Hi Guys,
Has anyone found a better description of the Modbus Protocol than the
one at http://www.modicon.com/techpubs ?  I have one of those serial
port "sniffers" connected to two PLCs that are talking on the modbus.
The master says (in RTU hex):
01    (start of header)
03    (??)
0f    (slave source address MSB)
f6    (slave source address LSB)
00   (??)
01   (number of bytes)
67   (rev 16bit CRC)
2c  (??)

I am trying to find out what the (??) bytes are as shown above.
After I understand the protocol a bit, I hope to make a PIC a slave.
Anyone been down this path?

Thanks a bunch,
Greg