diff --git a/Source/HBIOS/Config/RCZ180_sc126.asm b/Source/HBIOS/Config/RCZ180_sc126.asm
index 1b44e8c7..e90a52f3 100644
--- a/Source/HBIOS/Config/RCZ180_sc126.asm
+++ b/Source/HBIOS/Config/RCZ180_sc126.asm
@@ -9,6 +9,8 @@ Z180_CLKDIV	.SET	1		; 0=OSC/2, 1=OSC, 2=OSC*2
 Z180_MEMWAIT	.SET	0		; MEMORY WAIT STATES TO INSERT (0-3)
 Z180_IOWAIT	.SET	1		; IO WAIT STATES TO INSERT (0-3)
 ;
+DIAGPORT	.SET	$0D		; DIAGNOSTIC PORT ADDRESS
+;
 CPUOSC		.SET	18432000	; CPU OSC FREQ
 DEFSERCFG	.SET	SER_38400_8N1	; DEFAULT SERIAL LINE CONFIG
 ;
diff --git a/Source/HBIOS/acia.asm b/Source/HBIOS/acia.asm
index 966474df..48e54a92 100644
--- a/Source/HBIOS/acia.asm
+++ b/Source/HBIOS/acia.asm
@@ -101,7 +101,7 @@ ACIA_INITUNIT:
 	LD	HL,ACIA_DEV		; POINT TO CURRENT UART DEVICE NUM
 	LD	A,(HL)			; PUT IN ACCUM
 	INC	(HL)			; INCREMENT IT (FOR NEXT LOOP)
-	LD	(IY),A			; UDPATE UNIT NUM
+	LD	(IY),A			; UPDATE UNIT NUM
 ;
 #IF (INTMODE == 1)
 	; ADD IM1 INT CALL LIST ENTRY
@@ -171,11 +171,10 @@ ACIAA_INT00:
 	LD	E,A			; SAVE BYTE READ
 	LD	A,(ACIAA_BUFCNT)	; GET CURRENT BUFFER USED COUNT
 	CP	ACIAA_BUFSZ		; COMPARE TO BUFFER SIZE
-	;RET	Z			; BAIL OUT IF BUFFER FULL, RCV BYTE DISCARDED
 	JR	Z,ACIAA_INT2		; BAIL OUT IF BUFFER FULL, RCV BYTE DISCARDED
 	INC	A			; INCREMENT THE COUNT
 	LD	(ACIAA_BUFCNT),A	; AND SAVE IT
-	CP	ACIAA_BUFSZ - 5		; BUFFER GETTING FULL?
+	CP	ACIAA_BUFSZ / 2		; BUFFER GETTING FULL?
 	JR	NZ,ACIAA_INT0		; IF NOT, BYPASS DEASSERTING RTS
 	LD	A,ACIA_RTSOFF		; VALUE TO DEASSERT RTS
 	OUT	(C),A			; DO IT
@@ -216,11 +215,10 @@ ACIAB_INT00:
 	LD	E,A			; SAVE BYTE READ
 	LD	A,(ACIAB_BUFCNT)	; GET CURRENT BUFFER USED COUNT
 	CP	ACIAB_BUFSZ		; COMPARE TO BUFFER SIZE
-	;RET	Z			; BAIL OUT IF BUFFER FULL, RCV BYTE DISCARDED
 	JR	Z,ACIAB_INT2		; BAIL OUT IF BUFFER FULL, RCV BYTE DISCARDED
 	INC	A			; INCREMENT THE COUNT
 	LD	(ACIAB_BUFCNT),A	; AND SAVE IT
-	CP	ACIAB_BUFSZ - 5		; BUFFER GETTING FULL?
+	CP	ACIAB_BUFSZ / 2		; BUFFER GETTING FULL?
