I have one gateway linked to fire alarm control panel, and I need transmit events from control panel to two pc with the same gateway. I need a multiplexer RS485 Modbus. Have you found a solution about that?
Way too little information to go on.
1. Gateway is a very generic term. What exactly is the existing gateway? What is this gateway capable of? Why can't it do what you're asking?
2. What protocol do the 'two pc' (sic) talk? Do these PC's have any Modbus functionality already?
3. What is the function of the presumed 485 mulitplexor? What is being multiplexed and where and how is the multiplexed data being demulitplexed?
4. Is the fire alarm panel a Modbus master? Slave?
5. Multidrop Modbus/RTu over RS-485 is a sequential polling protocol, slaves are polled in sequence. Why isn't that sufficient?
I'm not sure that hardware is your problem . . .