Nevilleh wrote:It would require a failure that turned on the Tx opto permanently. That's fairly unlikely.
Or a fault that continually transmitted gibberish. But yes, probably unlikely. Most likely failures are probably in comms wiring.
So you have only 2 comms connection pads on each PCB? We'd need 4.
When you ask how the cascaded thingy would work, what level of description are you looking for? Hardware, bits, bytes, packets? Do you see why it has to be only one opto per board with that scheme now? Opto on output, resistors on input.
We should definitely look more closely at the AVR Tiny25 and consider saving that 50c per board. Is there a free assembler development system for it?
Your packet scheme seems very sensible. Can you upload a Protel file for your master and slaves? Or point us to schematics elsewhere in this huge thread?
Yes, I really do know how the cascade thing works, I thought you might might have come up with something else, but your "resistors on input" doe NOT provide 2.5 kV isolation - in fact, none at all.
The Tiny25 is the "most bang for the buck" processor that I have been able to find. You can download a free assembler and a free C compiler (WinAVR) from Atmel's web site, plus heaps of app notes and other stuff.
The ISP feature makes it a piece of cake to get going and Atmels emulator even lets you single step your code. I've never needed anything else to make it work. Edit: PonyProg supply a good, free programmer too.
If you order the "V" of the chip, it works from 1.8V to 5.5V and you can get a 20 MHz version for the same price. Doing away with voltage regulators saves money and all you need do is filter the supply properly, like a decoupling resistor or even a small choke and an electro and a ceramic. Mind you, making the comms work at 1.8V might be a bit of a mission.
The data packet scheme I invented way back in '97 to handle comms with poker machines - it included encryption then - and it is now used in about 60% of the poker machines in NSW under the aegis of "eBet" who bought my company. So it is a pretty robust thing.
I'll dig out the schematics for the boards in the next day or so and stick them up here.
edit: Guys, I thought by now you would realise that daisy chained comms is for the birds! Let's not waste any more time and effort discussing it, if it was any good the Internet would use it
What does the Internet use? Something rather like the packet structure I have described - OK, a tad more complex, but pretty similar. And we all know how well that works! If I buy any more Viagra, I can start a pharmacy