 	JR	NZ,ACIAB_INT0		; IF NOT, BYPASS DEASSERTING RTS
 	LD	A,ACIA_RTSOFF		; VALUE TO DEASSERT RTS
 	OUT	(C),A			; DO IT
@@ -291,9 +289,8 @@ ACIAA_IN:
 	LD	A,(ACIAA_BUFCNT)	; GET COUNT
 	DEC	A			; DECREMENT COUNT
 	LD	(ACIAA_BUFCNT),A	; SAVE SAVE IT
-	CP	5			; BUFFER LOW THRESHOLD
+	CP	ACIAA_BUFSZ / 4		; BUFFER LOW THRESHOLD
 	JR	NZ,ACIAA_IN0		; IF NOT, BYPASS SETTING RTS
-	
 	LD	C,(IY+3)		; C := ACIA CMD PORT
 	LD	A,ACIA_RTSON		; ASSERT RTS
 	OUT	(C),A			; DO IT
@@ -318,9 +315,8 @@ ACIAB_IN:
 	LD	A,(ACIAB_BUFCNT)	; GET COUNT
 	DEC	A			; DECREMENT COUNT
 	LD	(ACIAB_BUFCNT),A	; SAVE SAVE IT
-	CP	5			; BUFFER LOW THRESHOLD
+	CP	ACIAB_BUFSZ / 4		; BUFFER LOW THRESHOLD
 	JR	NZ,ACIAB_IN0		; IF NOT, BYPASS SETTING RTS
-	
 	LD	C,(IY+3)		; C := ACIA CMD PORT
 	LD	A,ACIA_RTSON		; ASSERT RTS
 	OUT	(C),A			; DO IT
@@ -458,46 +454,40 @@ ACIA_QUERY:
 ;
 ;
 ACIA_DEVICE:
-	LD	D,CIODEV_ACIA	; D := DEVICE TYPE
-	LD	E,(IY)		; E := PHYSICAL UNIT
-	LD	C,$00		; C := DEVICE TYPE, 0x00 IS RS-232
-	XOR	A		; SIGNAL SUCCESS
+	LD	D,CIODEV_ACIA		; D := DEVICE TYPE
+	LD	E,(IY)			; E := PHYSICAL UNIT
+	LD	C,$00			; C := DEVICE TYPE, 0x00 IS RS-232
+	XOR	A			; SIGNAL SUCCESS
 	RET
 ;
 ; ACIA DETECTION ROUTINE
 ;
 ACIA_DETECT:
-	;LD	C,ACIA_BASE	; BASE PORT ADDRESS
-	LD	C,(IY+3)	; BASE PORT ADDRESS
-	CALL	ACIA_DETECT2	; CHECK IT
-	JR	Z,ACIA_DETECT1	; FOUND IT, RECORD IT
-	;LD	C,ACIA_ALTBASE	; ALT BASE PORT ADDRESS
-	;CALL	ACIA_DETECT2	; CHECK IT
-	;JR	Z,ACIA_DETECT1	; FOUND IT, RECORD IT
-	LD	A,ACIA_NONE	; NOTHING FOUND
-	RET			; DONE
-;
-ACIA_DETECT1:
-	; ACIA FOUND, RECORD IT
-	;LD	A,C		; BASE PORT ADDRESS TO A
-	;LD	(IY+3),A	; SAVE ACTIVE BASE PORT
-	LD	A,ACIA_ACIA	; RETURN CHIP TYPE
-	RET			; DONE
+	LD	C,(IY+3)		; BASE PORT ADDRESS
+	CALL	ACIA_DETECT2		; CHECK IT
+	JR	Z,ACIA_DETECT1		; FOUND IT, RECORD IT
+	LD	A,ACIA_NONE		; NOTHING FOUND
+	RET				; DONE
+;	
+ACIA_DETECT1:	
+	; ACIA FOUND, RECORD IT	
+	LD	A,ACIA_ACIA		; RETURN CHIP TYPE
+	RET				; DONE
 ;
 ACIA_DETECT2:
 	; LOOK FOR ACIA AT PORT ADDRESS IN C
-	LD	A,$03		; MASTER RESET
-	OUT	(C),A		; DO IT
-	IN	A,(C)		; GET STATUS
-	OR	A		; CHECK FOR ZERO
-	RET	NZ		; RETURN IF NOT ZERO
-	LD	A,$02		; CLEAR MASTER RESET
