Brimesh Modbus Proxy
We have installed a Brimesh GY-G300 Modbus Gateway as a proxy between systems. This device is probably more powerful than I give it credit for, but much of the documentation is poor. The User Interface is not totally translated into English, and the manual, even in Chinese is missing significant amounts of documentation. This gateway has three RS232/RS485 ports and an ethernet port. In my case I poll an RS232 Network, an RS485 network and another TCP Modbus network, aggregating the data within the single device. It can then be polled itself. There are indications that it can upload Modbus, or send it via MQTT, but these options are poorly documented and I have not been able to get them to work.
The only downside with this unit is that it seems to do a restart every 24 hours or so, resetting all the read data to zero.
The cute thing is that you can see the status of all the devices you are monitoring in real time.