DUART: try not to step on 16x50 UART configuration

Source/HBIOS/duart.asm

@@ -562,6 +562,29 @@ DUART_DEVICE:
 ;
 DUART_DETECT:
 ;
+	; FIRST SEE IF IT LOOKS LIKE A 16X50-STYLE UART
+	LD	A,(IY + 3)		; GET BASE PORT OF CHIP
+	ADD	A,6			; BASE + 6 = CTU (DUART), MSR (1ST 16X50)
+	LD	B,A			; B := CTU/MSR PORT ADDRESS
+	INC	A			; BASE + 7 = CTL (DUART), SCR (1ST 16X50)
+	LD	D,A			; D := CTL/SCR PORT ADDRESS
+	ADD	A,7			; BASE + 14 = STCR (DUART), MSR (2ND 16X50)
+	LD	E,A			; E := STCR/MSR PORT ADDRESS
+	LD	A,$A5			; TEST VALUE
+	LD	C,B			; CTU
+	OUT	(C),A			; WRITE TEST VALUE TO CTU
+	LD	A,$FF			; LARGE VALUE TO PREVENT CTL FROM ROLLING OVER WHILE WE TEST
+	LD	C,D			; CTL
+	OUT	(C),A			; WRITE LARGE VALUE TO CTL
+	LD	C,E			; STCR
+	IN	A,(C)			; START COUNTER/TIMER (LATCH CTU, CTL)
+	LD	C,B			; CTU
+	IN	A,(C)			; READ BACK TEST VALUE
+	CP	$A5			; CHECK FOR TEST VALUE
+	JR	NZ,DUART_DETECT_NONE	; NO, PROBABLY NOT A DUART
+	IN	A,(C)			; CHECK TEST VALUE AGAIN,
+	CP	$A5			; ... IN RARE CASE DELTAS IN MSR WERE SET TO OUR TEST
+	JR	NZ,DUART_DETECT_NONE	; ALMOST CERTAINLY NOT A DUART
 	; SEE IF MR1 AND MR2 ARE DISTINCT
 	LD	A,DUART_CR_MR1		; SET MR POINTER TO MR1
 	DUART_OUTP(DUART_CR)		; SEND COMMAND