Did my GPS update yesterday. I constructed the cable to connect to the unit itself using the parts listed in this thread. I'd also ordered a spare unit in the event I wasn't able to get the update to work/something went wrong, etc (that shipped with the newest firmware, 4.40/20191007). In the interest of saving some other folks some time here are some things I discovered.
TL;DR: Updating at the GPS itself is reasonably easy, I used 4.40 from the website (links that the bottom), its working perfectly fine. Just use a normal RS232 adapter of good quality, it'll all work fine, a standard FTDI serial cable is a solid choice. You don't need to build the adapter cable, .1" IDC (Dupont, Breadboard Jumper Wires) female connectors will fit the pins and clear the socket fine. The devices power fine at 5V0 (which is roughly what I measured at the connector for the car), 12V0 seems fine as well, but I suspect you're working their voltage regulators a bit hard.
I have supplies to build another few cables for this update approach if people are interested; I can assemble them and loan them out. Send me a DM. If you're in the area (all 3 upstate NY owners) we can probably arrange a time for me to help you do it.
Having worked with the AmpSeal connectors before (whats used on the VMS), they're not designed for a lot of cycles and get brittle/fragile as they age. So I opted to do the update at the GPS itself.
Access through the wheel well to the connector was pretty easy. With the cable and info I have now I believe I could do the job in 60-90 minutes without issue. I was able to do it without needing to remove the wheel. I turned the wheel full right lock and reached in between the wheel and fender. Removed the light access hatch. Removed the front fasteners on the arch liner and gently bent it back (soft plastic) to the shock. Then removed the plastic front wheel arch bulkhead (several nuts and bolts).
I've had that area apart a few times before, so its possible my connectors were not exactly in the factory locations. I think my stuff is close to factory locations, as the GPS wiring shares the loom with the headlight connectors.
The signaling is 0-5V (which is fine for low voltage serial). This is noted in the
data sheet for the unit on page 12:
"The GPS 18x LVC/18x-5Hz interfaces to a serial port. The unit accepts TIA-232-F (RS-232) level inputs and transmits voltage levels from ground to the input voltage, TIA-232-F (RS-232) polarity."
The key language here is that it accepts TIA-232-F (standard serial, in both directions). Given that the unit is designed for low voltage applications, I wanted to see if I could get it to work with a TTL serial adapter. You can, however, because of it being TIA-232-F compatible the signaling in both directions is effectively inverted. As observed by me on my scope and noted in this
stack exchange post.
As is noted in that post, if you have a real FTDI based TTL serial adapter you can use
FT_PROG on Windows (or one of several Linux/Mac utilities) to reconfigure the device into inverted mode. Doing this I was able to interact with the spare unit I purchased without issue. However, this didn't work reliably in the car (dirty contacts? excessive resistance due to cable length? unclear); I'd get partial NEMA sentences, with occasional garage data in them. Giving up and swapping to a normal serial adapter resolved the issue. Some TTL serial adapters are already configured this way (supposedly, the "DSD TECH SH-U09C5 USB to TTL UART Converter Cable with FTDI Chip Support 5V 3.3V 2.5V 1.8V TTL" is; though I've not validated this).
Assuming your serial cable and power supply have a common ground, you don't need the data ground pin (they're commoned at the device), so you really only need 4 wires (VCC, GND, TXD, RXD). As the unit will run at 5V0 with quite low current requirements, tapping from USB is fine.
Yes, most of the other cables people have made used full voltage serial, so I should have started there, however I wanted to better understand why that choice was made...
As others have observed, you'll want the
SNSRXCFG software and the
latest firmware. I found the SNSRXCFG software worked better under Windows 10 if run as admin. The
update instructions are more or less correct, though I did find under Win10 that I needed to run SNSRXCFG as admin (the Windows install/update detection compatibility laye, will attempt to UAC elevate the updater and fail otherwise).
Thanks for the help! Let me know if folks have questions.