RuggedCom RS400 manual Worked Example, Use of Turnaround Delay

Serial Protocols

an exception to the originator. If sending exceptions has not been enabled, the Server Gateway will not send any response.

2.2.2.2 A Worked Example

A network is constructed with two Masters and 48 RTUs on four Server Gateways. Each of the Masters is connected to a Client Gateway with a 115.2 Kbps line. The RTUs are restricted to 9600 bps lines. The network is Ethernet based and introduces an on average 3 ms of latency. Analysis of traces of the remote sites has determined that the min/max RTU think times were found to be 10/100 ms. What time-out should be used by the Master?

The maximum sized Modbus message is 256 bytes in length. This leads to a transmission time of about 25 ms at the Master and 250 ms at the RTU. Under ideal circumstances the maximum round trip time is given by: 25 ms (Master->client) + 3 ms (network delay) + 250 ms (server- >RTU) + 100 ms (Think time) + 250 ms (RTU->server) + 3 ms (network delay) + 25 ms (client- >Master). This delay totals about 650 ms.

Contrast this delay with that of a “quick” operation such as reading a single register. Both request and response are less than 10 bytes in length and complete (for this example) in 1 and 10 ms at the client and server. Assuming the RTU responds quickly, the total latency will approach 35 ms.

The server can already be busy sending a request when the request of our example arrives. Using the figures from the above paragraph, the server being busy would increase the end-to- end delay from 650 to 1250 ms (additional 250 ms (server->RTU) + 100 ms (Think time) + 250 ms (RTU->server)).

The preceding analysis suggests that the Master should time-out at some time after 1250 ms from the start of transmission.

2.2.2.3 Use of Turnaround Delay

Modbus protocol uses the concept of a turnaround delay in conjunction with broadcast messages. When the host sends a broadcast message (that does not invoke an RTU response), it waits for a turnaround delay time. This delay ensures that the RTU has enough time to process the broadcast message before it receives the next poll.

When polling is performed over TCP, network delays may cause the broadcast and next poll to arrive at the remote server at the same time. Configuring a turnaround delay at the server will enforce a minimum separation time between each message written out the serial port.

Note that turnaround delays do not need to be configured at the host computer side and may be disabled there.

RS400 61 ROS™ v3.5