-	OUT	(C),A		; DO IT
+	LD	A,$03			; MASTER RESET
+	OUT	(C),A			; DO IT
+	IN	A,(C)			; GET STATUS
+	OR	A			; CHECK FOR ZERO
+	RET	NZ			; RETURN IF NOT ZERO
+	LD	A,$02			; CLEAR MASTER RESET
+	OUT	(C),A			; DO IT
 	; CHECK FOR EXPECTED BITS:
 	;   TDRE=1, DCD & CTS = 0
-	AND	%00001110	; BIT MASK FOR "STABLE" BITS
-	CP	%00000010	; EXPECTED VALUE
-	RET			; RETURN RESULT, Z = CHIP FOUND
+	AND	%00001110		; BIT MASK FOR "STABLE" BITS
+	CP	%00000010		; EXPECTED VALUE
+	RET				; RETURN RESULT, Z = CHIP FOUND
 ;
 ;
 ;
diff --git a/Source/HBIOS/cfg_ezz80.asm b/Source/HBIOS/cfg_ezz80.asm
index 6be302c6..ec4b4eef 100644
--- a/Source/HBIOS/cfg_ezz80.asm
+++ b/Source/HBIOS/cfg_ezz80.asm
@@ -10,6 +10,8 @@ MEMMGR		.EQU	MM_Z2		; MM_NONE, MM_SBC, MM_Z2, MM_N8, MM_Z180
 RAMSIZE		.EQU	512		; SIZE OF RAM IN KB, MUST MATCH YOUR HARDWARE!!!
 DEFSERCFG	.EQU	SER_115200_8N1	; DEFAULT SERIAL LINE CONFIG (SHOULD MATCH ABOVE)
 INTMODE		.EQU	2		; 0=NONE, 1=INT MODE 1, 2=INT MODE 2
+DIAGENABLE	.EQU	FALSE		; TRUE FOR DIAGNOSTIC CODE PORT OUTPUT
+DIAGPORT	.EQU	$00		; DIAGNOSTIC PORT ADDRESS
 ;
 CRTACT		.EQU	FALSE		; CRT ACTIVATION AT STARTUP
 VDAEMU		.EQU	EMUTYP_ANSI	; DEFAULT VDA EMULATION (EMUTYP_TTY, EMUTYP_ANSI, ...)
diff --git a/Source/HBIOS/cfg_mk4.asm b/Source/HBIOS/cfg_mk4.asm
index 77e41df4..1063f810 100644
--- a/Source/HBIOS/cfg_mk4.asm
+++ b/Source/HBIOS/cfg_mk4.asm
@@ -10,6 +10,8 @@ MEMMGR		.EQU	MM_Z180		; MM_NONE, MM_SBC, MM_Z2, MM_N8, MM_Z180
 RAMSIZE		.EQU	512		; SIZE OF RAM IN KB, MUST MATCH YOUR HARDWARE!!!
 DEFSERCFG	.EQU	SER_38400_8N1	; DEFAULT SERIAL LINE CONFIG (SHOULD MATCH ABOVE)
 INTMODE		.EQU	2		; 0=NONE, 1=INT MODE 1, 2=INT MODE 2
+DIAGENABLE	.EQU	FALSE		; TRUE FOR DIAGNOSTIC CODE PORT OUTPUT
+DIAGPORT	.EQU	$00		; DIAGNOSTIC PORT ADDRESS
 ;
 CRTACT		.EQU	FALSE		; CRT ACTIVATION AT STARTUP
 VDAEMU		.EQU	EMUTYP_ANSI	; DEFAULT VDA EMULATION (EMUTYP_TTY, EMUTYP_ANSI, ...)
diff --git a/Source/HBIOS/cfg_n8.asm b/Source/HBIOS/cfg_n8.asm
index a885b13f..be578dbb 100644
--- a/Source/HBIOS/cfg_n8.asm
+++ b/Source/HBIOS/cfg_n8.asm
@@ -10,6 +10,8 @@ MEMMGR		.EQU	MM_N8		; MM_NONE, MM_SBC, MM_Z2, MM_N8, MM_Z180
 RAMSIZE		.EQU	512		; SIZE OF RAM IN KB, MUST MATCH YOUR HARDWARE!!!
