I have somewhat of an interesting question/problem. I am trying to interface a Handspring Visor (a very old PalmOS device) to the dashboard port on the OI. Now, the way I understand it, the OI’s port is a standard RS-232 port. That would imply it is signalling at, like, +/- 12 volts, wouldn’t it? The port on the Visor demands 3.3v, and I think that’s a maximum, not a minimum. Is there an easy way, other than trying to track down a serial dock, to do the hardware interface here? I know which pins are the serial pins on this device, but I don’t want to fry it with 12 volts. EDIT: Forgot to mention, I am definitely cracking the device open, so the physical interface is not a problem.
I am developing software for it, and this will be open-source for future teams if I get anywhere with it. It’s the perfect dashboard device–small, lightweight, battery lasts forever and has audible and visual feedback. And an open SDK to boot.
I just don’t know enough about the electrical side. I understand there’s a way to drop voltage with diodes/a resistor but I don’t understand it fully. Perhaps an experienced EE can help me…
Almost. But the MAX232 chips need a 5V supply and thus have 5V TTL outputs. Not toooo likely to fry your Visor, but if you’re already going to the trouble I think you more likely want something along the lines of the MAX3232E which can operate from a 3.3V supply.
EDIT: In line with Alan’s suggestion, there’s also the MAX3233E hat doesn’t require external caps but still runs off 3.3V
ONLY if you eliminate the capacitors that are suggested for the chip. They provide a “Charge Pump” that shifts the voltage levels from 0-5V to about -10v to +10v… then it’s just operating as a line driver…not it’s main design function. ( Output… oh, you mean the TTL side of the chip. since you have an RX and TX on both sides, you really can’t say one side is input and the other output. A little terminology cross talk there.) It’s a really nice chip! Not that this really matters because …
I agree… this would be the way to go. There are internal capacitors that do the voltage boost so you can use the TTL 0 -3.3V and get the RS232 on the output. Too bad you just can’t bypass the OI RS232 shift and just go TTL to TTL.