*******************************************************
*
* DW4Write  -  6809 Turbo Edition  115k / 230k
*    Send a packet to the DriveWire server.
*    Serial data format:  8-N-2
*
* Entry:
*    X  = address of the data to be sent
*    Y  = number of bytes to send
*
* Exit:
*    X  = address of last byte sent + 1
*    Y  = 0
*    All others preserved
*
*BBOUT	equ	$FF20		; bit banger output port

DWWrite	pshs	u,d,cc		; preserve registers
	orcc	#$50		; mask interrupts
	ldu	#BBOUT		; point U to output port
	lda	3,u		; read PIA 1-B control register
	anda	#$F7		; clear sound enable bit
	sta	3,u		; disable sound output

* Wait for serial input line to become idle
wait1	lda	2,u		; sample input
	ldb	#$80		; setup ACCB as a shift counter
wait2	eora	2,u		; look for changing input
	lsra			; shift 'changed' bit into Carry
	lda	2,u		; sample input
	rorb			; rotate 'changed' bit into ACCB
	bcc	wait2		; loop for 8 samples
	bne	wait1		; try again if changes seen in the input

* Byte transmitter loop
 	ldb	#2
   	lda	,x		; retrieve first byte from buffer
txByte	decb			; ACCB = 1 (start bit / byte incrementor)
	abx			; increment data ptr
	stb	,u		; send start bit
	asla			; move bit 0 into position
	nop
	sta	,u		; bit 0
	rora
	sta	,u+		; bit 1
	lsra
	sta	-1,u		; bit 2
	lsra
	sta	,-u		; bit 3
	lsra
	incb			; ACCB = 2 (stop bit)
	sta	,u		; bit 4
	lsra
	sta	,u+		; bit 5
	lsra
	sta	-1,u		; bit 6
	lsra
	sta	,-u		; bit 7
	lda	,x		; fetch next byte
	stb	,u		; send stop bit
	leay	-1,y		; decrement counter
	bne	txByte		; loop if more bytes to send

	puls	cc,d,u,pc		; restore registers and return