In sense of telegram length two types of telegrams can be distinguished: see here.
The length of telegrams could influence the communication between two devices, i.e. the ability to communicate depends on the actual communication partners & the communication type.
About the communication partners
Three combinations can be distinguished:
- Standard + Standard
- When both devices support standard frames then they always will be able to communicate with each other in either direction.
- Standard + Extended
- The ability for a device supporting standard frames communicates and a device supporting extended frames to communicate depends on the type of communication, but in general the device supporting the extended frames can always process frames sent by the device supporting standard frames.
- Extended + Extended
- The so called 'APDU length' needs to be taken into account for any communication between two devices supporting extended frames. If both devices have the same APDU length then they always will be able to communicate with each other in either direction. The ability for a device supporting a smaller APDU length (e.g. 50) and a device supporting a higher APDU length (e.g. 100) to communicate depends on the type of communication, but in general the device supporting the higher APDU length can always process frames sent by the device supporting the smaller APDU length.
About the communication type
Three such types can be distinguished:
- The ability for devices to communicate during runtime depends entirely on the Data point type used, see here. Since all Data point types defined so far can be handled within standard frames, runtime communication between either communication partner combination is guaranteed.
- Please note however that in the context of security nearly always extended frames will be used, even for most of the runtime telegrams.
- The communication partners during commissioning are the interface (representing ETS) and the target device. It is however possible that a number of couplers are placed between these communication partners. It is the task of ETS to find out what type of devices are used in the entire communication chain. In 'worst' case scenario ETS will send out all management telegrams as standard frames. If however possible ETS will send out the management telegrams as extended frames, the length of these frames is set according to the smallest APDU length found in the entire communication chain.
- Monitoring (diagnostics)
- During monitoring two aspects need to be considered: the APDU length of the interface and the type of telegrams sent on the bus. For runtime telegrams: any runtime telegram can be captured by any interface. For management telegrams: telegrams with higher length in relation to the APDU length of the interface cannot be captured.