diff --git a/main.c b/main.c
index 5c1c08e..433d1f1 100644
--- a/main.c
+++ b/main.c
@@ -15,12 +15,26 @@ static bool eeprom_valid = true; // if the EDID can be read from EEPROM
 // LED pin (sink for on)
 #define LED_PORT GPIO_PA
 #define LED_PIN PA3
-// pull ups for device facing I²C port
-#define SDA_PU_PORT GPIO_PC
-#define SDA_PU_PIN PC3
-#define SCL_PU_PORT GPIO_PC
-#define SCL_PU_PIN PC4
+// I²C pins (sink and source)
+#define SDA_SRC_PORT GPIO_PB
+#define SDA_SRC_PIN PB5
+#define SCL_SRC_PORT GPIO_PB
+#define SCL_SRC_PIN PB4
+#define SDA_SRC_PU_PORT GPIO_PC
+#define SDA_SRC_PU_PIN PC3
+#define SCL_SRC_PU_PORT GPIO_PC
+#define SCL_SRC_PU_PIN PC4
+#define SDA_SNK_PORT GPIO_PD
+#define SDA_SNK_PIN PD2
+#define SCL_SNK_PORT GPIO_PD
+#define SCL_SNK_PIN PD3
+#define SDA_SNK_PU_PORT GPIO_PD
+#define SDA_SNK_PU_PIN PD4
+#define SCL_SNK_PU_PORT GPIO_PA
+#define SCL_SNK_PU_PIN PA1
+
 static bool i2c_fwd = false; // if the I²C source lines are connected to sink
+static bool sink_present = false; // if he sink is present (tested using sink DDC)
 // external EEPROM write protect pin
 #define WP_PORT GPIO_PC
 #define WP_PIN PC5
@@ -213,30 +227,47 @@ void main(void)
 	UART1->BRR1.reg = 0x08; // set baud rate to 115200 (at 16 MHz)
 	UART1->CR2.fields.TEN = 1; // enable TX
 
+	// configure I²C lines (sink and source sides)
+	SDA_SRC_PU_PORT->ODR.reg |= SDA_SRC_PU_PIN; // pull up SDA line
+	SDA_SRC_PU_PORT->CR1.reg |= SDA_SRC_PU_PIN; // switch pin to push pull
+	SDA_SRC_PU_PORT->DDR.reg |= SDA_SRC_PU_PIN; // switch pin to output
+	SCL_SRC_PU_PORT->ODR.reg |= SCL_SRC_PU_PIN; // pull up SCL line
+	SCL_SRC_PU_PORT->CR1.reg |= SCL_SRC_PU_PIN; // switch pin to push pull
+	SCL_SRC_PU_PORT->DDR.reg |= SCL_SRC_PU_PIN; // switch pin to output
+	SCL_SRC_PORT->ODR.reg |= SCL_SRC_PIN; // release SCL line
+	SCL_SRC_PORT->CR1.reg &= ~SCL_SRC_PIN; // switch pin to open-drain
+	SCL_SRC_PORT->DDR.reg |= SCL_SRC_PIN; // switch pin to output
+	SDA_SRC_PORT->ODR.reg |= SDA_SRC_PIN; // release SDA line
+	SDA_SRC_PORT->CR1.reg &= ~SDA_SRC_PIN; // switch pin to open-drain
+	SDA_SRC_PORT->DDR.reg |= SDA_SRC_PIN; // switch pin to output
+	SCL_SNK_PORT->ODR.reg |= SCL_SNK_PIN; // release SCL line
+	SCL_SNK_PORT->CR1.reg &= ~SCL_SNK_PIN; // switch pin to open-drain
+	SCL_SNK_PORT->DDR.reg |= SCL_SNK_PIN; // switch pin to output
+	SDA_SNK_PORT->ODR.reg |= SDA_SNK_PIN; // release SDA line
+	SDA_SNK_PORT->CR1.reg &= ~SDA_SNK_PIN; // switch pin to open-drain
+	SDA_SNK_PORT->DDR.reg |= SDA_SNK_PIN; // switch pin to output
+	// test I²C forwarding
+	SCL_SNK_PORT->ODR.reg &= ~SCL_SNK_PIN; // assert SCL line
+	SDA_SNK_PORT->ODR.reg &= ~SDA_SNK_PIN; // assert SDA line
+	if (0 == (SCL_SRC_PORT->IDR.reg & SCL_SRC_PIN) || 0 == (SDA_SRC_PORT->IDR.reg & SDA_SRC_PIN)) {
+		i2c_fwd = true; // remember the I²C line(s) are forwarded
+		puts("I²C lines forwarded\r\n");
+	}
+	SCL_SNK_PORT->ODR.reg |= SCL_SNK_PIN; // release SCL line
+	SDA_SNK_PORT->ODR.reg |= SDA_SNK_PIN; // release SDA line
+	if (!i2c_fwd) {
+		wait_10us(5); // let sink 47k pull-up take effect
+		if ((SCL_SNK_PORT->IDR.reg & SCL_SNK_PIN) && (SDA_SNK_PORT->IDR.reg & SDA_SNK_PIN)) {
+			sink_present = true;
+		}
+	}
+
 	// configure I²C pull-ups
-	SDA_PU_PORT->DDR.reg |= SDA_PU_PIN; // switch pin to output
-	SDA_PU_PORT->CR1.reg &= ~SDA_PU_PIN; // switch pin to open-drain
-	SDA_PU_PORT->ODR.reg &= ~SDA_PU_PIN; // pull SDA line low
-	wait_10us(0); // wait shortly to clear tristate
-	SDA_PU_PORT->DDR.reg &= ~SDA_PU_PIN; // switch pin to input (already floating)
-	if (SDA_PU_PORT->IDR.reg & SDA_PU_PIN) { // line is pulled up
-		i2c_fwd = true; // remember the I²C line(s) are forwarded
-	}
-	SCL_PU_PORT->DDR.reg |= SCL_PU_PIN; // switch pin to output
-	SCL_PU_PORT->CR1.reg &= ~SCL_PU_PIN; // switch pin to open-drain
-	SCL_PU_PORT->ODR.reg &= ~SCL_PU_PIN; // pull SDA line low
-	wait_10us(0); // wait shortly to clear tristate
-	SCL_PU_PORT->DDR.reg &= ~SCL_PU_PIN; // switch pin to input (already floating)
-	if (SCL_PU_PORT->IDR.reg & SCL_PU_PIN) { // line is pulled up
-		i2c_fwd = true; // remember the I²C line(s) are forwarded
-	}
-	if (!i2c_fwd) { // I²C lines are not pulled up
-		SDA_PU_PORT->DDR.reg |= SDA_PU_PIN; // switch pin to output
-		SDA_PU_PORT->CR1.reg |= SDA_PU_PIN; // switch pin to push pull
-		SDA_PU_PORT->ODR.reg |= SDA_PU_PIN; // pull up SDA line
-		SCL_PU_PORT->DDR.reg |= SCL_PU_PIN; // switch pin to output
-		SCL_PU_PORT->CR1.reg |= SCL_PU_PIN; // switch pin to push pull
-		SCL_PU_PORT->ODR.reg |= SCL_PU_PIN; // pull up SCL line
+	if (i2c_fwd) { // I²C lines are not pulled up
+		SDA_SRC_PU_PORT->CR1.reg &= ~SDA_SRC_PU_PIN; // disable pull up
+		SDA_SRC_PU_PORT->DDR.reg &= ~SDA_SRC_PU_PIN; // switch pin to input
+		SCL_SRC_PU_PORT->CR1.reg &= ~SCL_SRC_PU_PIN; // disable pull up
+		SCL_SRC_PU_PORT->DDR.reg &= ~SCL_SRC_PU_PIN; // switch pin to input
 	}
 
