diff --git a/src/DuetNG/DueXn.cpp b/src/DuetNG/DueXn.cpp
index 4f23199fa5..cb529c6bab 100644
--- a/src/DuetNG/DueXn.cpp
+++ b/src/DuetNG/DueXn.cpp
@@ -24,11 +24,16 @@ namespace DuetExpansion
 	static ExpansionBoardType dueXnBoardType = ExpansionBoardType::none;
 
 	const uint8_t AdditionalIoExpanderAddress = 0x71;	// address of the SX1509B we allow for general I/O expansion
+	const uint8_t AdditionalIoExpanderAddress2 = 0x3F;
 
 	static SX1509 additionalIoExpander;
 	static bool additionalIoExpanderPresent = false;
 	static uint16_t additionalIoInputBits = 0;
 
+	static SX1509 additionalIoExpander2;
+	static bool additionalIoExpanderPresent2 = false;
+	static uint16_t additionalIoInputBits2 = 0;
+
 	static volatile bool taskWaiting = false;
 	static volatile bool inputsChanged = false;
 
@@ -161,6 +166,7 @@ namespace DuetExpansion
 	void AdditionalOutputInit()
 	{
 		I2C::Init();										// initialise I2C
+		delay(200);											// the SX1509B has an independent power on reset, so give it some time
 
 		bool ret;
 		unsigned int attempts = 0;
@@ -178,6 +184,24 @@ namespace DuetExpansion
 			additionalIoInputBits = additionalIoExpander.digitalReadAll();
 			additionalIoExpanderPresent = true;
 		}
+
+		bool ret2;
+		unsigned int attempts2 = 0;
+		do
+		{
+			++attempts2;
+			delay(50);
+			ret2 = additionalIoExpander2.begin(AdditionalIoExpanderAddress2);
+		} while (!ret2 && attempts2 < 5);
+		(void)I2C_IFACE.GetErrorCounts(true);				// clear the error counts in case there wasn't a device there or we didn't find it first time
+
+		if (ret2)
+		{
+			additionalIoExpander2.pinModeMultiple((1u << 16) - 1, INPUT_PULLDOWN);
+			additionalIoInputBits2 = additionalIoExpander2.digitalReadAll();
+			additionalIoExpanderPresent2 = true;
+		}
+
 	}
 
 	// Return the name of the expansion board, or nullptr if no expansion board
@@ -199,7 +223,21 @@ namespace DuetExpansion
 	// Return the name of the additional expansion board, or nullptr if no expansion board
 	const char* _ecv_array null GetAdditionalExpansionBoardName()
 	{
-		return (additionalIoExpanderPresent) ? "SX1509B expander" : nullptr;
+		if (additionalIoExpanderPresent && additionalIoExpanderPresent2)
+		{
+			return "SX1509B expander (0x71 & 0x3F)";
+		}
+		else if (additionalIoExpanderPresent)
+		{
+			return "SX1509B expander (0x71)";
+		}
+		else if (additionalIoExpanderPresent2)
+		{
+			return "SX1509B expander (0x3F)";
+		} else
+		{
+			return nullptr;
+		}
 	}
 
 	// Set the I/O mode of a pin
@@ -236,12 +274,16 @@ namespace DuetExpansion
 				dueXnExpander.pinMode(pin, mode);
 			}
 		}
-		else if (pin >= AdditionalIoExpansionStart && pin < AdditionalIoExpansionStart + 16)
+		else if (pin >= AdditionalIoExpansionStart && pin < AdditionalIoExpansionStart + 32)
 		{
-			if (additionalIoExpanderPresent)
+			if (additionalIoExpanderPresent && pin < AdditionalIoExpansionStart + 16)
 			{
 				additionalIoExpander.pinMode(pin - AdditionalIoExpansionStart, mode);
 			}
+			else if (additionalIoExpanderPresent2 && pin >= AdditionalIoExpansionStart + 16)
+			{
+				additionalIoExpander2.pinMode(pin - (AdditionalIoExpansionStart + 16), mode);
+			}
 		}
 	}
 
@@ -262,9 +304,9 @@ namespace DuetExpansion
 				return (dueXnInputBits & (1u << (pin - DueXnExpansionStart))) != 0;
 			}
 		}
-		else if (pin >= AdditionalIoExpansionStart && pin < AdditionalIoExpansionStart + 16)
+		else if (pin >= AdditionalIoExpansionStart && pin < AdditionalIoExpansionStart + 32)
 		{
-			if (additionalIoExpanderPresent)
+			if (additionalIoExpanderPresent && pin < AdditionalIoExpansionStart + 16)
 			{
 				// We don't have an interrupt from the additional I/O expander, so always read fresh data.
 				// If this is called from inside an ISR, we will get stale data.
@@ -275,6 +317,17 @@ namespace DuetExpansion
 
