more work towards IEC bus. Also don't corrupt $DD00 VIC-II bank bits in the process.

c64/kernal/iec_tx_byte.s

@@ -1,8 +1,44 @@
 iec_tx_byte:	
 
+	pha
+	
+	jsr iec_assert_clk_release_data
+
+	;; Give devices time to respond
+	jsr wait1ms
+
+	;; Did a device respond? (DATA will be pulled if so)
+	lda $DD00
+	bmi +
+
+	;; No devices present, so we can immediately return with device not found
+	pla
+	jmp kernalerror_DEVICE_NOT_FOUND
+*
+
+	;; Release CLK and wait for DATA to release.
+	jsr iec_release_clk_and_data
+	
+	;; Wait for DATA to be released
+	;; Listener is allowed to hold DATA for as long as they want.
+	;; i.e., this is the part of the protocol where the listener indicates
+	;; that they are busy doing something, like processing the previous
+	;; character.
+	jsr iec_wait_for_data_release
+
+	;; We then wait for 60 usec (less than 200usec so we don't
+	;; signal EOI), before asserting CLK again.
+	jsr iec_wait60us
+
+	;; Re-assert CLK line
+	jsr iec_assert_clk_release_data
+
+	
 	;; Do the actual transmission of 8 bits of data
 	ldx #8
 
+	pla
+	
 iec_tx_byte_nextbit:	
 	;; Is next bit 0 or 1?
 	lsr
@@ -40,7 +76,6 @@ iec_tx_byte_l1:
 	bne -
 
 	;; Timeout - DEVICE NOT PRESENT
-	cli
 	jmp kernalerror_DEVICE_NOT_FOUND
 *
 	;; All done.