 	// configure I²C
@@ -287,7 +318,9 @@ we will only be able to provide 1 extension block since we can only respond to o
 
 	rim(); // re-enable interrupts
 	// even if we don't pull up HPD ourself, we should be able to respond to I²C within 20 ms
-	puts("I²C ready\r\n");
+	if (!i2c_fwd) {
+		puts("I²C source ready\r\n");
+	}
 
 	// check if EDID should be copied
 	EDID_PORT->DDR.reg &= ~EDID_PIN; // switch pin to output
@@ -297,9 +330,19 @@ we will only be able to provide 1 extension block since we can only respond to o
 	} else if (i2c_fwd) { // we are not the only master on the I²C sink lines
 		puts("can't read EDID: I²C lines forwarded\r\n");
 		LED_PORT->ODR.reg &= ~LED_PIN; // switch LED on to indicate error
+	} else if (!sink_present) {
+		puts("can't read EDID: sink not present\r\n");
+		LED_PORT->ODR.reg &= ~LED_PIN; // switch LED on to indicate error
 	} else {
 		IWDG_KR = IWDG_KR_KEY_REFRESH; // reset watchdog
 		puts("reading sink EDID: ");
+		LED_PORT->ODR.reg &= ~LED_PIN; // switch LED on to indicate we are saving EDID
+		SDA_SNK_PU_PORT->ODR.reg |= SDA_SNK_PU_PIN; // pull up SDA line
+		SDA_SNK_PU_PORT->CR1.reg |= SDA_SNK_PU_PIN; // switch pin to push pull
+		SDA_SNK_PU_PORT->DDR.reg |= SDA_SNK_PU_PIN; // switch pin to output
+		SCL_SNK_PU_PORT->ODR.reg |= SCL_SNK_PU_PIN; // pull up SCL line
+		SCL_SNK_PU_PORT->CR1.reg |= SCL_SNK_PU_PIN; // switch pin to push pull
+		SCL_SNK_PU_PORT->DDR.reg |= SCL_SNK_PU_PIN; // switch pin to output
 		uint8_t i2c_rc = 1; // if the complete read succeeded
 		if (!softi2c_master_setup(1)) { // start the I²C master to talk to sink
 			i2c_rc = 2;