The ES-4 Gate Expander is a Eurorack module which works in tandem with the ES-4 to offer 8 on/off outputs for use as gates, triggers, clocks etc.

Up to five ES-4 Gate Expanders can be attached to one ES-4 - one per ES-4 channel. (The 8 outputs of the Gate Expander reflect the 8 bits of CV information passed to the ES-4's DACs.)

Therefore you can combine the modules to create a system with up to 40 independent gate outputs all driven from a single S/PDIF connection.

The ES-4 Controller software (part of Expert Sleepers Silent Way) also allows the Gate Expander outputs to be used for DIN sync or MIDI output* (note - not just MIDI clock output, but as an actual MIDI port, capable of sending any MIDI messages).

Since these outputs are driven from an audio connection (the S/PDIF port), they have sample-accurate timing, which no USB MIDI interface can claim.

The ES-4 Gate Expander's specifications are as follows:
Panel width: 4HP
Module depth: 24mm
Analogue outputs: 8x 3.5mm jacks, DC-coupled
Output voltages: 0V/5V or 0V/12V, selectable via jumper in groups independently for channels 1-4 and 5-8
Indicators: LED per channel

The front panel is drilled for both Doepfer and Analogue Systems mounting holes (using a 'slot' rather than a single round hole).