but you would want to try one first. In checking, I also noted an error. The Allegro chip should be a 2981 rather than the 2982, higher voltage rating. If someone wants to fund the protos, I'll get the artwork done soon. Otherwise there's no hurry, I don't have the cash in the kitty for protos anytime soon. I would like a set though.

Regards
cww
 
C
> You can't compete with a sheet metal shop that has a CNC punch machine and press brakes. They load up a machine and load the CAD/CAM file and it punches all of your holes and profiles unattended. I'm pretty sure the metal will be under $20. <

No, I can't compete for a qty order, and I'm not all that fond of sheet metal work anyway. But, it would be good to settle on something that could be fabbed locally for people in East Borneo or ? I found card guides that would work for sheet metal, also at Digikey. I can post the number. I will need to decide on a card spacing soon. I have 1.25" and I think probably the best compromise is 1". That allows space for leds and a double stack connector and would save on board cost for the backplane.

> I forgot to mention this, but the pluggable 3.5mm connectors I like are made by Weidmuller and are available from Digikey. I think the Right angle board header is about $5 and the removable spring cage plug is a bit more. I would not mind finding something that is cheaper in a very similar package (yeah, I think I could live with 0.2" (5.08mm) pitch for 16 contacts). <

I don't see how to use 3.5mm without a bigger board or less points. The 2.08mm are an Onshore Tech. part, There are several varieties but I want to use at least tin plated grade. They are wire clamps with a screwdriver slot. I like spring clamps, but I think they are bigger.

I'm happy to leave packaging to others, but we want to settle the parts that affect the board layout soon. I will set some mounting holes on the backplane as these would affect the package.

Regards
cww
 
In reply to Ken E:

Vector also sells self-standing card supports. They are die cast zinc or aluminum. Each support is individually mounted (i.e. two supports per board) via a screw down through the base of the support into the mounting surface. The support curves away at the base to provide clearance for a backplane. The I/O board would then slide into guide slots just like with the conventional plastic guides.

I don't recall the price, but they were a good deal more expensive than the plastic ones. This is probably something you would only want to consider if space was very tight and you only had a few boards.
 
W

William Sturm

Ken said "I feel that 24V should be supplied to the backplane"

I believe that this is exactly what Curt has in mind. Here is my interpretation: We will have terminals to supply 24VDC to the backplane. The 24VDC will get bussed to each card, where it will be regulated to 5VDC for the logic power. If I understand correctly, the backplane may have an extra 5 volt regulator with enough current to drive a processor board. (add it to the board layout, it can be optionally populated) This is where I suggested the TO-220 packaged switching power supply. So we will have 24VDC in and 5 VDC out on the backplane. Is this correct?

Bill Sturm
 
I apologize for the commercial nature of this post, but thought I'd mention that the company I work for makes a low cost micro-PLC that is based upon the open-source Free-RTOS, comes in an enclosure, and already has the regulatory certifications needed for a lot of industrial installations. We can provide all the hardware libraries needed as modifiable C source code and let you write your own control application in C, giving you a lot more control than just standard Ladder.

There is a 4-line or 2-line LCD, with a knob and 4 buttons to create a user interface. Modifiable libraries are supplied for a generic menuing system or you can roll your own.

I/O is somewhat limited at only 4 DI and 2 DO on-board, but there are 2 RS-232 and 2 RS-485 ports and both Modbus master and slave communication libraries making I/O expansion extremely simple, and for those who don't want to do it themselves we are working on an I/O expansion board that accepts any of the standard Grayhill plug-in modules.

http://www.rogue-engr.com/cportal2/images/cutsheets/1950-201.pdf
 
C
Hi all
>
> I feel that 24V should be supplied to the backplane, either through the CPU or a connector on the backplane that someone can get at in the field. From this Backplane supply, the CPU card would feed 5V logic supply to the daughter IO cards through separate traces. <

The backplane has a connection for the 24VDC bulk power. These are currently for large wires, but would accept terminals of some sort. I don't bus 5V through the backplane because we learned in 70s that with the low voltage and relatively high currents and switching transients, you have much cleaner power with local regulators and minimal interaction. Since the output current will dwarf the CPU current, the plan is to power the backplane and run the CPU from that. The local regulation also means that the bulk power, the 24V bus, is not very critical and we can confidently use any OTS 24V supply with an adequate current rating.

The CPU power, we have to be a bit careful with. Although the input voltage is specified as 5V for convenience, the board is full of logic that is running at 3.3 V or even less. So the noise margins are low. It is important that the CPU common be run directly to the "one point ground" in the system at the bus ground plane. And if we do obtain the CPU power from the backplane, a high quality regulator and filtering should be used. Fortunately, PLC speeds are so low that noise from the IO shouldn't be a problem. Noise from a switching regulator might be, due to proximity. The ultra low power electronics means high impedance and we would want to shield a switcher if we build it in.

> - Supplying 24V to the front panel of the CPU card makes it easy to wire, but limits the amount of connectors you can have on the front of the CPU. <

The way I have planned it so far, is that the CPU card will mount horizontally ( that is in the same plane as the backplane)in it's own 4" X 4" bay at the left side of the housing. None of the CPUs I've seen are set up to plug in to a BP and they have connectors around the edges for the various interfaces. So it wouldn't have a front panel as such, There would be a cover over this bay which would provide a convenient place for status LEDs, switches, LCD display, etc. I should have made a rough drawing at least, as is was many words ago that I described that. I will make a concept sketch and ask Bill to post it. But, it will look like almost every other rack type PLC. Which isn't a bad thing.

> - Supplying 24V to the backplane makes more requirements (potentially) on the mechanical layout since you have to leave room for a screwdriver to get into the end of the backplane. Depending on the path chosen this might not be a bad thing. <

Access to the CPU end of the backplane is easy through the CPU bay.

> Something like that TO-220 switcher would be great to get started, and then something cheaper could be developed out of discretes. <
----- snip -----

I'm still pondering. Even though it wastes power, a simple linear regulator is attractive from cost and noise standpoints. And if we have a metal housing we can thermally couple the regulator to that for heat sinking.

I'll post a sketch tonight if possible.

Regards
cww
 
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