The ES-40 is a Eurorack format module which converts a standard S/PDIF input to five channels of expansion headers, which are used in conjunction with the Expert Sleepers ESX range of expanders (currently including the ESX-8GT, the ESX-4CV, the ESX-8CV and the ESX-8MD). The ES-40 will connect to any audio device using the S/PDIF standard. This includes the digital audio output provided as standard on many recent Macs (more info here on the Mac connection).
The ES-40 also includes an S/PDIF output for sending data back to the computer. This output is used by expansion modules connected to the ES-40; currenty, the only such module is the ES-7 CV Input Expander.
The ES-40 is essentially an ES-4 with the DACs and CV outputs removed. Much of what has been written about the ES-4, and many tutorial videos etc., still apply to the ES-40.