RS232 convertor - Why?

And if you asked where this sort of question belongs, sci.electronics.basics then I likely would provide an answer.

This is not a design issue. This is a basic issue. Grasp what RS-232 is about, and you've got your answer.

Michael

schreef in bericht news: snipped-for-privacy@73g2000cwn.googlegroups.com...

Well,

There are lots of tastes in serial data transfer. The RS232 as used by the COM-ports in PCs are most commonly known by the general public. The signal level vary from +/-5V to +/-12V or something in between. Which is well according to the specifications. The serial data from cameras is at TTL level so 0-5V (or 3.3V these days.) AFAIK the Nikon D200 uses 0-5V. So the point is: What's on the GPS side? If it's true RS232, you'd frie the serial interface of your camera when connecting directly. If it's also TTL the should have the same levels or you may damage your equipment as well. Apparently your GPS speaks RS232 as it is unlikely (to say the least) you to have a working connection by now.

petrus bitbyter

The 232 trip level can be any where between -3 to 3 volts according to the specifications, but nearly all 232 receivers are designed to be TTL compatible. I make it a point not to use those 232 chips with charge pumps for short distance interface.

If you building something just for your own use, check the levels. If everything is above ground, skip the 232 chip. If one of the transmitters goes beneath ground, you need to insure that you don't forward bias the protection diode of whatever circuit you attach to that transmitter. Generally a series resistor will do the trick, but that does slow things down.

If you are trying to market some product, all bets are off.

NMEA I/O from a GPS receiver is TTL swing but opposite polarity to true RS232. So apart from the level shifting - which in MANY cases is not required - the inversion in a MAX 232 (et al) is the key ingredient in most of these circuits.

Regardless of actual signal levels, I'd be betting that the camera has true RS232 polarity at its serial port and can direct connect to a PC's com port. The odd-man-out in these schemes is the GPS receiver. The NMEA signalling system was designed to directly interface (and multi-drop) to other marine electronic devices such as autopilots, NOT personal computers. Some later GPS receivers have USB output to satisfy the market for direct-PC-connection.

NMEA-0183 is TTL levels, but with RS-232 polarity. Almost all PC serial ports will accept the 5 volt NMEA-0183 output from a GPS receiver with no problems.

To convert NMEA-0183 signals to true RS-232 levels, you need an inverter and a MAX 232 or other RS-232 line driver, as the line drivers invert the signals, as well as changing levels.

"NMEA0183" schreef in bericht news: snipped-for-privacy@4ax.com...

Overlooking my information so far I think you're right. If the serial camera interface was supposed to be connected to a PC directly (using the cable provided with the camera?), its polarity and levels should be true RS232. A lot of modern PCs may accept TTL levels (*not* polarity) but you can't count on it. All RS232 drivers I know are inverting ones, so you have to invert again when going back to TTL. (AFAIK TTL with RS232 polatity exists but is not very common). Nikon provides a cable, MC-35, with build in RS232 to TTL conversion ($150,--) apparently to convert the cameras RS232 to TTL and not the other way around as I was originally thinking. A circit like that can be build into a DB 9 connector cap using modern (SMD) components.

petrus bitbyter

NMEA0183 wrote: > On Sun, 03 Dec 2006 10:44:27 -0800, Peter Bennett

I'm 'glad' you can make the TTL work into RS-232 but as one of the guys who has to clean up after things like that, I'm not inpressed. Example: Rosetta Stone RS-232 to RS-422 converter (different I know but wait) uses a few transistors inside to do the inversions and level shifts required. It 'works' with a short cable but run it into a patch bay with longer cables and it falls apart - edges skewed, crosstalk TX to RX so that it's useless. So when I hear of somebody intentinally cheating to make HIS life easier at the expense of mine, I'm not impressed.

GG

Gee, Peter, I'll have to go back and rebuild all those GPSPC interfaces that I've built so far, and which are currently in service { ;^0>

I'm quite used to using TTL swings directly into RS232 devices, but the polarity is not as you indicate on the Rockwell/Conexant Jupiter receivers, for one. These receivers do output true NMEA signalling.

When outputting "NMEA" protocol data (as distinct from Garmin's proprietary protocol), receivers such as Garmin II+/III+ output NMEA sentence structure but NOT true NMEA signals, which are opposite polarity to RS232.

(snip)

(snip)

Whoa there Glenn. I *have* used TTL swings into RS232 devices (and vice versa) a lot, but only for my in-house stuff and NEVER for release outside my cage.

I am fully aware of how *iffy* it can be. So any *cheating* (as you put it) that I do certainly isn't at your expense or anyone else's. But thanks for caring.

Thanks guys. A lot of it is over my head for now, will have to go read up but the question was answered.

As pointed out, Nikon has a MC-35 which is stupidly expensive and bulky. I managed to make the same thing with a =A310 cable from ebay and =A35 worth of parts from Maplin. After building it, connecting it and waiting for the smoke, the camera started flashing GPS which means it found it. Yippeee.

Thanks again for the answers and to the two guys (gals?) having a fight

- chill :)

Cheers, Crispin

Don't buy anything from Maplins. My broadband started to go slow and my ISP said I needed a new filter. Maplins said I needed new cable. I bought new cable but the broadband speeded up again - must have been BT. So I took the cable back. But because it had been machine packed there is no way I would return it "in the same condition". The cable WAS "in the same condition". But the idiots said the packing had to be the same -- impossible!

If they want to get things back "correctly" they should use packing it is possible to pack.

Every other company I've taken anything back to, in living memory has accepted it back with full refund.

Except Maplins.

Don't even think of shopping with them.

I'll bet you're so cheap that you would try to return used condoms.