diff --git a/src/devices/machine/6821pia.cpp b/src/devices/machine/6821pia.cpp
index b5b6b123b0b..4171b9c6f82 100644
--- a/src/devices/machine/6821pia.cpp
+++ b/src/devices/machine/6821pia.cpp
@@ -237,11 +237,12 @@ uint8_t pia6821_device::get_in_a_value()
 		}
 	}
 
-	// For port A, when the port is in output mode, other devices can drive the pins too.
-	// If they load the lines so the voltage changes from what the PIA is outputting, that
-	// value will be read back.
-	// TODO: Figure out if any boards do this, and compensate.
-	ret = (m_out_a & m_ddr_a) | (port_a_data & ~m_ddr_a);
+	// - connected pins are always read
+	// - disconnected pins read the output buffer in output mode
+	// - disconnected pins are HI in input mode
+	ret = (~m_port_a_z_mask             & port_a_data) |
+			( m_port_a_z_mask &  m_ddr_a & m_out_a) |
+			( m_port_a_z_mask & ~m_ddr_a);
 
 	return ret;
 }