Signalling voltage question

Hey all,

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…:confused:

Thanks in advance,
JBot

EDIT2: Ohh! I could just use a MAX232 couldn’t I?

You are correct; the dashboard port on the RC signals at +/- 12V. It does this by using a little IC known as a MAX232.

The MAX232 is a voltage level translator specifically designed for the RS232 standard. A quick Google search on the chip number will turn up a datasheet.

Use is quite straightforward–after connecting a few required capacitors, connect the RS232 RX pin to the RC dashboard port, and the LVTTL RX pin to your handheld’s RX pin.

Best of all, you can request a free sample of the chip in a DIP package from Maxim’s website at www.maxim-ic.com!

Hope this helps…

Eldarion

EDIT: As mentioned below, the MAX3233E is definitely a better choice for this application. Having cut my teeth on the venerable MAX232, I tend to use it for everything… :slight_smile:

The RS-232 specification says valid signals are anywhere from +/- 3 to +/- 12.

The port on the Visor demands 3.3v, and I think that’s a maximum, not a minimum.

I don’t believe the Visor does standard RS-232 by itself. It needs external hardware (in a serial cradle or cable) to convert the levels on its connector.

Is there an easy way, other than trying to track down a serial dock, to do the hardware interface here?..EDIT2: Ohh! I could just use a MAX232 couldn’t I?

A MAX203 would require fewer external parts. Look here for a relevant project.

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.

Steve

Thanks. I have ordered a sample; I’ll see if it works for me when I get it.

JBot