Serial to Ethernet converters can come in different varieties.
Serial to Ethernet Connector that allows you to work with Modbus to Ethernet. It enables you to provide shared access to a local Modbus device over network, so other network users can reach its content and functionality as if it were attached directly to their machines. In a similar way, you too can access a remote Modbus over Ethernet regardless your physical proximity to the device.
Some converters, such as Lantronix, will simply receive serial data, encapsulate it in a TCP packet and send it to the desired IP address. In this case, the underlying Modbus protocol is still the serial "Modbus RTU" or "Modbus ASCII". You will want to use the "0 RTU (CRC16)" or "1 ASCII (LRC)" format in your Modbus Unit configuration for the MatrikonOPC Server for SCADA Modbus. If you are using the MatrikonOPC Server for Modbus (non SCADA) you will want to use the "TCP/IP Terminal Server".
Some converters, such as Moxa and Garretcom, will go one step further and convert the Modbus protocol from Modbus RTU to Modbus TCP. This is useful in cases where multiple masters, such as redundant OPC servers, need to access the data. Modbus TCP differs from Modbus RTU, in that its messages do not use a CRC/LRC, and each message is prefixed with a 6-byte header. Make sure you choose the format "2 RTU TCP" in the Modbus Unit. If you are using the MatrikonOPC Server for Modbus (non SCADA) you will want to use the "Modbus Ethernet PLC (TCP/IP)".
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You have to define the IP address of the Serial to Ethernet converter and the Modbus address. The Modbus address depends on how your converter is configured. When it's configured to have Modbus slave(s) connected, the Modbus address = Modbus address of the slave. Be aware that you set your OPC request speed at a level that can be handled by the converter and your Modbus device.
When it's configured to have a specific slave connected, the Modbus address of the slave is probably defined in the converter. You should read the converters manual how to address the slave.
When a Modbus Master is connected to the converter, your OPC server should be able to receive (unsolicited) data form the converter. I'm not sure if Matrikon OPC can do this.
There really are so many different ways to do this...
Some vendors (such as Schneider) provide ModbusTCP to ModbusRTU gateways.
Others like Perle / Moxa can also provide just pure serial<->ethernet converters with various features available.
I know that both Perle / Moxa have a driver that can be installed which can enable the converter to appear identical to a local serial port, down to DCD/CTS/DTR/RI (modem signal) emulation and all.. Moxa call this RealPort I believe.
In such a mode, it's possible to use it on a multidrop RS485 network (with multiple ModbusRTU masters if required).. the likes of Kepware / Matrikon typically support these just fine.
When using the RawTCP (aka Telnet) mode, then you just setup a straight TCP socket (often it can be configured who will be the initiator) and then bytes received on the serial port will just be sent across the TCP socket, and bytes received on the TCP socket will be transmitted out the serial port. Typically in this mode you lose the flexibility of dynamic changes to baud rate, or modem signal support, but you also have less software layers involved, so fault finding can be easier.
I think it is all about horses for courses... certain situations will exhibit a preference for one solution over another.