diff --git a/README.md b/README.md
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+++ b/README.md
@@ -13,4 +13,5 @@ This repository specifies the control protocol implemented by [gex-core](https:/
 - [UNIT_DO.md](UNIT_DO.md) - Digital Output
 - [UNIT_DI.md](UNIT_DI.md) - Digital Input
 - [UNIT_NEOPIXEL.md](UNIT_NEOPIXEL.md) - NeoPixel driver
+- [UNIT_SIPO.md](UNIT_SIPO.md) - Multi-cast Shift Register Driver
 
diff --git a/UNIT_SIPO.md b/UNIT_SIPO.md
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@@ -0,0 +1,52 @@
+# SIPO - Multi-cast Shift Register Driver
+
+*SIPO stands for Serial Input Parallel Output*
+
+- Used for building user interfaces (LED displays etc.)
+- Designed for driving 74xx595 or 74xx4094
+- Can drive multiple data lines in parallel
+- Offers direct pin access without sending data (no clock pulse)
+- Supports data latch on Store pulse (see the WRITE command for details)
+- Pin polarities are configurable in the unit settings.
+
+The `DIRECT_*` commands are mostly meant for debugging purposes when trying to communicate with an unfamiliar SIPO-based board.
+
+## Commands
+
+### WRITE (0x00)
+
+Sends data to the shift registers.
+
+After each byte, the *Shift* pin is pulsed. After the last byte and the *Shift* pulse, the *Latch* data is output followed by the *Store* pin pulse.
+
+The *Latch* data is normally 0 and doesn't matter; however, it's possible to design
+external circuitry that latches the data pins on the *Store* pulse, e.g. for brightness
+control or extra LEDs.
+
+*Request:*
+- u16 - latch data to output before the Store pulse - all outputs, packed
+- (u8 array) x num_outputs
+  - must be a multiple of the outputs count (if 1, simply the output data)
+
+### DIRECT_DATA (0x01)
+
+Direct write to the data pins, without any pulse.
+
+*Request:*
+- u16 - data to output, packed
+
+### DIRECT_CLEAR (0x02)
+
+Pulse the *Clear* output.
+
+### DIRECT_SHIFT (0x03)
+
+Pulse the *Shift* output.
+
+### DIRECT_STORE (0x04)
+
+Pulse the *Store* output.
+
+## Events
+
+*No events defined for this unit type.*