Serial - what happens if you tie +5v to rs232_txd (pin 8)..?

[this_mike]

Hi there, this is my first stupid question on xda-developers! In essence, does tying the rs232_txd pin high do any damage to the driver or device itself?

That may sound like a dumb question, but I am having no end of trouble with running a serial GPS mouse with a car holder/cradle I bought for my XDA IIs (unbranded, silver plastic eBay job with a speaker), read on if you can stand to!

The cradle takes power from the ciggie lighter socket and has a mini-usb type connector to drive the gps mouse. I could sometimes communicate with the gps, but not always and not often for more than 30 mins.

I took the cradle apart and found that only the XDA's rxd pin was connected to the gps, no txd and decided that looked iffy enough to alter.

Fortunately, we have a tiny baby ( :D ) so I was a bit too tired to do this right, and after tracing all the connections meticulously, wired txd to the +5v vcc feed. Not realising exactly what I had done (really, really tired - it made sense at the time!) I hooked all up to a dc psu and got rock solid gps connectivity. Nothing could shake it, it always worked... (Investigated with CommLog, WinFast Navigator, GPS Info & TomTom GPS software).

Further thought with a clearer head made me drop my towel! I don't seem to have done any harm (I left it running like that for a while) and everything else works ok so I don't think I've killed anything, but should I be doing this?!

It basically means putting a short from v_adp to txd... In the olden days of 'proper' rs232 (mumble into beard, slurp pint of bitter), that would have been a 'bad thing' as far as I understand, but I have no idea if it is a good idea or not with a Blue Angel...

To make matters more interesting, I noticed that the rxd connection that was in place originally was connected to the gps mouse's tx pin (according to the globalsat pinout I have) in the first place.

In the interest of discovery, I connected the rx and tx properly (nothing happens except the cradle led changes to indicate no gps connected) and swapped rx and tx (with exactly the same results).

If I slap +5v on TXD, it all works, but even the big friendly letters on the front cover don't calm me...

Help? Please? :lol:

[cruisin-thru]

I have just converted a digitraveller gps for use with xda 1 and I used pins 5-ground and 8 and 10 for rx and tx, I have no idea why you should get better performance and connectivity by adding 5volts onto the tx line unless power is fed through that pin for that particular mouse. There is no way that I would have tried it in the first place so fair play to you for the abberrant thought process that brought you a positive result. Check this link, it mentions tying voltage to the rxd to wake up the gps.

http://www.grundlgasse.at/thomas/digitraveler/

[this_mike]

Hiya cruisin-thru, thanks for the fast follow up and link (love the microTrak those Floridians made, I'll need one for our daughter in a few years!)! It's interesting, he is talking about tying the gps mouse's rxd pin high (so, presumably, the PDA's txd pin too) which is basically what I did by mistake. My grasp of the physical side of these things isn't too hot - am I right in thinking that that is what he means?

Aberrant thought processes just about sums it up tho... It was one of those days when you carefully trace the connection all the way through all 4 pairs of interfaces, making careful notes, then painstakingly hook up the wrong connector. :roll:

I guess it all comes down to my concern about the XDA's ability to handle 5v on the txd pin over long periods of time... If anyone has any experience of this I'd be really grateful to hear about it!

[this_mike]

So. The RS232 spec says that each pin should be able to cope happily with a voltage applied to it equal to the maximum operating voltage of the device hosting the connector. That means that if HTC makes them to spec, I haven't done any permanent damage!

It also means that tying txd high should be a feasible solution although I'm not going to do it. The cradle I was trying to fix is now fixed and can be thrown away with my conscience clear - it's one of those with an external speaker and the rf generated noise is getting to me...

Thought I ought to tidy up after myself and close the thread!