Re: RS232 convertor - Why?


"NMEA0183" <me@xxxxxxxxxxx> schreef in bericht
news:1lc4n2dopsmt1l5d2cdg8s52v2tv6nn4du@xxxxxxxxxx
On Sun, 3 Dec 2006 00:35:33 +0100, "petrus bitbyter"
<pieterkraltlaatditweg@xxxxxxxxxxxxxxxxx> wrote:


<crispin.proctor@xxxxxxxxx> schreef in bericht
news:1165096222.005283.227770@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
Hi All,


I used to enjoy electronics as a hobby many years ago but sort of left
it behind in the last 10 years.. :(

I have now rekindled the hobby by building myself a GPS attachment for
my camera (Nikon D200)
Soldering skills are not what they used to be but and a few attempts
later and some trips back to the shop for more veroboard and
components, I got it working.

To the question:
The GPS device pumps out serial data. Fine. Happy with that.
The Nikon accepts serial data. Fine.
Why is it that all the circuit diagrams I find on the net stay I need a
MAX232 IC in between? Are the levels different? If so, which is higher?
Would sticking the TX from the GPS straight into the camera cause a
loss of that ever important smoke (Electronic components work on smoke;
once it escapes, they work no more.)

Compliments of a few sites I have found on the net, I built the RS232
circuit and the camera detected the GPS data and all is well.

My question stems from the fact that I have this lovely little GPS
receive all with SMT and a smart little smoked box. Now I have to have
an ugly veroboard black box attached as well. (or stick everything
thing into a larger box)

TIA for any help...

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.

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.

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


.

Relevant Pages

  • Re: RS232 convertor - Why?
    ... I used to enjoy electronics as a hobby many years ago but sort of left ... my camera ... The GPS device pumps out serial data. ...
    (sci.electronics.design)
  • Re: RS232 convertor - Why?
    ... I used to enjoy electronics as a hobby many years ago but sort of left ... my camera ... The GPS device pumps out serial data. ...
    (sci.electronics.design)
  • Re: Nikon D200 / D300 and GPS
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  • Re: Nikon D200 / D300 and GPS
    ... Seems all the $100 cable does is us a $2 chip to convert the serial level RS-232 signals to TTL level. ... Well I have found a GPS that has TTL level output along with the serial level signals. ... is there a way to pull the power the GPS needs to run from the camera? ... If I can avoid making a battery pack for the GPS, that sure would make for a nice very small and simple solution to having a GPS on the camera instead of having it in your pocket with wires all over the place. ...
    (rec.photo.digital)