RS-232

RS-232

RS-232 is the serial communications protocol that won't die. It's been around since the 60's.

RS-232 is also known as RS232, RS-232-C, RS-232-D, EIA-232, TIA-232E, TIA-232-F -- and even V.28/V.24. These standards are not identical, but RS-232 will get you close to what you want. In theory, the RS-232C standard only specifies the signaling, not how you actually wire it up. The pinouts given in this article are common.

The RS-232C standard requires the transmitter to use +12V (0, logic low) and -12V (1, logic high). The receiver needs only see over +3V and under -3V. Most transmitting ports use only +5V and -5V. Those levels are compatible with most devices and host computers. Maxim is one of the more popular suppliers of RS-232 level shifters or level converters.

DE-9 pinout (AKA: D-sub9, TIA-574, DE-9, and DB-9)

diagram of a DE-9 connector

Technically this connector is called a D-sub 9 or a DE-9, but it is often incorrectly referred to as a DB-9. The D refers to the shape of the connector. The E specifies the size of the connector shell. I'm not sure why you couldn't just get that from the number of pins, but whatever...

 1 2 3 4 5
  6 7 8 9

DB-25 pinout

diagram of a DB-25 connector

The D refers to the shape of the connector. The B specifies the size of the connector shell. Note that DB-25 may carry two RS-232 lines, but this is rare.

 1   2   3   4   5   6   7  8   9  10  11  12  13

  14  15  16  17  18  19  20  21  22  23  24  25

3-wire RS-232

A minimal "3-wire" RS-232 connection consists only of TxD, RxD, and SGND.

pins 3,2,5 on DE-9
pins 2,3,7 on DB-25

	DE-9 pins	DB-25 pins
TxD	3	2
RxD	2	3
SGND	5	7

5-wire RS-232

A minimal RS-232 connection with hardware flow control: TxD, RxD, SGND, RTS, and CTS (pins 3,2,5,7,8 on DB-9 and pins 2,3,7,4,5 on DB-25).

RS-232 DB-25 to DE-9 pinout table

DB-25	DE-9	line	description	direction DTE is the computer DCE is the device
1		shield	ground shield
2	3	TxD	Transmit Data	OUT: DTE -> DCE
3	2	RxD	Receive Data	IN: DTE <- DCE
4	7	RTS	Request To Send	OUT: DTE -> DCE
5	8	CTS	Clear To Send	IN: DTE <- DCE
6	6	DSR	Data Set Ready	IN: DTE <- DCE
7	5	SGND	signal ground	< - >
8	1	DCD	Data Carrier Detect	IN: DTE <- DCE
9
10
11
12
13
14
15
16
17
18
19
20	4	DTR	Data Terminal Ready	OUT: DTE -> DCE
21
22	9	RI	Rind Indicator	IN: DTE <- DCE
23
24
25

Linux serial TTY communications -- stty

This section shows various ways to watch the serial ports to see what is going on with the signals.

/proc/tty/driver/serial

watch -n 1 cat /proc/tty/driver/serial

Every 1.0s: cat /proc/tty/driver/serial                 Tue Nov 17 11:39:23 2009

serinfo:1.0 driver revision:
0: uart:16550A port:000003F8 irq:4 tx:334 rx:214898 fe:2636 brk:254 RTS|DTR
1: uart:16550A port:000002F8 irq:3 tx:181 rx:12 RTS|CTS|DTR
2: uart:unknown port:000003E8 irq:4
3: uart:unknown port:000002E8 irq:3

setserial

watch -n 1 setserial -avg /dev/ttyS*

Every 1.0s: setserial -avg /dev/ttyS0 /dev/ttyS1 /d...  Tue Nov 17 11:40:02 2009

/dev/ttyS0, Line 0, UART: 16550A, Port: 0x03f8, IRQ: 4
        Baud_base: 115200, close_delay: 50, divisor: 0
        closing_wait: 3000
        Flags: spd_normal skip_test

/dev/ttyS1, Line 1, UART: 16550A, Port: 0x02f8, IRQ: 3
        Baud_base: 115200, close_delay: 50, divisor: 0
        closing_wait: 3000
        Flags: spd_normal skip_test

/dev/ttyS2, Line 2, UART: unknown, Port: 0x03e8, IRQ: 4
        Baud_base: 115200, close_delay: 50, divisor: 0
        closing_wait: 3000
        Flags: spd_normal skip_test

/dev/ttyS3, Line 3, UART: unknown, Port: 0x02e8, IRQ: 3
        Baud_base: 115200, close_delay: 50, divisor: 0
        closing_wait: 3000
        Flags: spd_normal

statserial

statserial /dev/ttyS0

Device: /dev/ttyS0

Signal  Pin  Pin  Direction  Status  Full
Name    (25) (9)  (computer)         Name
-----   ---  ---  ---------  ------  -----
FG       1    -      -           -   Frame Ground
TxD      2    3      out         -   Transmit Data
RxD      3    2      in          -   Receive  Data
RTS      4    7      out         1   Request To Send
CTS      5    8      in          0   Clear To Send
DSR      6    6      in          0   Data Set Ready
GND      7    5      -           -   Signal Ground
DCD      8    1      in          0   Data Carrier Detect
DTR     20    4      out         1   Data Terminal Ready
RI      22    9      in          0   Ring Indicator