diff --git a/src/devices/bus/ieee488/hp9895.cpp b/src/devices/bus/ieee488/hp9895.cpp
index 80aa16bc55c..e5b1f6de2ec 100644
--- a/src/devices/bus/ieee488/hp9895.cpp
+++ b/src/devices/bus/ieee488/hp9895.cpp
@@ -6,6 +6,9 @@
 
 	HP9895 floppy disk drive
 
+	Reference manual:
+	HP 09895-90030, feb 81, 9895A Flexible Disc Memory Service Manual
+
 *********************************************************************/
 
 #include "hp9895.h"
@@ -28,6 +31,7 @@ hp9895_device::hp9895_device(const machine_config &mconfig, const char *tag, dev
 #if 0
 ioport_constructor hp9895_device::device_input_ports() const
 {
+	// TODO: inputs=HPIB address, "S" & "W" switches, "loop" pin
 }
 #endif
 void hp9895_device::device_start()
@@ -170,8 +174,8 @@ ADDRESS_MAP_END
 static ADDRESS_MAP_START(z80_io_map , AS_IO , 8 , hp9895_device)
 	ADDRESS_MAP_UNMAP_HIGH
 	ADDRESS_MAP_GLOBAL_MASK(0xff)
-	// TODO: TEMP!
 	AM_RANGE(0x10 , 0x17) AM_DEVWRITE("phi" , phi_device , reg8_w) AM_READ(phi_reg_r)
+	// TODO: 60-67 range
 ADDRESS_MAP_END
 
 static MACHINE_CONFIG_FRAGMENT(hp9895)
diff --git a/src/devices/machine/phi.cpp b/src/devices/machine/phi.cpp
index 2ac82a44326..34fa8d6117c 100644
--- a/src/devices/machine/phi.cpp
+++ b/src/devices/machine/phi.cpp
@@ -19,6 +19,12 @@
 	* C1,C2,C3,C4,C5
 	* HP non-standard IDENTIFY sequence
 
+	Fun fact: PHI has no clock input, its FSMs are driven only by
+	changes in input signals and by a few internal monostables
+
+	Main reference for this ASIC:
+	HP 12009-90001, sep 82, HP12009A HP-IB Interface Reference Manual
+
 *********************************************************************/
 
 #include "emu.h"
@@ -514,10 +520,12 @@ uint8_t phi_device::get_dio(void)
 
 void phi_device::set_dio(uint8_t data)
 {
-	LOG(("DIO=%02x\n" , data));
-	m_dio = data;
-	if (!m_loopback) {
-		m_dio_write_func(~data);
+	if (data != m_dio) {
+		LOG(("DIO=%02x\n" , data));
+		m_dio = data;
+		if (!m_loopback) {
+			m_dio_write_func(~data);
+		}
 	}
 }