 DEFSERCFG	.EQU	SER_38400_8N1	; DEFAULT SERIAL LINE CONFIG (SHOULD MATCH ABOVE)
 INTMODE		.EQU	2		; 0=NONE, 1=INT MODE 1, 2=INT MODE 2
+DIAGENABLE	.EQU	FALSE		; TRUE FOR DIAGNOSTIC CODE PORT OUTPUT
+DIAGPORT	.EQU	$00		; DIAGNOSTIC PORT ADDRESS
 ;
 CRTACT		.EQU	FALSE		; CRT ACTIVATION AT STARTUP
 VDAEMU		.EQU	EMUTYP_ANSI	; DEFAULT VDA EMULATION (EMUTYP_TTY, EMUTYP_ANSI, ...)
diff --git a/Source/HBIOS/cfg_rcz180.asm b/Source/HBIOS/cfg_rcz180.asm
index 482435ae..40f31017 100644
--- a/Source/HBIOS/cfg_rcz180.asm
+++ b/Source/HBIOS/cfg_rcz180.asm
@@ -10,6 +10,8 @@ MEMMGR		.EQU	MM_Z180		; MM_NONE, MM_SBC, MM_Z2, MM_N8, MM_Z180
 RAMSIZE		.EQU	512		; SIZE OF RAM IN KB, MUST MATCH YOUR HARDWARE!!!
 DEFSERCFG	.EQU	SER_38400_8N1	; DEFAULT SERIAL LINE CONFIG (SHOULD MATCH ABOVE)
 INTMODE		.EQU	2		; 0=NONE, 1=INT MODE 1, 2=INT MODE 2
+DIAGENABLE	.EQU	TRUE		; TRUE FOR DIAGNOSTIC CODE PORT OUTPUT
+DIAGPORT	.EQU	$00		; DIAGNOSTIC PORT ADDRESS
 ;
 CRTACT		.EQU	FALSE		; CRT ACTIVATION AT STARTUP
 VDAEMU		.EQU	EMUTYP_ANSI	; DEFAULT VDA EMULATION (EMUTYP_TTY, EMUTYP_ANSI, ...)
diff --git a/Source/HBIOS/cfg_rcz80.asm b/Source/HBIOS/cfg_rcz80.asm
index 62bc40c0..e24e52b5 100644
--- a/Source/HBIOS/cfg_rcz80.asm
+++ b/Source/HBIOS/cfg_rcz80.asm
@@ -10,6 +10,8 @@ MEMMGR		.EQU	MM_Z2		; MM_NONE, MM_SBC, MM_Z2, MM_N8, MM_Z180
 RAMSIZE		.EQU	512		; SIZE OF RAM IN KB, MUST MATCH YOUR HARDWARE!!!
 DEFSERCFG	.EQU	SER_115200_8N1	; DEFAULT SERIAL LINE CONFIG (SHOULD MATCH ABOVE)
 INTMODE		.EQU	1		; 0=NONE, 1=INT MODE 1, 2=INT MODE 2
+DIAGENABLE	.EQU	TRUE		; TRUE FOR DIAGNOSTIC CODE PORT OUTPUT
+DIAGPORT	.EQU	$00		; DIAGNOSTIC PORT ADDRESS
 ;
 CRTACT		.EQU	FALSE		; CRT ACTIVATION AT STARTUP
 VDAEMU		.EQU	EMUTYP_ANSI	; DEFAULT VDA EMULATION (EMUTYP_TTY, EMUTYP_ANSI, ...)
diff --git a/Source/HBIOS/cfg_sbc.asm b/Source/HBIOS/cfg_sbc.asm
index 3fcc40d3..c972476e 100644
--- a/Source/HBIOS/cfg_sbc.asm
+++ b/Source/HBIOS/cfg_sbc.asm
@@ -10,6 +10,8 @@ MEMMGR		.EQU	MM_SBC		; MM_NONE, MM_SBC, MM_Z2, MM_N8, MM_Z180
 RAMSIZE		.EQU	512		; SIZE OF RAM IN KB, MUST MATCH YOUR HARDWARE!!!
