CVDU hang on probe issues

ECB SBC hangs if no CVDU card is installed. Added timeout.

Source/HBIOS/cvdu.asm

@@ -10,7 +10,6 @@
 ;   - IMPLEMENT SET CURSOR STYLE (VDASCS) FUNCTION
 ;   - IMPLEMENT ALTERNATE DISPLAY MODES?
 ;   - IMPLEMENT DYNAMIC READ/WRITE OF CHARACTER BITMAP DATA?
-;   - IMPLEMENT TIMEOUT ON PROBE
 ;
 ;======================================================================
 ; CVDU DRIVER - CONSTANTS
@@ -293,12 +292,18 @@ CVDU_VDARDC:
 ;
 CVDU_WR:
 	PUSH	AF			; SAVE VALUE TO WRITE
+	PUSH	BC			;
 	LD	A,C			; SET A TO CVDU REGISTER TO SELECT
 	OUT	(CVDU_REG),A		; WRITE IT TO SELECT THE REGISTER
+	LD	B,0
 CVDU_WR1:
 	IN	A,(CVDU_STAT)		; GET CVDU STATUS
 	BIT	7,A			; CHECK BIT 7
-	JR	Z,CVDU_WR1		; LOOP WHILE NOT READY (BIT 7 NOT SET)
+	JR	NZ,CVDU_WRR		; EXIT WHEN READY (BIT 7 SET)
+	DJNZ	CVDU_WR1
+	;				; IF WE GET HERE WE HAVE TIMED OUT
+CVDU_WRR:
+	POP	BC
 	POP	AF			; RESTORE VALUE TO WRITE
 	OUT	(CVDU_DATA),A		; WRITE IT
 	RET
@@ -320,20 +325,26 @@ CVDU_WRX:
 CVDU_RD:
 	LD	A,C			; SET A TO CVDU REGISTER TO SELECT
 	OUT	(CVDU_REG),A		; WRITE IT TO SELECT THE REGISTER
+	PUSH	BC
+	LD	B,0
 CVDU_RD1:
 	IN	A,(CVDU_STAT)		; GET CVDU STATUS
 	BIT	7,A			; CHECK BIT 7
-	JR	Z,CVDU_RD1		; LOOP WHILE NOT READY (BIT 7 NOT SET)
+	JR	NZ,CVDU_RDR		; EXIT WHEN READY (BIT 7 SET)
+	DJNZ	CVDU_RD1
+	;				; IF WE GET HERE WE HAVE TIMED OUT
+CVDU_RDR:
 	IN	A,(CVDU_DATA)		; READ IT
+	POP	BC
 	RET
 ;
-CVDU_RDX:
-	CALL	CVDU_RD			; GET VALUE FROM REGISTER IN C
-	LD	H,A			; SAVE IN H
-	INC	C			; BUMP TO NEXT REGISTER OF PAIR
-	CALL	CVDU_RD			; READ THE VALUE
-	LD	L,A			; SAVE IT IN L
-	RET
+;CVDU_RDX:
+;	CALL	CVDU_RD			; GET VALUE FROM REGISTER IN C
+;	LD	H,A			; SAVE IN H
+;	INC	C			; BUMP TO NEXT REGISTER OF PAIR
+;	CALL	CVDU_RD			; READ THE VALUE
+;	LD	L,A			; SAVE IT IN L
+;	RET
 ;
 ;----------------------------------------------------------------------
 ; PROBE FOR CVDU HARDWARE