I've been asked to think about a client-server, solution for a new life stage art project.
Where there could be around 240 slaves to control led's and some other stuff on a life stage.
Obviously 240 computers is to expensive, while the tasks are simple, so Arduino became their choice.
They require to have fast communication between server and client, and the client should also respond back, (each arduino has a sensor, to act on nearby actors / dancers).
The whole actor play depends a lot on this.
The server side will be a PC, not an Arduino.
Their special effects man thought about RS485, since those modules are cheap too.
But if that couldn't do the job then take cabled ethernet modules.
They could make the cables themselves, only the switches might then get go expensive (240 ports switch??) But if RS485 wouldn't be able to do it, then you program it for ethernet he told me.
So now I'm a bit in doubt of the design here.
Because if i would use RS485, and send broadcast commands (i hope arduino RS485 libraries do support that), to update all clients at once, would go reasonable fast. But if each client would need to reply back, and just lets assume that it would take 50ms to answer back, ea a signal acka a frame on rs485 will be 50ms from client to server.
( 50ms is a random number out of my head, but I need to have some safety here,
I saw 50ms in some documentation on a project with 4 slaves ).
Then letting each client talk back in a time-slot fashion (since there will be frequent communication).
Then each Arduino would use his node ID multiplied by 50ms, to wait, and then send a reply, then the total wait would be, 240 * 50 ms = 12000ms = 12 seconds. (25ms would still be 6 seconds)
Which would be way to slow for a life stage act.
Should i use perhaps 2 RS485, devices per Arduino, so a Signal and a reply line ?
Or should i go for Ethernet 100mbit/sec might be a lot faster.
Long stage cabling, combined with RS485 might drop speed on RS485 quite according to wikipedia around 100kbit/s, for distances more then 10 meter.
(Note cannot use Wifi since cities have many wifi signals, the signalling might get suppressed, or worse jammed.)