diff --git a/source/ParallelPrinter.cpp b/source/ParallelPrinter.cpp
index 18429666..ec5d3f19 100644
--- a/source/ParallelPrinter.cpp
+++ b/source/ParallelPrinter.cpp
@@ -142,11 +142,15 @@ static BYTE __stdcall PrintStatus(WORD, WORD, BYTE, BYTE, ULONG)
 }
 
 //===========================================================================
-static BYTE __stdcall PrintTransmit(WORD, WORD, BYTE, BYTE value, ULONG)
+static BYTE __stdcall PrintTransmit(WORD, WORD address, BYTE, BYTE value, ULONG)
 {
 	if (!CheckPrint())
 		return 0;
 
+	// only allow writes to the load output port (i.e., $C090)
+	if ((address & 0xF) != 0)
+		return 0;
+
 	BYTE c = value & 0x7F;
 
 	if (IsPravets(GetApple2Type()))