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.
Suggested combinations
The ES-40 does nothing by itself, but forms the basis of a very configurable system. Simply add the expansion modules you need. Here are some ideas:
8 general purpose CVs - an ES-40 plus an ESX-8CV gives you a flexible, general purpose modular controller.
8 CV/gate pairs - an ES-40, an ESX-8CV and an ESX-8GT gives you enough outputs for 8 classic monosynths - or to realise that 8 voice polyphonic modular you've been dreaming of.
8 drum triggers - an ES-40 and an ESX-8GT gives you 8 triggers for your rack of drum modules, for the ultimate in sample-accurate rhythm. Add up to four more ESX-8GTs for up to 40 triggers/gates/clocks!
MIDI & DINsync - an ES-40 and an ESX-8MD form a sample-accurate, jitter-free, multi-port MIDI interface.
digital audio - an ES-40 and an ES-7 gives you two channels of 24 bit analogue-to-digital conversion to get your modular audio into your DAW.