From: Joseph Legris
X-Mailer: Mozilla 4.77 [en]C-SYMPA (Win95; U)
Subject: Re: CAN Protocol and SAE J1850 Bus Interface Circuit
Date: Sat, 09 Nov 2002 12:31:16 -0500
NNTP-Posting-Date: Sat, 09 Nov 2002 12:31:20 EST
Organization: Bell Sympatico
> I am working on a project where I need to design a module which can
> get vehicle speed information from the communication bus on a car. I
> realized that there are several communication protocols which exist in
> different cars: CAN, J1850 PWM, VPW, and ISO9141.
> I plan to use Microchip PIC18F258, which has a built-in CAN
> controller. And I will write application for the J1850 PWM, VPW and
> ISO 9141. Since I only need to receive messages from the bus, and I
> don't need to send anything to the bus, I think I can build my own bus
> interface circuits rather than buying transceiver chip for each
> protocol. I've seen the bus interface circuit for other SAE standard
> that is recommended by the SAE. And I can't find the one for SAE J1850
> or CAN protocol.
> Other than that, does anyone know what is the update rate for the
> vehicle speed information for J1850? I've checked in the standards but
> not able to find it.
> Any suggestions and help is greatly appreciated!
J1850 doesn't necessarily send any data at all unless a request is sent
first by your device (typically an OBD-II Scantool). For example, a Ford
I recently tested sent only the transmission status on a regular basis -
about three time a second as I recall - otherwise the bus was silent.
Therefore, you MUST both send and receive.
For further info and a head start contact http://www.elmelectronics.com
and look under the CONNECTIONS link.