Presentation / operationPage 1-10 ABB Control - AC 311SBC 260400 R1001-b 06.99The central unit interrogates the remote units in order to establish an initial image of thesystem configuration during initialization.The remote units are then interrogated with each bus cycle. This enables the recognition ofnewly added or removed remote units and the updating of the diagnosis information.If the central unit receives a message indicating a CRC8 error it will not be signaledimmediately and the frame will not be taken into account. After nine consecutive transmissionerrors a “bus error” is signaled by the central unit. The bus error is also signaled to the remoteunits after a 250 ms time-out.3.4. Refresh times / reply timesBus refresh times and central unit reply times depend on the system configuration, the numberand the type of remote units on the CS 31 bus. The bus refresh and the user programs areexecuted in series.3.4.1. Bus refresh timesThe bus refresh time corresponds to the transmission time over the bus. The calculationcorresponds to the addition of all the communication times of the remote units on the bus andthe base time of the master central unit which is 2 ms.3.4.2. Central unit reply timesThe central unit reply time corresponds to the time taken to activate an output after theactivation of an input.The maximum reply time is obtained by adding the filtering time of the input, the bus refreshtime, the delay time for the output and twice the cycle time.The filtering time for an input on the 40 and 50 series is 5 ms.The delay time for a transistor output is considered as null in milliseconds and the delay timefor a relay output is 6 ms.Comment:The inputs / outputs of central units and their extensions can be accessed, for rapidprocessing, independently of the cycle time with the user program functions DI and DO (seechapter 6).The cycle time calculation is given in chapter 5 at the cycle time configuration level as well asthe communication times of each unit for the bus transmission times calculation.
