RS232 to J1962 Comms ideas/queries.

martin at mgass.demon.co.uk martin at mgass.demon.co.uk
Sun Mar 22 14:53:27 GMT 1998


This might be a bit off-subject for diy_efi,  but as it relates 
to J1962 I'll post it anyway.....

ISO9141 describes intialisation on J1962 as follows :
one start bit - logic 0,
8 address bits (LSB first to MSB last)
one stop bit - logic 1.
all at 5bps
With an opto isolated i/f between car and PC, in RS232 terms
I would think the easiest way to achive this is by holding DTR up and 
down for the required number of milliseconds and making sure this 
converts via the opto interface to the K line.....any comments ?

The key word format(from ECU) is one start bit (0), seven bits of key word 1, 
parity, stop bit (1), then a time delay, then next word etc.
In RS232 does the MSB or LSB of the byte go first ?
I saw some references in the archive to 9141 being inverted relative 
to rs232 ?   
Assuming a baud rate of 10.4K (which is near enough achievable by adjusting the 
divisor [by 11] when the mscomm setting (in VB) is 9600,8,n,1) what is the 
nearest mscomm setting to the above key word format, which is 1 start, 7 data, 1 stop 
with parity....or do i have to frig it as in the initialisation by 
very carefull timings using (i.e. send each bit pulse out seperate as 
opposed to using the RS232 TXD line) , say DTR.

Anybody able to offer any advice.

regards,

martin
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martin at mgass.demon.co.uk
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