W5100S is based on W5100, but it isn’t compatible with the W5100 at Hardware PIN-to-PIN.
W5100S is compatible with W5100 at only firmware.
The W5100S is configured the same as the W5100 Register Map for Firmware compatibility with the W5100, and its some Registers are deleted, modified, and added for improving some
For more information, See
W5100_W5100S Comparison Sheet