Omron ZW-C1*AT User Manual page 163

Communications Types of EtherCAT
EtherCAT provides the following two types of communication functions.
PDO communications are always updating data per communication cycle on EtherCAT, while SDO
communications are processed in between those updates.
● Process data communications functions (PDO communications)
This communication function is used to transfer process data in real time in a fixed-cycle.
By mapping logical process data space to each node by the EtherCAT Master Unit, it achieves fixed-cycle
communications among the EtherCAT Master Unit and Slave Units.
EtherCAT Master Unit
Ethernet frame
Ethernet
header
Logic process data
.
.
.
Data a
.
.
.
Data b
Data c
.
.
.
● Mailbox communications functions (SDO communications)
It refers to message communications.
At any timing, the EtherCAT Master Unit transmits commands to Slave Units and the Slave Units return
responses to the EtherCAT Master Unit.
It performs the following data communications:
• Read and write process data
ZW User's Manual
Slave Unit
Slave Unit
EtherCAT 1st EtherCAT 2nd EtherCAT
header datagram
Slave Unit
3rd EtherCAT
datagram datagram
Slave Unit
. . .
CRC
EtherCAT Connection
161