RuggedCom RS400 manual Rtu

Serial Protocols

Master	Client	Server	RTU
	Gateway	Gateway
	1		Transmission time from
1a		2	Master to Client Gateway
			Network transmission time
		3a
	3b	4	Queuing time
		5	Transmission time from
			Server Gateway to RTU

6RTU "think" and transmission times to Server Gateway

	7		Network transmission time
8		9a	Transmission time from
			Client Gateway to Master
	9c	9b	Time-out / Retransmissions
9d			complete, Exception sent

Figure 35: Sources of Delay and Error in an End-to-End Exchange

In step 1 the master issues a request to the Client Gateway. If the Client Gateway validates the message it will forward it to the network as step 2.

The Client Gateway can respond immediately in certain circumstances, as shown in step 1b. When the Client Gateway does not have a configuration for the specified RTU it will respond to the master with an exception using TCPModbus exception code 11 (“No Path”). When the Client Gateway has a configured RTU but the connection is not yet active it will respond to the master with an exception using TCPModbus exception code 10 (“No Response”). If the forwarding of TCPModbus exceptions is disabled, the client will not issue any responses.

Steps 3a and 3b represents the possibility that the Server Gateway does not have configuration for the specified RTU. The Server Gateway will always respond with a type 10 (“No Path”) in step 3a, which the client will forward in step 3b.

Step 4 represents the possibility of queuing delay. The Server Gateway may have to queue the request while it awaits the response to a previous request. The worst case occurs when a number of requests are queued for an RTU that has gone offline, especially when the server is programmed to retry the request upon failure.

Steps 5-8 represent the case where the request is responded to by the RTU and is forwarded successfully to the master. It includes the “think time” for the RTU to process the request and build the response.

Step 9a represents the possibility that the RTU is offline, the RTU receives the request in error or that the Server Gateway receives the RTU response in error. The Server Gateway will issue

ROS™ v3.5 60 RS400