We are designing a device that has very basic needs and am evaluating the Rabbit RCM2100 board (until we need analog) as well as a Netburner board. It needs to collect input from three digital inputs as well as possibly three analog inputs (such as temperature and flow).
It also needs to send collected information to an external web server every minute (without stopping the collection process of course). So it needs an Ethernet port and TCP/IP with DHCP abilities.
We only need to produce between 5 and 25 of these devices per year and want to keep the cost down. It seems like these types of boards are perfect for us right now.
We are new to the embedded space and could really use some help in pointing us to alternatives or feedback on these boards.
You haven't said what form your I/Os will take. E.g., are the digital inputs at (nominal) TTL levels? Or, do they require level translators/isolators, etc.? Are the analog inputs DC coupled (with simple scaling)? Or, for example, 4-20mA loops?
Nor have you explained the resolution that you need on them (analog) or the sample rates you need.
Are you producing these for resale or for in-house use? Do you have any regulatory agency requirements that must be satisfied? Will you be incorpporating (packaging) this in a larger assembly/enclosure? Or, will you provide a specific enclosure just for this device (at those volumes, I suspect it is not going to be a custom plastic case! :> )
Without knowing the specifics alluded to above, why not consider simply repurposing some other commercial (COTS) device to suit your needs? E.g., any of the linksys/cisco SOHO appliances will probably easily handle the networking needs you mention with a small daughtercard (your design) to provide the necessary I/O and signal conditioning that you require.
The digital inputs are from a WattNode device and is a pulse output of 5v. We aren't really sure about the analog inputs yet but we are looking to use flow meters and temperature sensors. Flow meters seem to be available with pulse output as well and I think temperature sensors come in a digital output as well so analog might not be needed.
From the WattNode site,
Pulse Output * Output waveform: square-wave, 50% duty cycle * Optoisolator (phototransistor) output handles up to 60 VDC at 1?A (microamp) to 5mA (milliamp).
Fully isolated to withstand 5000 volts
Frequencies at Full Scale Power * Standard (All Models): 4.00 Hz * Custom (Standard Bidirectional Outputs): 0.01 Hz to 600 Hz * Custom (Option P3, Option PV): 0.01 Hz to 150 Hz
The devices are not for resale but are being installed as a monitoring device in residential homes. We have found an aluminum cases that we are going to use that can contain the WattNode as well as the board/power supply/etc. They are about 11"x8"x3" or so and will have a place to plug in a four wire power connector (for the WattNode) as well as terminals to connect our various sensor inputs. So the only case customization is some cutouts for the terminals and plug we are going to use.
Ouch! I am going to be a little "light" in my answers here for fear that your device may place me in jeopardy of previous work agreements. :< (sorry)
You'll get the most variety in "field" devices (sensors) if you adopt 4-20mA as your standard. However, that will also *cost* you the most!
Pulse outputs are good as long as the frequency and duty cycle allow you to accurately capture the smallest transition you are likely to encounter. (e.g., you can have a 1Hz signal -- nice and SLOW -- but if it has a 0.001% duty cycle, you'll probably have some problems!
You can also add signal conditioning to extend the length of pulse outputs a lot easier (introducing less error) than with other "analog" outputs.
Keep in mind the distances between your sensors/transducers and your data acquisition system. E.g., if you mount the DAS on the outside of the residence and run 100 ft of cable to each sensor, you may find serious degradation in your signal. This can be compensated for with things like differential xmitter/receiver pairs but it all adds cost. Bottom line, think before doing!
This can easily be extended for the sorts of distances you are likely to encounter.
Hmmm... 4Hz at full power, so what is it at no load? (is this a "single appliance" type monitoring situation or a "whole house"?) What's the range of loads that are represented (i.e., will you be able to discriminate between 4Hz and 3.999999972Hz and 3.99999921Hz (etc) in the sampling interval that you expect to use? (you might want to measure *period* instead of frequency -- so, you need a reasonably stable high frequency timebase that you can "count")
OK. So you don't have to worry about making it "look pretty for The Masses".
I am unclear about your statement:
It also needs to send collected information to an external web server every minute (without stopping the collection process of course). So it needs an Ethernet port and TCP/IP with DHCP abilities.
First, I assume you have a special application running on that "web server" that gathers the information. (you might want to design that application to *pull* the data from each device instead of having the devices *push* it to the server). Then, that application somehow makes it available to the web server as a static page or part of a cgi script, etc.?
The device only needs DHCP support if you want to make setting IP addresses easy :> But, then you will need some other way of keeping track of which physical device (i.e., which residence) is currently at which IP address! I.e., if a node loses its IP address "lease" (power outage at the residence or the server glitches and drops *all* the leases), then they will (automatically) go looking for a new lease. The new IP that each node gets could be different from the node it had 5 seconds ago (before the glitch).
[NB, you can work around this by tracking the MAC addresses of the nodes instead of their IP addresses. Or, embedding some other unique identifier -- e.g., "1313 Mockingbird Lane" -- in the code running in each device]
I don't know where your server is located (in the neighborhood? or, maybe on the far end of a DSL line??) but you could also consider other low bandwidth mechanisms to link the monitors to the server...
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