second project session 19/01/10

Today we mainly did the paperwork for the project.
We selected a project leader, Tom Kempynckis was chosen for this job.
Later on we will post some of the intresting paper work on here.

We also had concerns about the data rates and latency.

The wireless system that we are using is zigbee at 2.4GHz.
So we have a data rate of 250 kb/s to count on and a latency of 16ms.

This was what we have no, what do we need?

Since we are working with obd-2 there are 5 different protocols and all have very different secifications.
Any given vehicle will likely only implement one of the protocols. Often it is possible to make an educated guess about the protocol in use based on which pins are present on the J1962 connector:

• SAE J1850 PWM (pulse-width modulation - 41.6 kB/sec, standard of the Ford Motor Company)
  • pin 2: Bus+
  • pin 10: Bus–
  • High voltage is +5 V
  • Message length is restricted to 12 bytes, including CRC
  • Employs a multi-master arbitration scheme called 'Carrier Sense Multiple Access with Non-Destructive Arbitration' (CSMA/NDA)
• SAE J1850 VPW (variable pulse width - 10.4/41.6 kB/sec, standard of General Motors)
  • pin 2: Bus+
  • Bus idles low
  • High voltage is +7 V
  • Decision point is +3.5 V
  • Message length is restricted to 12 bytes, including CRC
  • Employs CSMA/NDA
• ISO 9141-2. This protocol has an asynchronous serial data rate of 10.4 kBaud. It is somewhat similar to RS-232, but that the signal levels are different, and that communications happens on a single, bidirectional line without extra handshake signals. ISO 9141-2 is primarily used in Chrysler, European, and Asian vehicles.
  • pin 7: K-line
  • pin 15: L-line (optional)
  • UART signaling (though not RS-232 voltage levels)
  • K-line idles high
  • High voltage is V_batt
  • Message length is restricted to 12 bytes, including CRC
• ISO 14230 KWP2000 (Keyword Protocol 2000)
  • pin 7: K-line
  • pin 15: L-line (optional)
  • Physical layer identical to ISO 9141-2
  • Data rate 1.2 to 10.4 kBaud
  • Message may contain up to 255 bytes in the data field
• ISO 15765 CAN (250 kBit/s or 500 kBit/s). The CAN protocol is a popular standard outside of the US automotive industry and is making significant in-roads into the OBD-II market share. By 2008, all vehicles sold in the US will be required to implement CAN, thus eliminating the ambiguity of the existing five signalling protocols.
  • pin 6: CAN High
  • pin 14: CAN Low

Note that pins 4 (battery ground) and 16 (battery positive) are present in all configurations. Also, ISO 9141 and ISO 14230 use the same pinout, thus the connector shape does not distinguish between the two.

When we look at the data rates of ISO 15765 CAN and SAE J1850 PWM the data rate af our "medium" Zigbee is not sufficient.
So we will have to read the data of the OBD-2 via RS232.
Since OBD-2 to RS232 cables are widely and most likely used for this sort of applications we will use a cable like that.
The data rate of RS232 is 75 bits per second, so now we have plenty of speed provided by Zigbee.