 				return (additionalIoInputBits & (1u << (pin - AdditionalIoExpansionStart))) != 0;
 			}
+			else if (additionalIoExpanderPresent2 && pin >= AdditionalIoExpansionStart + 16)
+			{
+				// We don't have an interrupt from the additional I/O expander, so always read fresh data.
+				// If this is called from inside an ISR, we will get stale data.
+				if (!inInterrupt() && __get_BASEPRI() == 0)								// we must not call expander.digitalRead() from within an ISR
+				{
+					additionalIoInputBits2 = additionalIoExpander2.digitalReadAll();
+				}
+
+				return (additionalIoInputBits2 & (1u << (pin - (AdditionalIoExpansionStart + 16)))) != 0;
+			}
 		}
 
 		return false;
@@ -290,12 +343,16 @@ namespace DuetExpansion
 				dueXnExpander.digitalWrite(pin - DueXnExpansionStart, high);
 			}
 		}
-		else if (pin >= AdditionalIoExpansionStart && pin < AdditionalIoExpansionStart + 16)
+		else if (pin >= AdditionalIoExpansionStart && pin < AdditionalIoExpansionStart + 32)
 		{
-			if (additionalIoExpanderPresent)
+			if (additionalIoExpanderPresent && pin < AdditionalIoExpansionStart + 16)
 			{
 				additionalIoExpander.digitalWrite(pin - AdditionalIoExpansionStart, high);
 			}
+			else if (additionalIoExpanderPresent2 && pin >= AdditionalIoExpansionStart + 16)
+			{
+				additionalIoExpander2.digitalWrite(pin - (AdditionalIoExpansionStart + 16), high);
+			}
 		}
 	}
 
@@ -309,12 +366,16 @@ namespace DuetExpansion
 				dueXnExpander.analogWrite(pin - DueXnExpansionStart, (uint8_t)(constrain<float>(pwm, 0.0, 1.0) * 255));
 			}
 		}
-		else if (pin >= AdditionalIoExpansionStart && pin < AdditionalIoExpansionStart + 16)
+		else if (pin >= AdditionalIoExpansionStart && pin < AdditionalIoExpansionStart + 32)
 		{
-			if (additionalIoExpanderPresent)
+			if (additionalIoExpanderPresent && pin < AdditionalIoExpansionStart + 16)
 			{
 				additionalIoExpander.analogWrite(pin - AdditionalIoExpansionStart, (uint8_t)(constrain<float>(pwm, 0.0, 1.0) * 255));
 			}
+			else if (additionalIoExpanderPresent2 && pin >= AdditionalIoExpansionStart + 16)
+			{
+				additionalIoExpander2.analogWrite(pin - (AdditionalIoExpansionStart + 16), (uint8_t)(constrain<float>(pwm, 0.0, 1.0) * 255));
+			}
 		}
 	}
 
diff --git a/src/DuetNG/Pins_DuetNG.h b/src/DuetNG/Pins_DuetNG.h
index 11d2bdfce6..1df88a3bbb 100644
--- a/src/DuetNG/Pins_DuetNG.h
+++ b/src/DuetNG/Pins_DuetNG.h
@@ -102,7 +102,7 @@ constexpr Pin UsbVBusPin = PortCPin(22);			// Pin used to monitor VBUS on USB po
 #define I2C_IRQn	WIRE_ISR_ID						// The interrupt number it uses
 
 constexpr Pin DueXnExpansionStart = 200;			// Pin numbers 200-215 are on the I/O expander
-constexpr Pin AdditionalIoExpansionStart = 220;		// Pin numbers 220-235 are on the additional I/O expander
+constexpr Pin AdditionalIoExpansionStart = 220;		// Pin numbers 220-251 are on the additional I/O expander
 
 // The numbers of entries in each array must correspond with the values of DRIVES, AXES, or HEATERS. Set values to NoPin to flag unavailability.
 
