Jay, regarding number of nodes, you are correct that there are limitation in the protocol and by hardware. (According to EIA standard: RS485 can have max 32 drivers/receivers on line).
Some devices with Modbus support, use the whole 8 bit address range 1-255 (0=broadcast), others a limited number of adresses, like only 1-31 or 1-247.
The hardware/electric restriction can be avoided with use of repeaters.
A RS485-repeater can be used to:
- increase the lenght of a network
- increase number of nodes
- make network-segments
- make a legal ”tree” or ”star” structure. (Branched- and star-networks are not allowed with RS485, only line structure with a limited lenght of stub cable)
The driver/receiver on each side of the repeater must be counted according to the 32 limit, so with 7 repeaters on a straight line a maximum of 2*31 + 6*30 = 242 nodes can be used!
This is the hardware limitation, but as Pierre stated, the communication turnaround-time increases fast with increasing number of nodes. For best performance, more than 1 comm.port should be used.
Regards
Karl Egil Liaset