A total of 15 on a relatively short run of RS-485 in our experience to keep Vista live diagrams loading completely in under 5 seconds.
James is correct from a systems performance point of view. Indeed I would say for 5 second updates I'd go as low as 6 devices, depending somewhat on device type. Most of this is due to device communications latency and the PME comm architecture (multiple clients asking for different data from the same device) and not the RS-485 spec. If the system does not require reasonably fast real time response then more devices can be on one line.
From a hardware specification point of view the RS-485 spec allows for 32 devices per loop and a wiring length of 4000' / 1000 m.
As one gets gets more devices and longer wiring lengths strict adherence to the spec is required, i.e. no stubs or T's or taps are permitted, the cabling must be designed to RS-485 (correct impedance) etc.
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