 DEFSERCFG	.EQU	SER_38400_8N1	; DEFAULT SERIAL LINE CONFIG (SHOULD MATCH ABOVE)
 INTMODE		.EQU	0		; 0=NONE, 1=INT MODE 1, 2=INT MODE 2
+DIAGENABLE	.EQU	FALSE		; TRUE FOR DIAGNOSTIC CODE PORT OUTPUT
+DIAGPORT	.EQU	$00		; DIAGNOSTIC PORT ADDRESS
 ;
 CRTACT		.EQU	FALSE		; CRT ACTIVATION AT STARTUP
 VDAEMU		.EQU	EMUTYP_ANSI	; DEFAULT VDA EMULATION (EMUTYP_TTY, EMUTYP_ANSI, ...)
diff --git a/Source/HBIOS/cfg_zeta.asm b/Source/HBIOS/cfg_zeta.asm
index 2ac1a983..6a3888fa 100644
--- a/Source/HBIOS/cfg_zeta.asm
+++ b/Source/HBIOS/cfg_zeta.asm
@@ -10,6 +10,8 @@ MEMMGR		.EQU	MM_SBC		; MM_NONE, MM_SBC, MM_Z2, MM_N8, MM_Z180
 RAMSIZE		.EQU	512		; SIZE OF RAM IN KB, MUST MATCH YOUR HARDWARE!!!
 DEFSERCFG	.EQU	SER_38400_8N1	; DEFAULT SERIAL LINE CONFIG (SHOULD MATCH ABOVE)
 INTMODE		.EQU	0		; 0=NONE, 1=INT MODE 1, 2=INT MODE 2
+DIAGENABLE	.EQU	FALSE		; TRUE FOR DIAGNOSTIC CODE PORT OUTPUT
+DIAGPORT	.EQU	$00		; DIAGNOSTIC PORT ADDRESS
 ;
 CRTACT		.EQU	FALSE		; CRT ACTIVATION AT STARTUP
 VDAEMU		.EQU	EMUTYP_ANSI	; DEFAULT VDA EMULATION (EMUTYP_TTY, EMUTYP_ANSI, ...)
diff --git a/Source/HBIOS/hbios.asm b/Source/HBIOS/hbios.asm
index e54da998..425e5d20 100644
--- a/Source/HBIOS/hbios.asm
+++ b/Source/HBIOS/hbios.asm
@@ -67,14 +67,14 @@ MODCNT	.SET	MODCNT + 1
 ;
 ;
 ;
-#IF ((PLATFORM == PLT_RCZ80) | (PLATFORM == PLT_RCZ180))
-#DEFINE	DIAGP	$00
-#ENDIF
+;#IF ((PLATFORM == PLT_RCZ80) | (PLATFORM == PLT_RCZ180))
+;#DEFINE	DIAGPORT	$00
+;#ENDIF
 ;
-#IFDEF DIAGP
+#IF (DIAGENABLE)
 #DEFINE	DIAG(N)	PUSH AF
   #DEFCONT \	LD A,N
-  #DEFCONT \	OUT (DIAGP),A
+  #DEFCONT \	OUT (DIAGPORT),A
   #DEFCONT \	POP AF
 #ELSE
 #DEFINE	DIAG(N)	\;
@@ -301,7 +301,7 @@ HBX_ROM:
 HBX_ROM:
 	RLCA				; TIMES 2 - GET 16K PAGE INSTEAD OF 32K
 	OUT	(MPGSEL_0),A		; BANK_0: 0K - 16K
-	;OUT	(DIAGP),A		; *DEBUG*
+	;OUT	(DIAGPORT),A		; *DEBUG*
 	INC	A			;
 	OUT	(MPGSEL_1),A		; BANK_1: 16K - 32K
