Signalling voltage question

[JBotAlan]

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?

[Eldarion]

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... :)

[Alan Anderson]

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 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 (http://big.lol.free.fr/visor/uk/index.html) for a relevant project.

[Kevin Sevcik]

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 (http://www.maxim-ic.com/quick_view2.cfm/qv_pk/1780) which can operate from a 3.3V supply.

EDIT: In line with Alan's suggestion, there's also the MAX3233E (http://www.maxim-ic.com/quick_view2.cfm/qv_pk/2008) hat doesn't require external caps but still runs off 3.3V

[Steve_Alaniz]

Almost. But the MAX232 chips need a 5V supply and thus have 5V TTL outputs.

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 think you more likely want something along the lines of the MAX3232E (http://www.maxim-ic.com/quick_view2.cfm/qv_pk/1780) which can operate from a 3.3V supply.

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

[JBotAlan]

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

JBot