NMEA WiFi Bridge

!! Software and library updates in github Jan 14, 2016 !!  

Updated Fritzing schematic Apr 28,16

 

The second strip-board version of my NMEA WiFi Bridge. On my boat and several others, we are running OpenCPN for navigation software. Most of us run it on a laptop. The common inputs are GPS, AIS and some have all their instruments connected as well; depth, wind, speed & distance log, etc.

There are two problems that I am trying to solve. First, too many cables to the laptop providing the instrument data. And second, a way to share the instrument data with multiple devices. The release of opencpn for android tablets and smart phones makes it feasible to have multiple displays around the boat. Like in the captain's bunk to check on crew without getting out of bed.

So this friend that wants all of the instrument data over the network has a multiplexer that takes his SeaTalk and various NMEA 0183 signals and outputs a higher baud NMEA sentence data stream. And I have a combination AIS/GPS that has NMEA 0183 at 38400 baud. My laptop is getting old and if you touch the USB cable from the RS-232-to-USB it disconnects and reconnects using a different port configuration.

And besides it would be nice to have no cables, especially to a phone or tablet.

Along comes the ESP8266 Serial WiFi module with a modest price around $2.50 USD and other board configurations for a few dollars more.

So the plan is to create new firmware for the module, connect the rs-232 based NMEA 0183 signals to it and broadcast the sentences over the WiFi as UDP packets on the local subnet. So this will require a WiFi Access-point (AP) on the boat. (ED. It may be possible for the ESP to be the router too.)  Another recent development is the availability of cheap pocket routers such as the TP-LINK WR702N. I picked mine up in Singapore for less than $8.00 USD last year, but the market is full of competitors.

The +5v power (from a 12v automobile USB charger module) comes in through the switch, upper left then a 3.3v regulator and then to the ESP module's socket. It is a little sensitive, so pull-up resistors are recommended on the 3 data lines, GPIO2 and Rx) and the chip select and reset pins. Tx and Rx are needed for programming (flashing) the firmware, and as a development board I chose to bring those to the top right pins with a ground for a future test of a 16x2 serial display. 

The NMEA 0183 from a Garmin and my AIS sort of conforms to rs-232, they are -2.9v to 2.9v. Therefore, after a little search, I found a single transistor level shifter for the Rx input to make it conform to the 3.3v TTL levels of the ESP module.

The LEDs and pins at the bottom show the state of the 2 GPIO lines. The software configuration technique uses a short between these to go into config mode. The push button puts the module in flash mode.

And the schematic of the first rev of the board. It is also the one in the fritzing project on my github site along with the code in C.

Figure 2. Schematic_v2

Parts List:
 1 - ESP8266-ESP-01
 1 - LD1117-33 regulator (any 3.3v > 500ma  will do)
 1 - 1uF tantilum
 6 - 4.7K ohm pullup resisters
 1 - 330 ohm current limit for Tx buffer
 1 - 10K ohm for rs232 level convert 1 - 2n2222 for level convert
 1 - 1N5819 program Rx input
 1 - LED
 3 - momentary pushbuttons


The design of the code has three parts.

First, short the two GPIO lines(press the config button) and turn on the power. After a few seconds (5-6)  the LED comes on indicting the module is ready to configure. It starts an AP at 10.1.1.1 that you can connect to with a WiFi capable device (laptop or phone or tablet using DHCP), then access the webpage at that IP. You get this page:

Enter the ssid and password for your boat network, the UDP port to connect and the baud rate to listen for the NMEA data.

When you click configure, it displays a confirmation of your data.

If it is correct, turn off the board, connect it to you NMEA data, remove the short between the GPIO pins and turn on the power.

The second part of the code now runs. It listens for NMEA sentences and when it has a complete sentence, broadcasts it to the broadcast address of the subnet you provided in config. All devices on that subnet can read these UDP packets.
Now go to your opencpn configuration settings, then connections. Add a new connection, selecting: network, UDP and the port number from above (default is 10110).
You should be getting data.

The third part consists of a status page. You can look in a web browser at the IP address of the NMEAWiFiBridge and get a status page that updates the number of sentences broadcast every 10 seconds. It may severely impact the ability of the ESP to keep up with a busy data stream to monitor it, so it is only there to help with trouble shooting the connections.

See my github repository for code and schematics. For faster communication, I can be contacted through my github profile (click on my user name).

The configuring software is based upon the work of Forward Computing and Control. My hardware is opensource, my software is GPL3.

Xprotolab Scope Helps RS-232 Problems

I have been working on another project for a while. It involves reading serial data over an rs-232 interface. Actually from a Garmin GPS and a Ocean PC AIS.

I am sure that I read somewhere that Garmin's NMEA data out was not the rs-232 positive and negative voltage, but just 0v to 5volts. So I spent several days (part time only) failing to read the data. I was not sure if it was the logic-level shifting from 5v to 3.3volt for the chip I am using or a software problem.

I tried checking with my cheap digital volt meter and learned nothing.  I even got out my tiny Xprotolab oscilloscope. I was poring through the manual to verify the input voltage because I thought I remembered it being only a 3.3 volt part. BUT it is -14 volts to +20 for the scope part, the 8 channel logic analyzer part is indeed 3.3 volt.

Unfortunately, on the 128x64 bit 0.9 inch display, I decided it was indeed a 0-2.9 volt signal.

XMEGA Xprotolab Scope

Another day later and I became even more suspicious of the levels so I started looking for more ways to check. That's when I noticed that in the Xprotolab's manual (a pdf of course) they have a PC interface and a display program for the PC to make a little more readable output.

I downloaded the Linux version and very quickly learned that the Garmin 72H that I was listening to and my Standard Horizion radio w/AIS reciever both put out a plus/minus 3.3 volt rs-232 over the NMEA 0183 data lines.

That was it! So a little time searching the web, as I didn't want to include the MAX232 in this project. I found Alternatives of MAX232 which shows a single transistor, single line rs232 ->ttl converter.

Great! now back to the original problem and the small matter of software! A lot of days I wish I had a real oscilloscope, 200 khz on th e 1" display doesn't help with a lot of stuff. But it does help sometimes.