Serial Multiplexing is the main function of the SS20 SCADASwitch and is enabled by default. The SS20's multiplexing functions allow two independent asynchronous communication channels to share a third communication channel without contention of the channel or packet collisions.
The basic theory of operation is incoming messages from the two input com ports are routed to the output port (as both RS-232 & RS-485). After an incoming message (request) is sent out the output port, the SS20 waits for a response from the downstream device or network until either a response is recieved or the specified timeout period expires. If a new request comes in from the other input port while the SS20 is waiting to receive a response it will buffer the request until the first transaction is completed either by response or timeout. Once the transaction is complete the buffered request will by sent out the output port.
The SS20's multiplexing capabilities are only applicable to poll-response type protocols where every transaction is initiated by the host or master device poll, followed by a single response from the slave RTU. Modbus RTU and Fisher ROC protocols are good examples of protocols that follow this architecture. Protocols where the slave RTU can push unsolicited data back to the host (Report by Exception) or where a response to a poll is sent back in multiple packets are not compatible with the SS20.