Siemens SIMATIC ET 200AL System Manual page 1339

Transmission performance
IO-Link is a very robust communication system. This communication system operates with a
24V level. If transmissions fail, the frame is repeated two more times. The IO-Link master
only detects a break in communication after the failure of the second repeat attempt and
reports this to the higher-level CPU.
Data types
Four data types are available:
● Process data → Cyclic data
● PQI → Cyclic data
● Device data → Acyclic data
● Events → Acyclic data
Process data
The process data of the devices is transferred cyclically in a data frame with the process
data size being determined by the device. Process data from 0 to 32 bytes is possible per
device (input and output, respectively).
Port Qualifier Information (PQI)
Each port has a Port Qualifier Information (PQI). This PQI provides information on the device
status of the port and IO-Link (value status). The PQI is transmitted cyclically with the
process data. The PQI is disabled by default.
Device data
Device data can be parameters, identification data or diagnostic information. The data is
exchanged cyclically and at the request of the IO-Link master. Device data can be written to
the device and also read from the device.
Events
When an event occurs, the device signals to the master that an event is present. The master
then reads the event. Events can be error messages (for example, short-circuit) and
warning/maintenance data (for example, dirt, overheating).
Error messages are transferred from the device to the CPU or the HMI by the IO-Link
master.
The IO-Link master can also transfer events and states. Such events are, for example, wire
break, break in communication or overload.
Device parameters or events are transmitted independently of the cyclic transmission of
process data. The transmissions do not influence or impair each other.
IO-Link system
Function Manual, 12/2017, A5E31637677-AB
System overview
2.4 IO-Link protocol
15