 	RET				; DONE
@@ -774,9 +774,9 @@ HB_START:
 	DI				; NO INTERRUPTS
 	IM	1			; INTERRUPT MODE 1
 ;
-#IFDEF DIAGP
+#IF (DIAGENABLE)
 	LD	A,%00000001
-	OUT	(DIAGP),A
+	OUT	(DIAGPORT),A
 #ENDIF
 ;
 	LD	SP,HBX_LOC		; SETUP INITIAL STACK JUST BELOW HBIOS PROXY
@@ -2366,7 +2366,7 @@ HB_BADINT:
 	LD	HL,HB_BADINTCNT
 	INC	(HL)
 	LD	A,(HL)
-	OUT	(DIAGP),A
+	OUT	(DIAGPORT),A
 	OR	$FF
 	RET
 HB_BADINTCNT	.DB	0
diff --git a/Source/HBIOS/sio.asm b/Source/HBIOS/sio.asm
index 1dfacc13..0c0e85b1 100644
--- a/Source/HBIOS/sio.asm
+++ b/Source/HBIOS/sio.asm
@@ -175,7 +175,6 @@ SIOA_INT00:
 	LD	E,A			; SAVE BYTE READ
 	LD	A,(SIOA_CNT)		; GET CURRENT BUFFER USED COUNT
 	CP	SIOA_BUFSZ		; COMPARE TO BUFFER SIZE
-	;RET	Z			; BAIL OUT IF BUFFER FULL, RCV BYTE DISCARDED
 	JR	Z,SIOA_INT2		; BAIL OUT IF BUFFER FULL, RCV BYTE DISCARDED
 	INC	A			; INCREMENT THE COUNT
 	LD	(SIOA_CNT),A		; AND SAVE IT
@@ -221,7 +220,6 @@ SIOB_INT00:
 	LD	E,A			; SAVE BYTE READ
 	LD	A,(SIOB_CNT)		; GET CURRENT BUFFER USED COUNT
 	CP	SIOB_BUFSZ		; COMPARE TO BUFFER SIZE
-	;RET	Z			; BAIL OUT IF BUFFER FULL, RCV BYTE DISCARDED
 	JR	Z,SIOB_INT2		; BAIL OUT IF BUFFER FULL, RCV BYTE DISCARDED
 	INC	A			; INCREMENT THE COUNT
 	LD	(SIOB_CNT),A		; AND SAVE IT
@@ -309,7 +307,7 @@ SIOA_IN:
 	LD	A,(SIOA_CNT)		; GET COUNT
 	DEC	A			; DECREMENT COUNT
 	LD	(SIOA_CNT),A		; SAVE SAVE IT
-	CP	5			; BUFFER LOW THRESHOLD
+	CP	SIOA_BUFSZ / 4		; BUFFER LOW THRESHOLD
 	JR	NZ,SIOA_IN0		; IF NOT, BYPASS SETTING RTS
 	LD	A,5			; RTS IS IN WR5
 	OUT	(SIOA_CMD),A		; ADDRESS WR5
@@ -336,7 +334,7 @@ SIOB_IN:
 	LD	A,(SIOB_CNT)		; GET COUNT
 	DEC	A			; DECREMENT COUNT
 	LD	(SIOB_CNT),A		; SAVE SAVE IT
-	CP	5			; BUFFER LOW THRESHOLD
+	CP	SIOB_BUFSZ / 4		; BUFFER LOW THRESHOLD
 	JR	NZ,SIOB_IN0		; IF NOT, BYPASS SETTING RTS
 	LD	A,5			; RTS IS IN WR5
 	OUT	(SIOB_CMD),A		; ADDRESS WR5