-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 On Thu, Jun 21, 2007 at 09:28:24AM +0100, Alan B. Pearce wrote: > >Given that I need to have address discovery, then yes, 2 way > >comms has to be possible. I might want to put sensors in the > >network too later on. Being able to upgrade the firmware of > >the devices via bootloader would be really cool too, except > >that no 12F or 10F part supports it. > > Not sure if the small 16F parts support it. Would also give you enough pins > to drive multiple colours of LEDs at a node. Yeah, they don't unfortunately. The cheapest part that supports bootloaders is... Wait, now that's interesting. Using the paramatic search tool on Microchip you would assume "Self Write" is the option to search for. But the 16f818 chip doesn't show up, and I wrote a bootloader for it... I think the magic term is "Enhanced Flash" of which the 16f818 is the cheapest part with it. Even then, a 6 pin 10f part is enough pins to drive a three color led, 6 pins, 2 for power, one for communication and the other three for the led rgb. > >My understanding is that I'd be doing voltage modulation > >for both ways. The slaves would short Vcc/Data and GND to > >indicate zero's. Given, say, a 5k pull up and a short through > >a 200ohm resistor the slaves would be pulling the line > >down to 0.2V > > OK, that is a similar system to what I am suggesting except you are clamping > the voltage right down close to 0, which means you can use voltage sensing > at the power supply end. However this will mean that you will need to have a > power supply that has a sharp knee on its current limit, possibly even some > form of foldback current limit, and the pass element has the possibility of > getting very hot - remember this will have the whole power supply input > voltage across it while the node is shorting to ground, ant will need to be > current limited at a touch over 2A for 100 nodes at 20mA/node - >2A if you > want expandability. Not only that but the transistor at the node will have > to have this current capability as well. Note that unless you have some > large capacitance at each node, isolated from the supply by a diode, you may > see the transmission modulate the light level of any LEDs that are turned on > due to the chopping of the supply voltage. Ahh ok. None of that will be needed, because when the master expects communication from the slave, the master turns off the main, high current source, and leaves the data line connected pulled up with nothing more than a 4.7k resistor. A slave will easilly be able to short that resistor. That said, just realised that implies that all slaves must turn their leds off to back communication to work... oops, as I said differently in another post. > >I'm going to leave the checksum off as integrity isn't critical. > >This is just a light show. :) > > But you will want a checksum if you start doing firmware downloads. > Depending what you are using the LED display for, you may still want a > checksum of some sort, there is nothing more frustrating than having such a > system go AWOL in sequence, even if it is only a demonstration Christmas > tree lighting display. I'm just assuming that given that updates are every second, any incorrect color will only show up for a second anyway before it's corrected. The occasional bit of weirdness will be interpreted as character. :) Firmware downloads though, yes definetely. I'll likely even use a very simple error correcting "two out of three" type system. With a checksum, you can always just invoke the bootloader again, but a two out of three system would even allow the bootloading to succeed in the face of errors, as well as be very simple to program. > >Another problem with full cat 3 is visuals... it's big and ugly. > >If anything I'll may very well have to find a source of jet black > >twisted pair somewhere, skinny and unobtrusive will be important > >during the day. > > I would see if I could find some dark green sheath CAT wire - like used on > Christmas tree lights. As to the component cost, I would try hitting up some > FAEs for chips - I am sure you could get a couple hundred CAN drivers & PIC > chips for 'demonstration purposes' especially if you are prepared to write > an app note of the experience (student task - part of the project!). You I'll definetely look into that, although the whole design can't completely depend on such charity, as the idea is for other groups to be able to reproduce our setup, even at normal market prices. It's why I was quoting part prices from digikey for instance. > might even get the same consideration for a roll of CAT cable with a special > coloured sheath - would the army use CAT5 with camouflage green sheath? With > the right colour I think you would find that even CAT 5 would get reasonably > invisible. Definetely. Having said that, I think I found my wire... Turns out digikey sells solid two conductor twisted pair, with a pvc jacket, for $35/1000ft. Basically telephone wire, which I'd totally forgotten about. P/N C7023708-1000-ND. It's non-stock, but ships in a few weeks, and there's gotta be more like it from other distributors, like local electrical companies for telephone installation. That brings the per node wiring cost down to $1.75 and nobody has to go around cutting up CAT3. - -- http://petertodd.org -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.6 (GNU/Linux) iD8DBQFGfHH63bMhDbI9xWQRAt75AJwIKGOCpx4pV8G4V800XqFPLrd+dgCfXOLV dWgD9+vOuHYQzj7TNKaUljo= =TBX2 -----END PGP SIGNATURE----- -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist