How to determine 485 network is not busy in half-duplex

[ecoli-557]

Darrel-
I have studied the J1708 and feel I understand how it works and how I can use it for my own applications. My question (at least for now - grin) is there should be no limitation of 20 devices IF I use 1/4 or 1/8 loading 485 transceivers = correct? The 20 device load was designed when they were heavier on the line I am guessing?
I also plan on using 57.6k as my speed thus making my bit time 17.3 us for a total bus idle time of 173 us - also shouldn't be a problem?
Won't be using any PIDs but the MIDs will just be the address on the network - lower numbers get higher priority on arbitration.
I did also like the message delay priority - might try to use that!
Lastly, probably use just a simple checksum without the inversion and add 1 - don't think I need it.
Have you used this for something other than trailers?
Regards,
Steve

[Darrel Taylor]

I think the limitation of 20 nodes and 9600 baud is related to the huge caps on the A/B lines.
Trucks have a very noisy electrical environment that would benefit from the reduced slew rate.

If you are using the diode termination method (no caps) you should be able to get much high baud rates and many more nodes.
I've run DMX at 250K baud with the diode method. But of course that was a 1 master, many slave arrangement.

I've never used the full J1708 protocol ... just the arbitration process on a system that only had a max of 5 master nodes.