@@ -298,22 +298,38 @@ constexpr PinEntry PinTable[] =
 	{ 210,			PinCapability::rwpwm,	"duex.gp2" },
 	{ 209,			PinCapability::rwpwm,	"duex.gp3" },
 	{ 208,			PinCapability::rwpwm,	"duex.gp4" },
-	{ 220,			PinCapability::rwpwm,	"sx1509b.0" },
-	{ 221,			PinCapability::rwpwm,	"sx1509b.1" },
-	{ 222,			PinCapability::rwpwm,	"sx1509b.2" },
-	{ 223,			PinCapability::rwpwm,	"sx1509b.3" },
-	{ 224,			PinCapability::rwpwm,	"sx1509b.4" },
-	{ 225,			PinCapability::rwpwm,	"sx1509b.5" },
-	{ 226,			PinCapability::rwpwm,	"sx1509b.6" },
-	{ 227,			PinCapability::rwpwm,	"sx1509b.7" },
-	{ 228,			PinCapability::rwpwm,	"sx1509b.8" },
-	{ 229,			PinCapability::rwpwm,	"sx1509b.9" },
-	{ 230,			PinCapability::rwpwm,	"sx1509b.10" },
-	{ 231,			PinCapability::rwpwm,	"sx1509b.11" },
-	{ 232,			PinCapability::rwpwm,	"sx1509b.12" },
-	{ 233,			PinCapability::rwpwm,	"sx1509b.13" },
-	{ 234,			PinCapability::rwpwm,	"sx1509b.14" },
-	{ 235,			PinCapability::rwpwm,	"sx1509b.15" }
+	{ 220,			PinCapability::rwpwm,	"sx1509b.0,0x71.0" },
+	{ 221,			PinCapability::rwpwm,	"sx1509b.1,0x71.1" },
+	{ 222,			PinCapability::rwpwm,	"sx1509b.2,0x71.2" },
+	{ 223,			PinCapability::rwpwm,	"sx1509b.3,0x71.3" },
+	{ 224,			PinCapability::rwpwm,	"sx1509b.4,0x71.4" },
+	{ 225,			PinCapability::rwpwm,	"sx1509b.5,0x71.5" },
+	{ 226,			PinCapability::rwpwm,	"sx1509b.6,0x71.6" },
+	{ 227,			PinCapability::rwpwm,	"sx1509b.7,0x71.7" },
+	{ 228,			PinCapability::rwpwm,	"sx1509b.8,0x71.8" },
+	{ 229,			PinCapability::rwpwm,	"sx1509b.9,0x71.9" },
+	{ 230,			PinCapability::rwpwm,	"sx1509b.10,0x71.10" },
+	{ 231,			PinCapability::rwpwm,	"sx1509b.11,0x71.11" },
+	{ 232,			PinCapability::rwpwm,	"sx1509b.12,0x71.12" },
+	{ 233,			PinCapability::rwpwm,	"sx1509b.13,0x71.13" },
+	{ 234,			PinCapability::rwpwm,	"sx1509b.14,0x71.14" },
+	{ 235,			PinCapability::rwpwm,	"sx1509b.15,0x71.15" },
+	{ 236,			PinCapability::rwpwm,	"sx1509b2.0,0x3f.0" },
+	{ 237,			PinCapability::rwpwm,	"sx1509b2.1,0x3f.1" },
+	{ 238,			PinCapability::rwpwm,	"sx1509b2.2,0x3f.2" },
+	{ 239,			PinCapability::rwpwm,	"sx1509b2.3,0x3f.3" },
+	{ 240,			PinCapability::rwpwm,	"sx1509b2.4,0x3f.4" },
+	{ 241,			PinCapability::rwpwm,	"sx1509b2.5,0x3f.5" },
+	{ 242,			PinCapability::rwpwm,	"sx1509b2.6,0x3f.6" },
+	{ 243,			PinCapability::rwpwm,	"sx1509b2.7,0x3f.7" },
+	{ 244,			PinCapability::rwpwm,	"sx1509b2.8,0x3f.8" },
+	{ 245,			PinCapability::rwpwm,	"sx1509b2.9,0x3f.9" },
+	{ 246,			PinCapability::rwpwm,	"sx1509b2.10,0x3f.10" },
+	{ 247,			PinCapability::rwpwm,	"sx1509b2.11,0x3f.11" },
+	{ 248,			PinCapability::rwpwm,	"sx1509b2.12,0x3f.12" },
+	{ 249,			PinCapability::rwpwm,	"sx1509b2.13,0x3f.13" },
+	{ 250,			PinCapability::rwpwm,	"sx1509b2.14,0x3f.14" },
+	{ 251,			PinCapability::rwpwm,	"sx1509b2.15,0x3f.15" }
 };
 
 constexpr unsigned int NumNamedPins = ARRAY_SIZE(PinTable);
diff --git a/src/RepRap.cpp b/src/RepRap.cpp
index bffe8faf4a..9a68c733a7 100644
--- a/src/RepRap.cpp
+++ b/src/RepRap.cpp
@@ -726,6 +726,8 @@ void RepRap::Diagnostics(MessageType mtype) noexcept
 	platform->MessageF(mtype, "%s version %s running on %s", FIRMWARE_NAME, VERSION, platform->GetElectronicsString());
 	const char* const expansionName = DuetExpansion::GetExpansionBoardName();
 	platform->MessageF(mtype, (expansionName == nullptr) ? "\n" : " + %s\n", expansionName);
+	const char* additionalExpansionName = DuetExpansion::GetAdditionalExpansionBoardName();
+	platform->MessageF(mtype, (additionalExpansionName == nullptr) ? "\n" : " + %s\n", additionalExpansionName);
 #elif defined(__LPC17xx__)
     platform->MessageF(mtype, "%s (%s) version %s running on %s at %dMhz\n", FIRMWARE_NAME, lpcBoardName, VERSION, platform->GetElectronicsString(), (int)SystemCoreClock/1000000);
 #else