diff --git a/idzio/idzio.h b/idzio/idzio.h
index 5c14c55..395b012 100644
--- a/idzio/idzio.h
+++ b/idzio/idzio.h
@@ -1,5 +1,17 @@
 #pragma once
 
+/* INITIAL D ZERO CUSTOM IO API
+
+   This API definition allows custom driver DLLs to be defined for the
+   emulation of Initial D Zero cabinets. To be honest, there is very
+   little reason to want to do this, since driving game controllers are a
+   mostly-standardized PC peripheral which can be adequately controlled by the
+   built-in DirectInput and XInput support in idzhook. However, previous
+   versions of Segatools broke this functionality out into a separate DLL just
+   like all of the other supported games, so in the interests of maintaining
+   backwards compatibility we provide the option to load custom IDZIO
+   implementations as well. */
+
 #include <windows.h>
 
 #include <stdint.h>
@@ -19,21 +31,76 @@ enum {
 };
 
 struct idz_io_analog_state {
+    /* Current steering wheel position, where zero is the centered position.
+
+       The game will accept any signed 16-bit position value, however a real
+       cabinet will report a value of approximately +/- 25230 when the wheel
+       is at full lock. Steering wheel positions of a magnitude greater than
+       this value are not possible on a real cabinet. */
+
     int16_t wheel;
+
+    /* Current position of the accelerator pedal, where 0 is released. */
+
     uint16_t accel;
+
+    /* Current position of the brake pedal, where 0 is released. */
+
     uint16_t brake;
 };
 
+/* Get the version of the IDZ IO API that this DLL supports. This
+   function should return a positive 16-bit integer, where the high byte is
+   the major version and the low byte is the minor version (as defined by the
+   Semantic Versioning standard).
+
+   The latest API version as of this writing is 0x0100. */
+
 uint16_t idz_io_get_api_version(void);
 
+/* Initialize JVS-based input. This function will be called before any other
+   idz_io_jvs_*() function calls. Errors returned from this function will
+   manifest as a disconnected JVS bus.
+
+   All subsequent calls may originate from arbitrary threads and some may
+   overlap with each other. Ensuring synchronization inside your IO DLL is
+   your responsibility.
+
+   Minimum API version: 0x0100 */
+
 HRESULT idz_io_jvs_init(void);
 
+/* Poll the current state of the cabinet's JVS analog inputs. See structure
+   definition above for details.
+
+   Minimum API version: 0x0100 */
+
 void idz_io_jvs_read_analogs(struct idz_io_analog_state *out);
 
+/* Poll the current state of the cabinet's JVS input buttons and return them
+   through the opbtn and gamebtn out parameters. See enum definitions at the
+   top of this file for a list of bit masks to be used with these out
+   parameters.
+
+   Minimum API version: 0x0100 */
+
 void idz_io_jvs_read_buttons(uint8_t *opbtn, uint8_t *gamebtn);
 
+/* Poll the current position of the six-speed shifter and return it via the
+   gear out parameter. Valid values are 0 for neutral and 1-6 for gears 1-6.
+
+   idzhook internally translates this gear position value into the correct
+   combination of Gear Left, Gear Right, Gear Up, Gear Down buttons that the
+   game will then interpret as the current position of the gear lever.
+
+   Minimum API version: 0x0100 */
+
 void idz_io_jvs_read_shifter(uint8_t *gear);
 
-void idz_io_jvs_read_coin_counter(uint16_t *total);
+/* Read the current state of the coin counter. This value should be incremented
+   for every coin detected by the coin acceptor mechanism. This count does not
+   need to persist beyond the lifetime of the process.
 
-// TODO force feedback once that gets reverse engineered
+   Minimum API version: 0x0100 */
+
+void idz_io_jvs_read_coin_counter(uint16_t *total);