K
Ken E
Wow, stay out of work for one sick day and look what happens! )
I'm just going to reply globally to all of the posts so far...
Here is what I feels strongly about:
- Develop an SPI based rack with electrical and mechanical specifications.
- The rack can be used with a CPU or a communications module (i.e. remoteIO).
* Lets not get too detailed on what these are *Yet* There will undoubtedly be many options here.
* Using SPI as the [rack] bus will allow many options, from completely embedded AVR/Arduino to Marvell processors.
- The rack should be as small as we can make it (This is to compete with the bloated solutions that already exist).
- Make it as inexpensive for basic IO modules as possible. We want to leverage this system to be cheap when you have high IO counts. $50-$100 for 16 bits of IO is very reasonable and achievable in any decent quantities.
I think that developing the IO rack in itself is enough of a project for now. I don't think looking at embedded linux computers is a bad idea at this point because that is where we want to go, but lets not get bogged down with the exact System on Module right now. I think I prefer the modules that are more "stamp"-like since they let you dictate where the front panel connectors (USB, ethernet) will be located on the "carrier" module. We don't want to design a rack based system around one CPU module. The "linux stamp" is such a device, but so far it seems limited to a 200MHz ARM, which should be fine for most things, but not for everything.
I have developed IO boards based on a backplane before and I know some of the problems in doing it on your own. I was able to get it down to roughly a 3" x 3" board size while still using 16 IO points and 24VDC feed. I'd like to do some of the mechanical layout and board design if my "home time" allows for it (I have to young sons, so time is not that available). I've found that DIP devices are very much hard to find in some areas, so working with SMD is almost required. If we keep with 0.050" pitch SMD devices you can still hand solder them and use a 2 layer design.
Bill: I've not used a lot of free schematic/PCB packages, have you considered the Advanced Circuits free PCB tool? It is one of those deals where it is free software if you order the boards from them, but they are one of the cheapest in the USA for prototypes. I know a lot of folks use Eagle, but their board size limitation (for the free version) is limiting. I'll check out the software packages you mentioned.
BTW, the plug computer keeps saying it will be released for $50, but this seems to be a marketing thing for them to demonstrate that you could build a Marvell based "PC" for $50 in quantity. So far I don't know of anyone that is offering anything based on that chip for $50.
BTW, what do you guys think about bussing out the 24V on the backplane as an option? I've seen some of the slice IO guys doing this and it can shave down the front panel connector requirements (and wiring). We could limit the amount of current that the "bussed" 24V backplane could supply for power budget reasons, and anybody requiring more power would have to supply it to the front of the board. Just an option, and I don't think it is critical, but it would be nice to consider up front.
KEJR
I'm just going to reply globally to all of the posts so far...
Here is what I feels strongly about:
- Develop an SPI based rack with electrical and mechanical specifications.
- The rack can be used with a CPU or a communications module (i.e. remoteIO).
* Lets not get too detailed on what these are *Yet* There will undoubtedly be many options here.
* Using SPI as the [rack] bus will allow many options, from completely embedded AVR/Arduino to Marvell processors.
- The rack should be as small as we can make it (This is to compete with the bloated solutions that already exist).
- Make it as inexpensive for basic IO modules as possible. We want to leverage this system to be cheap when you have high IO counts. $50-$100 for 16 bits of IO is very reasonable and achievable in any decent quantities.
I think that developing the IO rack in itself is enough of a project for now. I don't think looking at embedded linux computers is a bad idea at this point because that is where we want to go, but lets not get bogged down with the exact System on Module right now. I think I prefer the modules that are more "stamp"-like since they let you dictate where the front panel connectors (USB, ethernet) will be located on the "carrier" module. We don't want to design a rack based system around one CPU module. The "linux stamp" is such a device, but so far it seems limited to a 200MHz ARM, which should be fine for most things, but not for everything.
I have developed IO boards based on a backplane before and I know some of the problems in doing it on your own. I was able to get it down to roughly a 3" x 3" board size while still using 16 IO points and 24VDC feed. I'd like to do some of the mechanical layout and board design if my "home time" allows for it (I have to young sons, so time is not that available). I've found that DIP devices are very much hard to find in some areas, so working with SMD is almost required. If we keep with 0.050" pitch SMD devices you can still hand solder them and use a 2 layer design.
Bill: I've not used a lot of free schematic/PCB packages, have you considered the Advanced Circuits free PCB tool? It is one of those deals where it is free software if you order the boards from them, but they are one of the cheapest in the USA for prototypes. I know a lot of folks use Eagle, but their board size limitation (for the free version) is limiting. I'll check out the software packages you mentioned.
BTW, the plug computer keeps saying it will be released for $50, but this seems to be a marketing thing for them to demonstrate that you could build a Marvell based "PC" for $50 in quantity. So far I don't know of anyone that is offering anything based on that chip for $50.
BTW, what do you guys think about bussing out the 24V on the backplane as an option? I've seen some of the slice IO guys doing this and it can shave down the front panel connector requirements (and wiring). We could limit the amount of current that the "bussed" 24V backplane could supply for power budget reasons, and anybody requiring more power would have to supply it to the front of the board. Just an option, and I don't think it is critical, but it would be nice to consider up front.
KEJR