The goal of this project is to create a circuit that will listen to the Harley Davidson's data bus, J1850 VPW, then communicate data that would otherwise not be on the instrument cluster to the rider via LEDs or some other type of visual indicator. At a very minimum, I'd like to know engine/water temperature and have a shift light indicator that turns on at a specified RPM.
Rather than writing code to read the data bus in arduino and then porting the code to Atmel Studio for use on another microprocessor, I've managed to capture the J1850 VPW data and decode it using my Saleae Logic Analyzer and Microsoft Excel. As a test, I captured 5 seconds of data using my logic analyzer shown below.
To interface the 5v logic analyzer with a signal from the data bus that had a high of 7v, I used an emitter follower, and took advantage of the fact that the output on the emitter follower would be a diode drop lower than the input on the base of the transistor. I used the below schematic to read the bus using the logic analyzer.
The +12v was connected to the motorcycle's +12 on the J1850 connector. The grounds between the motorcycle and the logic analyzer were connected together.
After I captured the data on my $10 logic analyzer, I exported to CSV, selecting "between the specified times" in the "samples to export" dialogue box. This way it only exported the change in state and timings rather than every sample that was collected.
I opened the csv and created formulas within excel that would take the timings, determine if they were active or passive 1's or 0's, then combine every 8 bits within a frame, and finally convert the binary to hex.
The first step in this project was to scope the signal on the data bus, which uses J1850 VPW, to verify I had the right source for the data, verify the voltage, and verify the signal itself. I had a few automotive 3-pin connectors lying around, so I spliced in a connector on the ECM tuning cable. The connector was 4-pin, but only 3 pins were used: GND, +12v, and Data.
In the above image you can see one transmission that was captured on the data bus. The high signal was 7 volts, as it should be.
Inverting and printing the image, I was able to decode the signal, which was transmitted as a total of 7 bytes. The header was 3 bytes, the message was another 3 bytes, and there was 1 byte for CRC (cyclic redundancy check) which I have yet to figure out if I want to verify the integrity of the message received.
The next step in the project will be to create some code to automatically capture and decode this data, then send it via serial com (most likely using an Arduino for simplicity) so I can see the HEX values of the data that was sent.
28 1b 10 02 xx xx : rpm, xxxx = rotations/minute * 4
48 29 10 02 xx xx : speed, xxxx = km/h * 128
48 3b 40 xx : gear in neutral if (xx & 0x20),
: clutch engaged if (xx & 0x80)
48 da 40 39 xx : turn signals, xx = 1,2,3 for left/right/both
68 88 10 03 : check engine indicator off
68 88 10 83 : check engine indicator on
a8 3b 10 03 xx : current gear, xx = 1,3,7,15,31,63 for gears 1-6
a8 49 10 10 xx : engine temperature, xx = degrees Fahrenheit
a8 69 10 06 xx xx : odometer, xxxx = ticks, each tick = 0.4 meters
a8 69 10 86 xx xx : same as above, but a wraparound occured
a8 83 10 0a xx xx : fuel consumption, xxxx = ticks, each tick = 0.000040 liters
a8 83 10 8a xx xx : same as above, but a wraparound occured
a8 83 61 12 dx : fuel gauge, x = level (0-15)
A visual display interface , also called a visual display unit or VDU and colloquially as a screen, is any electronic visual display device for computers. VDUs are used for visual data presentation, for example, computer monitors and television sets for computer video display, or for other purposes. A visual display unit is a general-purpose computer peripheral which generates visual images.