EtherCAT, a popular communication protocol based on Ethernet, with a mechanism to transfer data on the fly that enables high speed with high efficiency, plays an essential role in real-time Ethernet-based communication protocols. However, in network-based control systems, it is impossible to transmit a control command to multiple devices at the same time due to network latency. This problem is of particular importance in motion control. In the present work, we analyzed several aspects concerning EtherCAT network latency and carried out some related experiments. As the speed at which the embedded programmable logic controller (PLC) gave commands was increased, the EtherCAT I/O became more likely to respond with signal errors. Moreover, some incorrect signals appeared when short-duration PLC commands were given.