6551 ACIA pinout

DIP-28. RS-232 serial (Plus/4 only)

Asynchronous serial interface for the Plus/4 user port, up to 19,200 bps. Vdd is on pin 15, not 28. Any 6551 or 65C51 works as a replacement.

Pins

Pin	Name	Type	Signal
1	Vss	ground	0 V.
2	CS0	ctrl	Chip select, active HIGH.
3	/CS1	ctrl	Chip select, active LOW. Both must be true.
4	/RES	ctrl	Reset.
5	RxC	clock	Receive clock (external) or baud-rate clock out.
6	XTAL1	clock	Crystal in (1.8432 MHz).
7	XTAL2	clock	Crystal out.
8	/RTS	serial	Request To Send.
9	/CTS	serial	Clear To Send.
10	TxD	serial	Transmit data.
11	/DTR	serial	Data Terminal Ready.
12	RxD	serial	Receive data.
13	RS0	addr	Register select 0.
14	RS1	addr	Register select 1.
15	Vdd	power	+5 V supply (note: pin 15, not 28).
16	/DCD	serial	Data Carrier Detect.
17	/DSR	serial	Data Set Ready.
18	D0	data	Data bus bit 0.
19	D1	data	Data bus bit 1.
20	D2	data	Data bus bit 2.
21	D3	data	Data bus bit 3.
22	D4	data	Data bus bit 4.
23	D5	data	Data bus bit 5.
24	D6	data	Data bus bit 6.
25	D7	data	Data bus bit 7.
26	/IRQ	ctrl	Interrupt request.
27	Φ2	clock	System clock phase 2.
28	R/W	ctrl	Read/Write.