# Fieldbus Data Types

Each fieldbus has its own capacity regarding the amount of data it could transfer over the bus.

- Cyclic data: Cyclic data (also called as "implicit" or "essential" data) is typically PLC data/process data such as Start, Stop, OK, NOK signals and so on. Cyclic data transfers occur on a regular basis (for example, 100ms) between the master and the controller fieldbus and must be updated as often as possible as it contains commands that are crucial to the operation.

- Acyclic data: Acyclic data (also called as "information") is usually tightening data. Acylic data is outside the cyclic data fieldbus mapping and an acyclic data transfer occurs only on a request from the master to the controller fieldbus.

- Async data: Async data is tightening data that is part of the cyclic data transfer and is part of the mapping and is meant to be for those fieldbus types that do not support acyclic data (CC Link). The async data function can be used in order to send the tightening results with cyclic data in packages over the fieldbus to the PLC. The PLC then merges all the packages into one tightening result. The async data from the controller is communicated with the I/O signal Fieldbus AsyncData from PF and the async data is communicated to the controller with the I/O signal Fieldbus AsyncData to PF. These signals are used for the async data handshake which is the same type of handshake done with acyclic data. The size of the Fieldbus AsyncData from PF can be considered as the instance size or record size and indicates how many bytes of data is transferred for each async data handshake request.

  For example, if the total acyclic data configuration is 1024 bytes and Fieldbus AsyncData to PF is set as 64 bytes, then the total async data will be sent 16 times X 64 bytes each time. That is, for each request from the PLC, the controller fills theFieldbus AsyncData to PF with the next 64 bytes of data from the total async data.

  In theory, if you define Fieldbus AsyncData from PF as 2 bytes long, then you can send upto 32K bytes of data (which will need 16K data requests).

Relevant Information

- Fieldbus Sizes