diff --git a/Machines/AtariST/AtariST.cpp b/Machines/AtariST/AtariST.cpp
index 904163c0a..24d0e459d 100644
--- a/Machines/AtariST/AtariST.cpp
+++ b/Machines/AtariST/AtariST.cpp
@@ -654,8 +654,14 @@ class ConcreteMachine:
 					GPIP 0: centronics busy
 			*/
 			mfp_->set_port_input(
-				0x80 |
-				((keyboard_acia_->get_interrupt_line() || midi_acia_->get_interrupt_line()) ? 0x0 : 0x10)	// Interrupts are active low.
+				0x00 |	// b7: Monochrome monitor detect (1 = is monochrome).
+				0x40 |	// b6: RS-232 ring indicator.
+				0x20 |	// b5: FD/HS interrupt (0 = interrupt requested).
+				((keyboard_acia_->get_interrupt_line() || midi_acia_->get_interrupt_line()) ? 0x0 : 0x10) |	// b4: Keyboard/MIDI interrupt (0 = interrupt requested).
+				0x08 |	// b3: Unused
+				0x04 |	// b2: RS-232 clear to send.
+				0x02 |	// b1 : RS-232 carrier detect.
+				0x00	// b0: Centronics busy (1 = busy).
 			);
 		}