What is an appropriate number of instruments per bus segment?

The impact of host system design factors on subnet design Users’ requirements regarding host system design often have a direct or indirect influence on subnet design. Taking the number of field instruments connected to a network segment as an example, various aspects of the host system design can affect the design of that network segment: The system’s risk management design specifies the number of control loops in each network segment, and may even prohibit instruments belonging to other loops from being connected to the network segment where a particular control loop is located. In this way, the network segment may ultimately have only 2 to 3 field instruments connected to it. 　　The system’s limit on the number of function blocks that can be used per network segment, or the limit on the number of VCRs per network segment, can both become constraints on the number of instruments that can be connected. The system limits each IP range to 64 function blocks that can be used. Some instruments have 8 function blocks, or even more. 　　The system’s time limit on network segments can directly become a limit on the number of instruments that can be connected. For example, for a time requirement of 1 second, the maximum number of instruments that can be connected is 12, including 4 valves ； For network segments requiring an execution time of 0.5 seconds, the maximum number of instruments is 6, including 2 valves ； For a network segment requiring an execution time of 0.2 seconds, only 3 instruments equipped with 1 valve each can be connected.