;+
;	Serial Input routine for the 6809
;	I/O port configured for Radio Shack color computer
;-
bitime	=	0x176	;Time constant for 1200 baud bit interval
halfbit	=	0x29	; 1/2 bit time for center bit sync
IPORT	=	0xFF22	; Color computer i/o port address
IMASK	=	1	; Bit position of input bit in port
;
;+
;	input call to receive a character
;-
serinp:
	.byte	0x1A,0x50	;orcc 50, kill interrupts
	bsr	inwait		;get character
	clr	b
	.byte	0x1E,0x89	;Exg B,A
	rts			;exit, character in b
;
;+
;	inwait	waits for an input character to start and
;	receives it.
;-
inwait:
	mov	IPORT,a
	and	$IMASK,a		; Get the bit out
	beq	inwait		; wait on line = 0 
;
serget:
	.byte	0x34,0x34	;PSHS X,Y,B
	.byte	0x10,0x8E,0,8	;LDY #8 = bitcounter
	.byte	0x8E
	.word	halfbit	;LDX bitime /2 
	clr	a		; clear receive register
;+
;	sync up on the start bit transition
;-
sync:
	mov	IPORT,b	;get port value
	and	$IMASK,b		;Test bit value
	bne	sync		;wait on start bit = 1
start:
	.byte	0x30,0x1F	;LEAX -1,X
	beq	assemble	;one half bit time has expired
	mov	IPORT,b
	and	$IMASK,b
	beq	start		;count down the start bit
	mov	$-1,a		; Glitch on start,return error
	.byte	0x35,0x34	;PULS X,Y,B
	rts			;error exit
assemble:
	.byte	0x8E
	.word	bitime		; set one bit time wait
assemb1:
	.byte	0x30,0x1F	; LEAX -1,x ,dec bit timer
	bne	assemb1
	mov	IPORT,b
	and	$IMASK,b
	beq	assemb2
	or	$0x80,a		; or in the bit
assemb2:
	.byte	0x31,0x3F	;LEAY -1,Y decr. bit count
	beq	seriox		; exit if done
	lsr	a		; move the bit down for the next pass
	bra	assemble
seriox:
	.byte	0x35,0x34	;PULS X,Y,B
	rts			; character in A
ter in A
cracter