Introduced in 2001, the Erostek ET-312 electro-stimulation box is still considered to be the top of the line in estim devices. It has 2 output channels, with inputs via line-in audio, external microphone, and RS-232 serial communication.
ET-312 Serial Protocol Documentation
ET-312 Firmware Disassembly
ET-312 Board Schematics
ET-312 Board Images
The buttshock library provides serial port control functions for the Erostek ET-312 and ET-232, and the Estim Systems 2b. It can be used for setting patterns and intensities, loading new patterns onto hardware that allows for it, etc...
Buttplug supports the Erostek ET-312 via the following message types: