INS8250 asynchronous communications element (UART) pinout

DIP-40. Serial port: one UART with a built-in baud rate generator and modem control lines.

Converts bytes to and from the serial stream of the RS-232 port, with its own baud rate divider (16 x clock out on pin 15) and the modem lines DTR, RTS, CTS, DSR, RLSD and RI. Registers are picked with A0-A2; DLAB in the line control register switches A0-A1 to the divisor latch. On the PCjr it sits on the planar (ZM9) and interrupts on INTRPT. IBM asks for a short delay between back-to-back I/O to it. A dead 8250 gives a serial port that sends or receives nothing; check the RS-232 drivers (1488/1489) first.

Open the interactive 8250 pinout

Pins

Pin	Name	Type	Signal
1	D0	data	Data bus bit 0.
2	D1	data	Data bus bit 1.
3	D2	data	Data bus bit 2.
4	D3	data	Data bus bit 3.
5	D4	data	Data bus bit 4.
6	D5	data	Data bus bit 5.
7	D6	data	Data bus bit 6.
8	D7	data	Data bus bit 7.
9	RCLK	clock	Receiver clock, 16 x the receive baud rate (often tied to /BAUDOUT).
10	SIN	serial	Serial data in from the line receiver.
11	SOUT	serial	Serial data out to the line driver. Marking (HIGH) when idle.
12	CS0	ctrl	Chip select, active HIGH.
13	CS1	ctrl	Chip select, active HIGH.
14	/CS2	ctrl	Chip select, active LOW.
15	/BAUDOUT	clock	16 x baud rate clock from the divisor.
16	XTAL1	clock	Crystal or external clock input.
17	XTAL2	clock	Crystal output (not connected on the PCjr, which feeds XTAL1).
18	/DOSTR	ctrl	Data output (write) strobe, active LOW.
19	DOSTR	ctrl	Data output (write) strobe, active HIGH; tied LOW when /DOSTR is used.
20	VSS	ground	Ground.
21	/DISTR	ctrl	Data input (read) strobe, active LOW.
22	DISTR	ctrl	Data input (read) strobe, active HIGH; tied LOW when /DISTR is used.
23	DDIS	ctrl	Driver disable: LOW while the CPU reads the chip.
24	CSOUT	ctrl	Chip select out: HIGH while the chip is selected.
25	/ADS	ctrl	Address strobe: LOW latches A0-A2 and the selects. Tied LOW on the PCjr.
26	A2	addr	Register select bit 2.
27	A1	addr	Register select bit 1.
28	A0	addr	Register select bit 0.
29	NC	nc	No connection (the one pin the IBM pin description does not list). UNVERIFIED
30	INTRPT	ctrl	Interrupt request, HIGH when an enabled interrupt is pending.
31	/OUT2	ctrl	User output 2 (modem control register bit 3).
32	/RTS	serial	Request to send.
33	/DTR	serial	Data terminal ready.
34	/OUT1	ctrl	User output 1 (modem control register bit 2).
35	MR	ctrl	Master reset, active HIGH.
36	/CTS	serial	Clear to send.
37	/DSR	serial	Data set ready.
38	/RLSD	serial	Received line signal detect (carrier detect).
39	/RI	serial	Ring indicator.
40	VCC	power	+5 V.

Fitted in

IBM PCjr 4860 ZM9 (PCjr Technical Reference, sheet 8 of 17); INS8250N on the photographed board

INS8250 asynchronous communications element (UART) pinout

Pins

Fitted in

Sources