MC6850 asynchronous comms adapter pinout

DIP-24. Asynchronous serial interface (ACIA).

The serial chip for the HERO Jr RS-232 accessory (U216). It answers at $D880-$DFFF with A7 on CS0 and A0 on RS; /CS2 comes from the U202 decode and CS1 from the reset net. Rx Data comes from level shifter Q201, Tx Data goes out through driver U219. /CTS and /DCD are strapped to ground and /RTS is unused. The ROM sets 7 data bits, even parity, 1 stop bit; baud rate comes from U217, U218 and jumper SW201 dividing the 1 MHz E clock. With no accessory fitted, nothing answers at $D880.

Open the interactive ACIA pinout

Pins

Pin	Name	Type	Signal
1	VSS	ground	Ground.
2	Rx Data	serial	Receive serial data in.
3	Rx CLK	clock	Receive clock (1, 16 or 64 times the bit rate).
4	Tx CLK	clock	Transmit clock (1, 16 or 64 times the bit rate).
5	/RTS	serial	Request to send, set by the control register.
6	Tx Data	serial	Transmit serial data out.
7	/IRQ	ctrl	Interrupt request, open drain.
8	CS0	ctrl	Chip select 0, active high.
9	/CS2	ctrl	Chip select 2, active low.
10	CS1	ctrl	Chip select 1, active high.
11	RS	ctrl	Register select: low = control/status, high = data.
12	VCC	power	+5 V supply.
13	R/W	ctrl	Read/write from the CPU.
14	E	clock	Enable clock from the CPU.
15	D7	data	Data bit 7.
16	D6	data	Data bit 6.
17	D5	data	Data bit 5.
18	D4	data	Data bit 4.
19	D3	data	Data bit 3.
20	D2	data	Data bit 2.
21	D1	data	Data bit 1.
22	D0	data	Data bit 0.
23	/DCD	serial	Data carrier detect input; high stops the receiver and can interrupt.
24	/CTS	serial	Clear to send input; high inhibits the transmit-data-register-empty flag.

Fitted in

HERO Jr U216 (accessory)
BBC Micro Model B IC4
BBC Master 128 IC45

Sources

Motorola MC6850 datasheet DS9493R4 (1985), pin assignment p. 1