It depends on the specifications and combinations of Ethernet devices.
Communication ports of Ethernet devices have either of the following specifications.
1) AUTO MDI/MDI-X supported (Auto-switching of straight/cross cable)
2) AUTO MDI/MDI-X not supported (No-auto-switching of straight/cross cable)

Applicable cable types when directly connecting the above devices are as follows:
・Combination of 1) devices:
　Both straight cables and cross cables are usable.
・Combination of 1) device and 2) device
    Both straight cables and cross cables are usable.
・Combination of 2) devices:
　Only cross cables are usable.

The specifications of SX series PLCs are as follows.
1) (AUTO MDI/MDI-X supported)
     NP1L-WS1,
     Ethernet built in SPH3000 series and SPH5000 series
2) (AUTO MDI/MDI-X not supported)
　NP1L-ET1, NP1L-WE1,
    Ethernet built in SPH2000 series,
　NA3LA-ET1, NA0LA-ET1

 

[FYI]
These days, many commercially available personal computers support AUTO MDI/MDI-X.