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Serial Port Expander for Modbus Master Device
I Need RS-485 Port Expander for use on Modbus Slave Device and send data through single port to 4 Nos. Master Devices.

I need to connect a slave device with 4 mater devices. Slave device has only one RS-485 port. I need to poll data from this slave device by 4 master devices. Is there any device available that can be connected on RS-485 port of slave device to expand one RS-485 port of slave device to 4 Nos. RS-485 or RS-232 ports to send data to 4 master devices?

Modbus RTU is a single master network. If you want to use multiple masters you will need a device capable of sharing slaves with multiple masters like the TSH series from ICP DAS. In this case you will need 3 TSH devices to connect the 4 masters to the slave. Possibly you can find other suppliers with devices to connect more masters.
A second solution is the use of a modbus TCP to RTU gateway but you can only use this solution if your modbus masters can handle modbus TCP. Modbus TCP can handle multiple masters.