Using the FT232R to connect a USB port to the Garmin RS232 port
Posted by Neil Rush at
I'm currently working on a design for a project to connect the RS232 port on a Garmin eTrex GPS receiver to the USB port on a computer. I've settled on using the FTDI FT232R as the bridge between the RS232 and USB ports. I've currently got pinouts for the Garmin RS232 port (as it uses a proprietary plug, not a DB-9) and a USB port, and I've also located a datasheet for the FT232R. The FT232R datasheet has application examples which told me basically all the wiring that I need to do, but when it comes to connecting the RS232 pins, I encountered a problem:
According to the datasheet, the RS232 pins on the FT232R should be connected to an RS232 level converter. After researching what this was, I discovered that it is an IC used to convert the voltages provided by an IC (0 - +5) to those used for RS232 communication (-14 - +14 apparently), however, the GPS in question only uses 3V, so do I still use this converter, or do I use one with differant voltages (-3 - +3v?), or do I just omit the converter?
I've attached a GIF showing my circuit diagram so far. It shows the circuit based on the application example from FTDI including the level converter.
The CBUS pins have the following assignments:
CBUS0: TXLED
CBUS1: RXLED
CBUS2: GPIO2
CBUS3: GPIO3
CBUS4: SLEEP#
I've also attached the datasheet for the FT232R in PDF.
Replies
-
Hi Neil,
I have just had a quick look at this (I have an etrex and was looking into something similar a few years back!).
The GPS has a power input which I believe is limited to 3V or something very close.
The RS232 signals are not true RS232 levels as far as I can determine from web searches, but are "inverted TTL data" (which I interpret as TTL voltage levels but the data levels are inverted) so without the actual RS232 level translating chip and suitable for short distances use only.
Have a look at http://www.avrfreaks.net/index.php?name=PNphpBB2&file=printview&t=63469&start=0 for some hobbiest info.Garmin seem to indicate it is RS232 compatible with many instruments so a quick check with a scope could help!
Regarding rs232, the line levels are in the RS232 spec and not affected by the instrument power supply voltage. This is indicated by your example of the 5V powered level translator providing +-12V outputs.
-
So, just to confirm; I can connect the FT232R's pins direct to the GPS? But those pins on the FT232R are active low, so do I need an inverter/NOT gate?
-
Neil, this is where you need to research more. My understanding is the eTrex is TTL compatible but whether you need the inverter, I just don't know!
After posting yesterdays reply I also thought you may need to check the FTI device levels as some are 3.3V levels now which you may need to account for.
While doing a web search yesterday on this there were lots of posts where people were investigating whether to invert or not, but unfortunately most threads ended with no results being confirmed! The frustration of modern information! However the link I posted seemed to be the most complete and I hope that helps you to solve the design requirements.
Please let us know how you get on and even share your design?